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Discovery and its tectonic significance of the Proterozoic Miaowan ophiolites in the southern Huangling anticline, western Hubei, China

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Studies of strongly deformed and metamorphosed mafic-ultramafic rocks from the Proterozoic Miaowan Formation in the high-grade metamorphic Kongling Group in the southern Huangling anticline show that the mafic rocks are composed of layered fine-grained aniphibolites, nietagabbro rockbodies and dykes and diabase dykes, while the ultramafic rocks are mainly composed of serpentinized dunite and harzburgite, cropping out as tectonite slices and blocks within aniphibolites. The geochemical features of the fine-grained layered amphibolite rocks including: TiO2=1.14%-1.48%, slightly depleted-flat type PvEE curve with no obvious Eu anomaly, (La/Yb)N=0.87-1.12, average La/Nb, Ce/Zr, Zr/Nb, Zr/Y, Ti/Y are respectively 1.04, 0.15, 18.78, 2.53, 290.51, and the average of Nb/Th is 9.88. These features show that these aniphibolites are N-MOPvB type tholeiites typical of those formed in mid-ocean ridge tectonic settings. The metagabbro have typical cumulate texture, flat REE distribution and obvious positive Eu anomaly; The REE characteristics of serpentinized dunite show a U-shape of slight loss of middle REE, representing cumulates meta-somatized by LREE slightly enriched mande. All these features indicate that the mafic-ultramafic rocks from the Proterozoic Miaowan Formation within Kongling Group in the southern part of Huangling anticline, are a structurally dismembered ophiolite resting above an ophiolitic melange, sitting on top of the Proterozoic shelf sequence on the Yangtze Craton. The discovery of the Proterozoic Miaowan ophiolite supplies important evidence for the existence of an Mesoproterozoic oceanic basin, as well as tectonic events of Columbia supercontinent aggregation and break-up.
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鄂西黄陵背斜南部元古宙庙湾蛇绿岩的发现
及其构造意义
彭松柏1
2
李昌年 1
Kusky Timothy M2
2
3
张先进 1
2
蒋幸福 1
2
熊承仁 2
PENG Song-bai1,2, LI Chang-nian1, KUSKY Timothy M2, WANG Lu2
3,
ZHANG Xian-jin1,2, JIANG Xing-fu1,2, XIONG Cheng-ren2
1. 中国地质大学武汉地球科学学院湖北 武汉 430074;
2. 中国地质大学/教育部长江三峡库区地质灾害研究中心湖北 武430074;
3. 中国海洋大学海洋地球科学学院山东 青岛 266100
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摘要#对鄂西黄陵背斜南部宜昌太平溪邓村一带崆岭岩群中的元古宙庙湾岩组强烈变形变质超镁铁镁铁质岩的研究表明
镁铁质岩主要为似层状细粒斜长角闪岩辉长岩岩岩脉及辉绿岩岩脉超镁铁质岩则主要为蛇纹石化纯橄榄岩方辉橄
榄岩呈构造岩片岩块分布于斜长角闪岩之中细粒斜长角闪岩 TiO2=1.14%~1.48%稀土元素配分型式为略亏损平坦型
明显的 Eu 异常(La/Yb)N=0.87~1.12La/NbCe/ZrZr/NbZr/YTi/Y 平均分别1.040.1518.782.53290.51Nb/Th
均为 9.88显示为大洋中脊构造环境形成的 N-MORB 型拉斑玄武岩变辉长岩具典型的堆晶结构特征稀土元素配分型式为
平坦型具明显的 Eu 正异常蛇纹石化纯橄榄岩的稀土元素配分型式具中稀土元素略亏损的 U形特征显示为 LREE 略富
的地幔岩上述特征表明黄陵背斜南部崆岭岩群中的元古宙庙湾岩组实际上是一套混杂堆积的古大洋蛇绿岩残片元古宙庙
湾蛇绿岩的发现为华南扬子克拉通存在中元古代洋盆和哥伦比亚超大陆聚合裂解构造事件提供了重要的证据
关键词湖北黄陵背斜元古宙庙湾蛇绿岩超镁铁镁铁质岩构造意义
中图分类号$P534.3;P542+.4 文献标志码$A文章编号$1671-2552%2010&01-0008-13
Peng S B, Li C N, Kusky T M, Wang L, Zhang X J, Jiang X F,Xiong C R. Discovery and its tectonic significance of the
Proterozoic Miaowan ophiolites in the southern Huangling anticline, western Hubei, China.Geological Bulletin of China,
2010, 29(1):8-20
Abstract: Studies of strongly deformed and metamorphosed mafic- ultramafic rocks from the Proterozoic Miaowan Formation in the
high-grade metamorphic Kongling Group in the southern Huangling anticline show that the mafic rocks are composed of layered
fine-grained amphibolites, metagabbro rockbodies and dykes and diabase dykes, while the ultramafic rocks are mainly composed of
serpentinized dunite and harzburgite, cropping out as tectonite slices and blocks within amphibolites. The geochemical features of the
fine-grained layered amphibolite rocks including: TiO2=1.14%-1.48% , slightly depleted-flat type REE curve with no obvious Eu
anomaly, (La/Yb)N= 0.87-1.12, average La/Nb, Ce/Zr, Zr/Nb, Zr/Y, Ti/Y are respectively 1.04,0.15,18.78,2.53,290.51, and the
average of Nb / Th is 9.88. These features show that these amphibolites are N-MORB type tholeiites typical of those formed in
收稿日期$2009-11-13"修订日期$2009-12-07
基金项目$中国地质大学%武汉&长江三峡库区地质灾害研究中心开放基金项目和中国地质调查局活动构造与区域地质环境调查成果
集成与综合研究(项目%编号#1212010670104&联合资助
作者简介#彭松柏%1963- &)))从事岩石大地构造*显微构造和活动构造研究+E-mail:psb200301@yahoo.com.cn
·基础地质·
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鄂西黄陵背斜地区是扬子克拉通内前南华纪变
质杂岩系出露的重要窗口以出露华南最古老的前
南华纪变质基底而闻名是研究扬子克拉通前南华
纪大地构造演化的关键地区前人对黄陵背斜地区
前南华纪变质杂岩系的研究主要集中在北部但黄
陵背斜南部宜昌太平溪邓村之间呈北西西向带状
展布的崆岭岩群庙湾岩组超镁铁镁铁质岩岩石组
成因时代及构造演化意义一直未引起足够的重
最近笔者对黄陵背斜南部元古宙庙湾岩组超镁
镁铁质岩进行了野外地质调查和岩石学地球
化学研究发现黄陵背斜南部崆岭岩群中的元古宙
庙湾岩组实际上为一套被构造肢解的古大洋蛇绿岩
残片其形成时代可能为中元古代
"研究区地质背景
黄陵地区前南华纪变质杂岩分布于著名的黄陵
背斜的核部主体为一套太古宙元古宙中深变质
11K 灰色片麻岩和变质表壳岩系R"&ST后被新元古
代黄陵花岗杂岩侵入分为南U个区并被南华
系不整合覆盖黄陵背斜南部的崆岭岩群大致可分
V套岩系下部古村坪杂岩以太古宙 11K 灰色
片麻岩为主夹少量斜长角闪岩变粒岩构成的表壳
岩系中部小渔村岩组为一套孔兹岩系其下段以含
石墨和富铝矿物的片麻岩为主夹云英片岩大理
石英岩上段以黑云变粒岩浅粒岩为主上部庙
湾岩组主体为层状细粒斜长角闪岩其间夹有少量
薄层大理岩和石英岩蛇纹石化纯橄岩方辉橄榄
辉长岩辉绿岩等呈独立岩片或岩块沿北西西向
断续出露于庙湾岩组内RWQST
早在 UX YXZX 年代湖北省第十地质队
"!YY"!Z""!ZV就对黄陵背斜南部崆岭岩群庙湾
岩组的太平溪超基性岩体进行了铬铁矿矿产的普查
和详查工作宜昌地质矿产研究所"!ZS对太平溪
超基性岩的岩石学矿物学特征进行了初步研究
研究多限于找矿勘查过程中涉及的地质问题和将其
作为独立岩体进行的区域地质调查!X 年代后
西地质"!!""S万区域地质调查中比照
花岗岩岩石谱系单元填图方法对黄陵背斜南部崆
岭岩群的超镁铁镁铁质岩进行了分解将其划为
中元古代梅子厂序列并利用 [#Q\% 全岩等时线
法在庙湾组获得斜长角闪岩的形成年龄 "YX]H*±
]"H*徐云鹏等RYQZT对宜昌太平溪一带蛇纹石
化橄榄岩的矿物学特征进行了研究王岳军等R]T
太平溪梅子厂超镁铁质岩的地球化学特征进行了初
步研究认为其属阿尔卑斯型残留地幔岩但对其成
因及构造意义未作进一步研究彭松柏等UXXZ
中南地区基础地质综合研究总结报告中认为
岭岩群中庙湾岩组斜长角闪岩具大洋玄武岩的地
球化学特征推测黄陵地区前南华纪可能存在中
元古代洋盆
黄陵背斜南部崆岭岩群庙湾岩组即庙湾蛇绿
的超镁铁镁铁质岩主要分布于宜昌太平溪
村一带总体呈北西西向带状展布")。 蛇纹石化
超镁铁质岩连续出露的最大长度达 "VF#宽度近 U
F#似层状镁铁质岩及变沉积岩则分布于蛇纹石化
超镁铁质岩南北两侧超镁铁质岩以蛇纹岩蛇纹石
化纯橄岩方辉橄榄岩为主镁铁质岩以似层状细粒
斜长角闪岩为主块状变辉长岩岩体岩脉和
辉绿岩岩脉则分布于似层状细粒斜长角闪岩和蛇纹
石化纯橄岩方辉橄榄岩之间与超镁铁镁铁质岩
空间上紧密相伴的变沉积岩主要为一套透镜状的薄
层大理岩石英岩蛇纹石化超镁铁质岩变形变质
层状块状变辉长岩和似层状细粒斜长角闪岩之间
呈构造接触并被新元古代黄陵花岗岩侵入和肢解
最后为南华系所覆盖总体构成一套呈北西西向展
布的构造蛇绿混杂岩系U)。 超镁铁镁铁质岩
均经历了强烈韧性和脆性变形变质作用的改造
镁铁质岩早期的韧性构造变形面理走向北西西向
彭松柏等鄂西黄陵背斜南部元古宙庙湾蛇绿岩的发现及其构造意义 !
地质通报
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2黄陵背斜南部庙湾蛇绿岩地质剖面15万地质资料修编
Fig. 1 Geological section of Miaowan ophiolites from southern Huangling anticline area
1黄陵背斜南部地区地质构造略图15万地质图修编
Fig. 1 Geological sketch map of southern Huangling anticline area
10
2! "
倾角近直立倾向变化较大总体以向北倾斜为主
而晚期脆性变形断裂面产状变化较大镁铁质岩总
体呈面状出露韧性变形强烈叠加褶皱发育
变化复杂
#蛇绿岩主要岩石单元的岩石学特征
本次重点对小溪口$梅子厂薄刀岭$茅垭剖面
进行了野外地质调查和分析测试研究研究表明
陵背斜南部崆岭岩群庙湾蛇绿岩中的超镁铁镁铁
质岩普遍遭受了韧性变形变质作用的改造
铁质岩主要为蛇纹石化纯橄岩方辉橄榄岩
质岩主要为似层状斜长角闪岩以及呈岩体
互相穿插的变形变质层状和块状辉长岩辉绿岩
尽管这些岩石大都遭受了强烈变形变质作用的改
但在一些变形较弱的地段主要岩石类型的结
构构造和矿物组合特征仍能清楚地观察到
对庙湾蛇绿岩超镁铁铁质石单
地质特征作简要的描述
"蛇纹石化方辉橄榄岩
主要见于古村坪薄刀岭梅子厂一线与蛇
纹石化纯橄岩紧密共生呈透镜状岩块岩片产出
岩石深灰黑色灰绿色他形半自形柱状结构
橄结构反应边结构网状结构蛇纹石化强烈矿物
具定向排列糜棱面理发育状构$%)。
主要矿物为辉石 &’()’*(橄榄+’()&’(
+()’(磁铁矿 ,()#(蚀变矿物主要为滑石
纹石和绿泥石橄榄石呈半自形自形柱状粒径
+)’--多已被蛇纹石滑石所取代并常见包橄结
$%)。 辉石主要为斜方辉石常蚀变为透
辉石角闪石主要由辉石退变而成呈半自形柱
粒径 ’),*--长轴具定向分布特征
#蛇纹石化纯橄岩
主要见于古村坪薄刀岭梅子厂一线与蛇
纹石化方辉橄榄岩紧密共生呈透镜状岩块岩片产
岩石为深灰黑色灰绿色他形粒状结构网状结
图版$.)、填隙结构蛇纹石化强烈矿物具定
向排列糜棱面理发育并见有豆荚状铬铁矿
$./$/0),块状构造主要矿物为橄榄
+*()&*(蛇纹石 ’*()1*(方辉2()+(
铁矿 ,()+(橄榄石呈他形粒状晶体较粗粒径可
+)’--沿网状裂隙大多橄榄石蚀变为蛇纹石
滑石呈残余孤岛状蚀变较弱的部位可见橄榄石呈
线状排列斜方辉石为半自形他形粒状粒径大小
,)+--几乎全被蛇纹石透闪石绿泥石交代呈假
偶见柱状辉石被叶蛇纹石置换成绢石局部可
见透辉石穿插包裹橄榄石随交代变质作用增
橄榄石向透辉石蛇纹石斜硅镁石菱镁矿
特别是滑石转化岩石颜色明显由深绿色变为灰
黑色灰绿色
+变辉长岩
主要见于古村坪薄刀岭梅子厂一线
于蛇纹石化纯橄岩方辉橄榄岩南侧呈岩体岩脉
产出岩石呈深灰色变余堆晶结构层状韵律构造
图版$34)、块状构造部分强烈韧性变形的岩石
具条带眼球状构造显微镜下可见变余辉长结构
图版$34)。 主要矿物为普通辉石 +’()&*(
性斜长石 &*()&’(普通角闪石 ’(),*( 磁铁矿
,()#(普通辉石一般为自形板柱状粒径一
’)5--多退变为角闪石纤闪石绿帘石绿
泥石等少数呈孤岛状残留常包嵌自形柱状斜长
有的呈半包嵌结构或熔蚀港湾结构斜长石呈柱
形程较高粒径比辉石略小一般为 +)
’--斜长石主要为拉长石角闪石主要由辉石退
变而成呈半自形柱粒状粒径一般为 #)+--
&变辉绿岩
主要见于古村坪薄刀岭梅子厂一线
于蛇纹石化纯橄岩方辉橄榄岩的南侧与变辉长岩
密切共生呈岩脉或岩墙产出岩石为深灰绿色
余辉长$辉绿结构块状构造部分强烈韧性变形的
岩石具条纹状构造主要矿物为普通辉石 +’()
&*(基性斜长石 &*()&’(普通角闪石 ’(),*(
铁矿 ,()#(普通辉石一般为他形不规则状粒径
一般为 ,)#--多退变为角闪石绿帘石绿泥石
少数呈孤岛状残留斜长石呈柱粒状自形
半自形粒径一般为 *6’),--主要为拉长石
斜长角闪岩
主要分布于古村坪薄刀岭梅子厂蛇纹石化
纯橄岩方辉橄榄岩变辉长岩和变辉绿岩的南
两侧似层状产出岩石为深灰色变余斑状结构
条带状构造普遍经历了韧性变形变质作用
$78)。 变余斑晶斜长石的粒度一般为 #)&--
部分变余斑晶表现为角闪石斑晶角闪石矿物集合
但仍保留有橄榄石和辉石的形态特征图版$
78)。基质为阳起石更长石绢云母粒径一般
彭松柏等鄂西黄陵背斜南部元古宙庙湾蛇绿岩的发现及其构造意义 ,,
地质通报
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a蛇纹石化方辉橄榄岩中的强韧性变形面理b蛇纹石化纯橄榄岩中的豆荚状铬铁矿c蛇纹石化纯
橄榄岩中的强变形定向排列状铬铁矿d变辉长岩与变辉绿岩的互相侵入穿插关系e变辉长岩与变辉
绿岩的层状韵律构造f变辉长岩的典型堆晶构造g变形斜长角闪岩h强变形斜长角闪岩
图版Plate
12
29 1
图版Plate
a蛇纹石化方辉橄榄岩中的包橄结构正交镜);b蛇纹石化纯橄榄岩中的网状结构正交镜c蛇纹石化纯橄
榄岩中豆荚状铬铁矿蚀变环单偏光);d蛇纹石化纯橄榄岩中的蚀变豆荚状铬铁矿反射光e变辉长岩中变
辉长结构辉石退变为角闪石正交镜);f变辉长岩中变辉长结构斜长石发生强烈退变质和交代蚀变
正交镜);g变形斜长角闪岩中的变余残斑结构单偏光h强变形斜长角闪岩中的变余残斑结构单偏光
彭松柏等鄂西黄陵背斜南部元古宙庙湾蛇绿岩的发现及其构造意义 13
地质通报
!"#$#!%&’$ ()$$"*%+ #, &-%+. 2010
3斜长角闪岩的分类[10]和系列判别图[11]
Fig. 3 Classification diagram and discriminant diagram for the amphibolites in Miaowan ophiolites
(a) Zr/TiO2vs.Nb/Y 分类图; (b) FeO*-Na2O+K2O-MgO 判别图!
TH-拉斑玄武岩!CA-钙碱性玄武岩!-茅垭斜长角闪岩
0.1~0.3mm"基质主要矿物为#普通角闪石 40%~
45%$35%~40%$透辉石 1%~2%$石英 5%~10%$
绢云母 2%~3%$磁铁矿 2%~3%"普通角闪石呈短柱
%颗粒边缘多呈圆滑状%波状消光明显%偶见透辉
石交代残晶保留短柱状辉石的外形"斜长石呈板状%
多被钠&更长石$绢云母$绿泥石呈假象交代石英
常呈透镜状和扁豆状%定向排列%玻状消光明显%
亚晶粒发育"&更长石则呈微粒状$透镜状集合体
相间分布%定向排列%显示斜长角闪岩经历了强烈的
韧性剪切变形"
3蛇绿岩主要岩石单元的地球化学特征
庙湾蛇绿岩岩石单元样品采自黄陵背斜南部太
平溪至邓村公路薄刀岭-茅垭地质构造剖面的崆岭
岩群庙湾岩组"岩石主量元素测试在武汉综合岩矿
测试中心完成!微量元素测试由中国地质大学 (
)地质过程与矿产资源国家重点实验室 LA-ICP-
MS 分析实验室完成%分析精度优于 1%~3%*蛇绿岩
岩石单元样品的主量元素+微量元素和稀土元素分
析结果列于表 1%数据处理采用路远发[9]的地球化学
工具软件包 GeoKit "由于样品经历了角闪岩相
变质%因此%本文主要用变质变形作用过程中不易迁
移的高场强元素(HFSE)Ti$Zr$Nb$Ta$Hf$Th%
(REE)和过渡金属元素(Sc$V$Cr$Ni)进行岩
石分类$成因和形成环境的判别"
3.1 斜长角闪岩的地球化学特征
3.1.1 主量元素
黄陵背斜南部崆岭岩群庙湾蛇绿岩中的似层
状镁铁质岩主要为细粒斜长角闪岩%Winchester
[10]的火山岩 Zr/TiO2-Nb/Y 岩石化学分类图上%
细粒斜长角闪岩的样品点落入亚碱性玄武岩范围
(3-a)% Nb/Y 比值较低(0.13~0.15)%故其原岩属
亚碱性玄武岩"Irvine [11]的玄武岩系列 FeO*-
(Na2O+K2O)-MgO 分类判别图中%样品点落入拉
斑玄武岩系列范围(3-b)" 从主量元素分析结果
可以看出%细粒斜长角闪岩 MgO 含量中等(MgO=
7.22% ~8.65% %Mg#=47~57)%CaO/Al2O3=0.75~0.88%
TiO2=1.01%~1.48%%TiO2量中(1.24%)%
与洋中脊拉斑玄武岩(MORB)TiO2(
1.5%)非常接近"Al2O3MgO 呈负相关关系%表明
没有明显的斜长石分离结晶"因此%细粒斜长角闪岩
的原岩为亚碱性拉斑玄武岩"
Pearce [12]TiO2-K2O -P2O5(
4-a)%斜长角闪岩的样品点均落入 OT 范围"
Glassley[13]TFeO/Mg-TiO2(4 -b )%
品点落入 MORB 范围"因此%从主量元素构造环境
14
29 1
1黄陵背斜南部庙湾蛇绿岩岩石单元主量元素微量元素和稀土元素分析结果
!"#$% 1&"’()* +)",% %$%-%.+ "./ 012 "."$3+4, )%56$+5 (7 +8% )(,9 6.4+5 4. &4"(:". (;84($4+%5 7)(- <6".=$4.= ".+4,$4.%
主量元素含量%稀土元素和微量元素含量 10-6
彭松柏等鄂西黄陵背斜南部元古宙庙湾蛇绿岩的发现及其构造意义 15
地质通报
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4斜长角闪岩主量元素构造判别图解
Fig. 4 Major elements tectonic discriminant diagrams for the amphibolites in Miaowan ophiolites
aTiO2-K2O-P2O5图解[19];(bTFeO/MgO-TiO2[13]OT大洋玄武岩CT大陆玄武岩
MORB洋中脊玄武岩IAT岛弧拉斑玄武岩OIB洋岛玄武岩-茅垭斜长角闪岩
判别图中可以看出这些镁铁质岩总体上应属大洋
中脊MOR构造环境形成的拉斑玄武岩
3.1.2 微量元素和稀土元素
从黄陵背斜南部庙湾蛇绿岩中似层状细粒斜长
角闪岩样品的稀土元素配分型式5-a可以看
细粒斜长角闪岩的稀土元素配分型式为轻稀土
元素略亏损平坦型稀土元素总量为 36.50×10-6~
49.05 ×10 -6LREE/HREE =1.57 ~1.91(La/Yb)N=
0.87 ~1.12(La/Sm)N=1.22 ~1.56La/Nb =0.86 ~1.19
1.04),Ce/Zr=0.13~0.170.15),Zr/Nb=
17.31~20.41平均 18.78),Zr/Y=2.42~2.64平均 2.53),
Ti/Y=283.89~300.00290.51无明显的 Eu
异常δEu=0.94~1.06显示其稀土元素配分特征与
N-MORB 相似这表明细粒斜长角闪岩形成于亏
损型大洋中脊N-MOR的构造环境[14-18]
Pearce[19]的洋中脊玄武岩MORB归一化微
量元素分布图5-b斜长角闪岩中 LILSr
Ba对于 MORB 明显富集LIL 元素Th
在蚀变和变质过程中是活动元素对变质火山岩
往往没有明确的构造背景意义而不活泼元素Th
HFSE REE基本上与标准 MORB 布型
表明它们很可能来源于 N-MORB 或类似 N-
MORB 的地幔源区玄武岩基本上没有受到地壳物
质的混染
斜长角闪岩样品具有比较一致的 Ti/V
20.00~22.20),MORB [20]Wood[21]
Hf-Ta-Nd-判别图6-a样品点落入亏损型
地幔洋中脊玄武岩N-MORBPearce [22]
Ti-Zr-Sr 判别图6-b落入洋中脊拉斑玄武
ORTBMeschede[23]Nb-Zr-Y
样品点落入 N-MORB 俯冲带上山岛
武岩IABJenner [24]Nb/Th-Y 火山弧
和非火山弧环境判别图中斜长角闪岩的Nb/Th=
9.12~10.619.88 Nb/Th 比值大于7.4与原
始地幔相比均显示正异常Th 明显比 Nb 亏损
显示其为大洋中脊构造环境形成的拉斑玄武岩
上述结果表明黄陵背斜南部庙湾蛇绿岩中似
层状细粒斜长角闪岩的主量微量和稀土元素地球
化学特征与 N-MORB 型大洋中脊构造环境形成
的拉斑玄武岩相似因此庙湾岩组中似层状细粒斜
长角闪岩的原岩属略亏损地幔源区形成的大洋中脊
拉斑玄武岩
3.2 变辉长岩的地球化学特征
黄陵背斜南部庙湾岩组镁铁质侵入岩主要以层
状和块状变辉长岩岩体岩脉及辉绿岩岩脉产出
经历了强烈的变形和变质作用但一些地段仍可见
有典型的层状堆晶结构以及辉长岩与辉绿岩互相
侵入穿插的关系表明变辉长岩变辉绿岩的形成时
16
!! "
#蛇绿岩岩石单元稀土元素球粒陨石标准化分布图!$"
微量元素 %&’( 标准化蛛网图!)"
*+,- . /01234+5672148$9+:63 ’;; 3+$,4$8< =$> $23 %&’(72148$9+:63
54$?6 6968625 <@+36473+$,4$8< =)> A14 506 41?B C2+5< +2 %+$1D$2 1@0+19+56<
#茅垭斜长角闪岩$#薄刀岭变辉长岩$#薄刀岭%马滑沟蛇纹石化纯橄榄岩
代基本一致&反映它们是同一地质事件的产物
%+339681<5 EF#G 的火成岩 H+&I7!J$I& KLF&"M4N
O+&F7J)NP 岩石化学分类图上&变辉长岩 J)NP
值很低!Q-QRSQ-QT"&样品点落入亚碱性辉长岩区(
变辉长岩样品的稀土元素配分型式可以看出&
元素配分型式呈平坦分布!#7$ "( 稀土元素总量
#-UV "WQ 7RSX-R! "WQ 7R&Y’;;NZ’;; [W-X. SI -QV&
=Y$NP)>J[W-UX 7W-U!&=Y$NH8>J[W-XX SW-X!$Y$NJ) [
Q-XRSW-W!!W-QU"&/6NM4[Q-WVSQ-WT!Q-W."&
M4NJ)[WT-VWSIQ-UW !WX-TX"&M4NP[I-UISI-RU
!I-.V"&O+NP[IWX-RISI..-VI!IVR-!T"&O0 N
P) [Q -W! S Q- IX !Q- IU "&J)NP)[Q-RTSQ-XW!
Q-TU"( %重稀土元素分异较弱&具明显的 ;C
异常!δ;C[I-QXSI-!V"&表明有明显的斜长石堆晶作
用存在&这与大洋地幔部分熔融形成的层状堆晶
辉长岩一致(上述特征表明&岩组%
块状辉长岩实际上是地幔部分熔融形成的堆晶岩
EWR7WX\ FR7]QG(
]-] 蛇纹石化纯橄榄岩的地球化学特征
黄陵背斜南部庙湾蛇绿岩中的超镁铁质岩以岩
%岩片产出的蛇纹石化纯橄岩%方辉橄榄岩和蛇纹
岩最为常见(蛇纹石化程度一般可达 .Q^SRQ^(
纹石化纯橄岩中的橄榄石属镁橄榄石&*1 [!W-]XS
!F-QQER7TG(蛇纹石化纯橄岩主量元素的平均含量 H+&F
_Q-FT^%‘9F&]W-_]^%*6F&]T-QT^ %*6&
W-T.^%%,& ]R-WT^%/$& Q-W.^%J$F&Q-Q]^%
%2& Q-WW#%O+&FQ-Q]$%aF&.Q-QW^&镁铁
比值 %,b[%,&N=%,&KO*6&>XTS!Q 之间&
彭松柏等)鄂西黄陵背斜南部元古宙庙湾蛇绿岩的发现及其构造意义 WT
地质通报
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6斜长角闪岩微量元素 Hf-Th-Nb 图解(a)[21]Ti-Zr-Sr 构造判别图解(b)[22]
Fig. 6 Trace element tectonic discriminant diagrams for the amphibolites in Miaowan ophiolites
N-MORB!亏损型洋脊玄武岩"E-MORB!富集型洋脊玄武岩"IAT !岛弧拉斑玄武岩"CAB!钙碱性玄武岩"
WPT!板内拉斑玄武岩"WPAB!板内碱性玄武岩"ORTB !洋中脊拉斑玄武岩"ITB!岛弧玄武岩"!茅垭斜长角闪岩
洋壳底部变质地幔橄榄岩的特征一致#为弱亏损
的地幔橄榄岩$暗示可能与 N-MORB 的成因有
%蛇纹石化纯橄岩稀土元素&REE 总量较低
&1.77 ×10-6$(La/Yb)N=4.17$(La/Sm)N=3.04 ~6.40$
显示轻稀土元素略富集$δEu=0.92$呈不明显的弱
负异常$稀土元素配分型式表现为中稀土元素略
亏损的 U形特征&5-a$这与造山带蛇绿岩中
变质地幔橄榄岩&包括二辉橄榄岩(橄榄
纯橄岩的特征相似[31-33]%蛇纹石化橄榄岩 REE
量比地幔岩的稍高$轻稀土元素略富集$可能是后
期强烈蛇纹石化蚀变吸附较多轻稀土元素&LREE
的结果[26, 31, 33-35]%
4构造意义
黄陵背斜南部太平溪)邓村一带前南华纪崆岭
岩群中的庙湾蛇绿岩位于扬子克拉通北部边缘$
露的超镁铁!镁铁质岩(玄武岩及硅质岩(大理岩岩
石组合与典型的蛇绿岩岩石单元组合一致$而且各
岩石单元之间为构造接触关系$表明为一套具密切
成因关系的构造蛇绿混杂岩系%似层状细粒斜长角
闪岩具大洋中脊拉斑玄武岩的岩石地球化学特征$
明显不同于板内玄武岩$也不同于许多元古宙具俯
冲带岛弧特征的低 Ti 的钙碱性玄武岩$这种情况通
常出现在大洋中脊构造环境[17, 36-41]%变辉长岩具典型
的层状堆晶结构和明显的 Eu 正异常$变辉长岩(
辉绿岩呈岩脉状互相穿插$指示辉长岩和辉绿岩
的形成时代一致$显示地幔部分熔融形成的堆晶
岩浆岩的特征%蛇纹石化纯橄岩的稀土元素配分
式具型的 U形特征$与洋壳底部地幔橄榄岩
的稀土元素配分型式相似$而与层状(块状变辉长
岩和细粒斜长角闪岩的稀土元素配分型式明显不
$表明蛇纹石化纯橄岩很可能是经历了地幔部分
熔融的地幔岩[31-33]%超镁铁!镁铁质岩的上述特征
表明$黄陵背斜南部太平溪!邓村一带前南华纪崆
岭岩群庙湾岩组的超镁铁!镁铁质岩$实际上是由
变质地幔岩&蛇纹石化橄榄岩(方辉橄榄岩’(地幔岩
部分熔融聚集形成的堆晶岩浆岩 &变辉长岩(辉绿
和大洋中脊构造环境形成的拉斑玄武岩&斜长角
闪岩所构成的蛇绿岩岩石组合$经后期变形变质作
用改造成为构造蛇绿岩残片%
对于黄陵背斜南部崆岭岩群中庙湾蛇绿岩的形
成时代$鄂西地质大队&1991在庙湾岩组&即庙湾蛇
绿岩似层状斜长角闪岩中利用 Sm-Nd 全岩等时
线法获得的形成年龄为 1608Ma±81Ma$但其测年的
方法和精度有待改进和提高%马大铨等[4]对崆岭岩
群小渔村岩组(庙湾岩组前人所测的 Rb-Sr
18
2! "
!#$%& 法等同位素年龄分析后认为"庙湾岩组的
形成时代可能在 "’()*+")),- 之间#近年来"
华南前寒武纪地质研究的深入"特别是对黄陵背
斜北部崆岭杂岩地质热事件年代学的研究发现"
扬子克拉通在古元古代晚期$"’()*+))),-%
一次重要的构造.变质.岩浆热事件"并可能最终
形成扬子陆块的统一基底/0+.0(1#这一重要地质热事
件与华北克拉通古元古代造山作用事件的时间基
本一致/02.0!1"可能与哥伦比亚$3456789-%
聚合有密切关系/(:$("1#黄陵背斜南部太平溪&邓村一
带崆岭岩群元古宙庙湾蛇绿混杂岩的发现"为扬子
克拉通存在古中元古代的洋盆提供了重要的物质
记录"并且暗示扬子克拉通目前的范围可能是由不
同地块经元古宙的洋$陆俯冲$碰撞拼贴最终形成
"但黄陵背斜南部元古宙庙湾蛇绿岩是古元古代
$"’(:*+:::,-%$碰撞拼贴之前形成的"还是在
$碰撞拼贴之后的中元古代裂解作用中形成
"尚需精确的年代学研究#
综上所述"黄陵背斜南部崆岭岩群中的元古宙
庙湾岩组超镁铁镁铁质岩实际上是一套被构造肢
解的蛇绿岩残片"其形成时代可能为中元古代#黄陵
背斜南部崆岭岩群元古宙庙湾蛇绿岩的发现"为重
新认识扬子克拉通乃至华南前寒武纪大地构造演化
提供了重要的信息#但黄陵背斜南部元古宙庙湾蛇
绿岩是在哥伦比亚$3456789-%超大陆俯冲$碰撞聚
合作用之前形成的"还是与俯冲$碰撞聚合之后再
裂解形成洋盆的作用有关等关于华南前寒武纪的重
大地质问题尚需进一步深入研究"这对于华南前寒
武纪大地构造演化的研究无疑具有重要意义#
致谢(在野外工作期间得到王建雄高级工程师
等的帮助和支持"并与张旗研究员&刘观亮研究员&
高山教授&江麟生高级工程师&周忠友高级工程师进
行了有益的讨论"张旗研究员&高山教授还对本文提
出了许多有益的建议和意见"在此一并表示真诚的
感谢)
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... It is composed of voluminous Mesoproterozoic to Neoproterozoic igneous intrusions containing minor mafic to ultramafic rock associations along the margins, while Paleoproterozoic to Archean rocks crop out mainly in the Huangling dome that is located about 200 km away from the northern margin of the craton (Li et al., 2008;Wang et al., 2013a;Zheng et al., 2013;Guo et al., 2014;Xu et al., 2014, and references therein). A study of the sedimentary rocks associated with the late Mesoproterozoic-early Neoproterozoic mafic to ultramafic rock associations can provide an excellent opportunity to resolve outstanding questions about Precambrian tectonothermal events that occurred during the evolution of the Yangtze craton (Jiang et al., 2002;Xiong et al., 2004), especially for the assembly and break-up of the Rodinia Zhou et al., 2007;Zhang et al., 2008;Peng et al., 2010;Zhang and Zheng, 2013) and the Columbia supercontinents (Zhang et al., 2006;Xiong et al., 2008;Wu et al., 2009;Peng et al., 2012c). Peng et al. (2010) showed that the mafic to ultramafic rock association, formerly called the Miaowan Formation, in the southern Huangling dome in the Yangtze craton represents a Proterozoic ophiolite, named the Miaowan Ophiolite Complex (MOC) (Fig. 1b). ...
... A study of the sedimentary rocks associated with the late Mesoproterozoic-early Neoproterozoic mafic to ultramafic rock associations can provide an excellent opportunity to resolve outstanding questions about Precambrian tectonothermal events that occurred during the evolution of the Yangtze craton (Jiang et al., 2002;Xiong et al., 2004), especially for the assembly and break-up of the Rodinia Zhou et al., 2007;Zhang et al., 2008;Peng et al., 2010;Zhang and Zheng, 2013) and the Columbia supercontinents (Zhang et al., 2006;Xiong et al., 2008;Wu et al., 2009;Peng et al., 2012c). Peng et al. (2010) showed that the mafic to ultramafic rock association, formerly called the Miaowan Formation, in the southern Huangling dome in the Yangtze craton represents a Proterozoic ophiolite, named the Miaowan Ophiolite Complex (MOC) (Fig. 1b). Subsequent studies Jiang et al., 2012;Peng et al., 2012a) suggested that the MOC formed in a forearc tectonic setting at an intra-oceanic supra-subduction zone at ca. 1.0 Ga. ...
... The latter can be further subdivided into three major suites: (1) gray granitic gneisses with few amphibolite lenses; (2) graphite and aluminous gneiss; and (3) mafic-ultramafic rocks. The third suite is dominated by metamorphosed dunite, harzburgite, layered and massive gabbros, massive diabase, basalt, sheeted dikes, plagiogranite, and a small amount of mylonites (Peng et al., 2010Deng et al., 2012). ...
... The Miaowan Ophiolite Complex has been recognized in the northern margin of the Yangtze Craton, China ( Fig. 1; Peng et al., 2010Peng et al., , 2012a. Peng et al. (2010Peng et al. ( , 2012a initially proposed that the ophiolite complex includes spatially and temporally associated metamorphic rocks including serpentinized harzburgite and serpentinized dunite, isotropic and layered gabbros, diabasic sheeted dikes, plagiogranite, basalt and sedimentary rocks. ...
... The Miaowan Ophiolite Complex has been recognized in the northern margin of the Yangtze Craton, China ( Fig. 1; Peng et al., 2010Peng et al., , 2012a. Peng et al. (2010Peng et al. ( , 2012a initially proposed that the ophiolite complex includes spatially and temporally associated metamorphic rocks including serpentinized harzburgite and serpentinized dunite, isotropic and layered gabbros, diabasic sheeted dikes, plagiogranite, basalt and sedimentary rocks. On the basis of geochemical data and zircon U-Pb ages, it was suggested that this ophiolite complex formed in a forearc setting during late Mesoproterozoic to early Neoproterozoic times (1120-970 Ma) (Peng et al., 2012a). ...
... The Miaowan Complex (previously defined as Miaowan Group, Ma et al., 1997) mainly consists of layered fine-grained metabasites with intermittently exposed serpentinized harzburgite, serpentinized dunite, gabbro, diabase and basalt with minor metasedimentary units (Fig. 1b;Peng et al., 2012a). Peng et al. (2010Peng et al. ( , 2012a proposed that the Miaowan Complex is a Proterozoic ophiolite complex. However, in this contribution, based on differences of intensity of deformation, ages of igneous rocks, and geochemical characteristics of mafic-ultramafic rocks, we divide the Miaowan Complex into two ...
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The Miaowan Complex in the Northern Yangtze Craton consists mainly of layered fine-grained metabasites, pillow lavas, sheeted dikes, gabbros, sepentinized harzburguite, and sepentinized dunite, with rare metasedimentary rocks in the metabasite section, and was initially regarded as an ophiolite complex. In this study, we divide the Miaowan Complex into two suites including the Miaowan Ophiolite Suite and a Late Magmatic Suite, based on differences in the degree of deformation, age and geochemical characteristics. The Miaowan Ophiolite Suite (MOS) mainly consists of ductily deformed serpentinized harzburgite, sepentinized dunite, gabbro, sheeted dikes, basalt, plagiogranite, and layered metasedimentary rocks. All these units were then intruded by the Late Magmatic Suite (LMS) consisting of pegmatitic-isotropic gabbro and massive diabase. Magmatic zircons from a deformed gabbro in the MOS yield an age of ca. 1115 Ma, consistent with the whole-rock Sm-Nd errochron age (1135±54 Ma) of harzburgite, gabbro and basalt in the MOS. Hence, the formation age of the MOS is interpreted to be ca. 1115 Ma. Harzburgites in the MOS are characterized by smooth LREE-depleted and flat MREE-HREE patterns; whereas dunites in the MOS display U-shape REE patterns. Deformed gabbro and basalt in the MOS display flat to slightly LREE enriched patterns, and low Th/Yb ratios and a lack of Nb anomalies, showing N-MORB affinities. The initial εNd (t) values of rock units in the MOS range from +6.6 to +7.8, indicating that the MOS was derived from a strongly depleted mantle source. Accordingly, the harzburgite, gabbro and basalt in the MOS are interpreted to have formed in an oceanic spreading center and the dunites in the MOS were formed by reaction between the harzburgites and the subduction-related boninitic melts when the MOS was trapped with the harzburgites as a part of the mantle wedge above a subduction zone in response to the initiation of an intra-oceanic subduction system. Igneous zircons from late pegmatitic-isotropic gabbros in the LMS yield weighted mean ²⁰⁷Pb/²⁰⁶Pb ages of 973±15 Ma, 999±17 Ma and 1002±19 Ma, respectively, suggesting that the LMS was intruded between ca. 1000 Ma and ca. 970 Ma, consistent with their whole-rock Sm-Nd errochron age (1007±62 Ma). Pegmatitic-isotropic gabbro and diabase in the LMS are characterized by enriched-LREE patterns with high Th/Yb ratios and negative Nb and Zr anomalies, consistent with a subduction-related setting. The initial εNd (t) values of rock units in the LMS range from +6.0 to +7.2. It is suggested that the LMS formed in a depleted fore-arc setting. Collectively, the Miaowan Complex consists of two magmatic suites including an older MORB-type ophiolitic suite and a younger arc-related magmatic suite, recording the evolution of geodynamic settings in a Proterozoic ocean.
... 1a). The Miaowan Ophiolite Complex, defined by Peng et al. (2012Peng et al. ( , 2010, crops out in the southern Huangling dome (Jiang et al., 2016;Gao et al, 2011;Ma et al., 1997;Fig. 1b). ...
... The older ophiolite suite, occurs along a WNW strike for 13 km long and with nearly 4 km wide, which has tectonic contact with the Archean TTG (tonalite-trondhjemite-granodiorite) (Deng et al., 2017;Zhang et al., 2011), showing location of the Huangling dome. (b) Geological map of the Miaowan ophiolite complex in the southern Huangling dome (Jiang et al., 2018;Deng et al., 2017;Peng et al., 2012Peng et al., , 2010 gneisses and Paleoproterozoic metasedimentary rocks and has intrusive contact with the Neoproterozoic Huangling granitoids (Fig. 1b). The suite has a formation age of ~1 115 Ma and consists from base to the top of ultramafic rocks, metagabbro and metadiabase, sheeted dike complex, metabasalt, pillow lava and metasedimentary rocks (Deng et al., 2017;Jiang et al., 2016). ...
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Coronitic microstructures have been used to interpret the late-stage solidification history of igneous rocks and to constrain the corresponding chemical and/or physical changes. Coronas with three shells were also recognized in the Miaowan olivine norite, Yangtze Craton, South China. In our study, orthopyroxene intergrowth with vermicular magnetite in the inner shell is in optical continuity with magnetite-free orthopyroxene in the middle shell. In the outer shell of brown amphibole remaining magnetite-free orthopyroxene inclusions sporadically occur. Meanwhile Mg# values of orthopyroxene (76–80) in the inner and middle shells are basically consistent with olivine (78–81). In this paper, we propose a multi-stage genetic model for the formation of coronas in the Miaowan olivine norite. In the first stage, the magnetite-free orthopyroxene shell formed through reaction between primocrystal olivine with the residual Si-rich melt at 990–1 053 °C and 6.2–6.5 kbar. In the second stage, the orthopyroxene-magnetite symplectite shell formed when primocrystal olivine reacted with the late-stage residual Fe-rich melt promoted by high oxygen fugacity condition at 927–1 035 °C and 6.0–6.5 kbar. In the third stage, the brown amphibole shell formed as the presence of residual hydrous melt and replaced the middle shell at 821–900 °C and 5.5–6.0 kbar.
... The South Kongling Complex mainly consists of the Mesoproterozoic-Neoproterozoic Miaowan ophiolitic complex (Peng et al., 2010Jiang et al., 2016;Deng et al., 2017;Lu et al., 2020) and Archean TTG gneisses and Paleoproterozoic metasedimentary rocks (Gao et al., 2011). The South Kongling Complex was largely intruded by the Neoproterozoic Huangling granite, which separated the South Kongling Complex from the North Kongling Complex. ...
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Ultramafic massifs are minor but meaningful components of orogens because they record the mantle properties and the processes of plate generation, subduction, accretion, and collision, thus providing key information about crust-mantle interaction processes and the evolution of orogens. Here, we present an integrated study of field observations, whole-rock and mineral geochemistry, zircon U-Pb ages, and whole-rock Sm-Nd and Re-Os isotopes for serpentinized harzburgites and olivine pyroxenites from the Paleoproterozoic Shuiyuesi mélange belt, North Kongling Complex, to constrain the tectonic evolution of the Yangtze craton, South China. The serpentinized harzburgites are characterized by enrichment of light rare earth elements (LREEs) and negative εNd(t) (−4.3 to −3.9) values, but they have high olivine Mg# (91.7−93.1) and very low Re contents (<0.09 ppb) and negative γOs(t) (−5.1 to −4.0) values. The Archean Re-Os model ages (TRD = 2.82 Ga) of the harzburgites that have the most depleted Re-Os isotope composition represent estimates for the age of the initial melt extraction. Metasomatic zircons from the serpentinized harzburgites yield a weighted average age of ca. 2.04 Ga, which is interpreted to represent the age of metasomatism during a subduction event. These geochemical and isotopic features and geochronologic results suggest that the protolith of the serpentinized harzburgites in the North Kongling Complex originated as fragments of late Mesoarchean (ca. 2.82 Ga) subcontinental lithospheric mantle and experienced melt/fluid metasomatism in a subarc mantle wedge at 2.04 Ga. The olivine pyroxenites are cumulates, and they are characterized by high MgO, Ni, and Cr contents and depleted to slightly enriched LREEs but depletion in high field strength elements (HFSEs). They have positive εNd(t) (+2.0 to +6.6) values and Paleoproterozoic Nd model ages (2.41−2.05 Ga), suggesting that the olivine pyroxenites were derived from a Paleoproterozoic depleted mantle source. Metamorphic zircons from the olivine pyroxenites indicate subsequent metamorphism at 2.0 Ga, in response to a collision. The olivine pyroxenites together with the metabasites in the Shuiyuesi mélange belt represent the dismembered fragments of oceanic lithosphere that formed in a suprasubduction zone during the Paleoproterozoic. Combined with the Archean and Paleoproterozoic rocks in the North Kongling Complex, it appears that the Mesoarchean subcontinental lithospheric mantle peridotites and Paleoproterozoic ophiolitic rocks were simultaneously accreted during a Paleoproterozoic orogenic event. We further propose that a subduction channel may have played an important role in the architecture of the accretionary orogen, in which both oceanic lithospheric materials and subcontinental lithospheric mantle wedge materials would have been simultaneously scraped off and accreted during slab subduction-accretion.
... It is a typical dome structure, which has continuous sedimentary strata and an abundant fossil record. This area has always been a hotspot for studying the frontier issues of geoscience, such as the growth and evolution of the Early Precambrian continental crust of the South China Yangtze Craton, and intracontinental compression, uplift and extensional tectonics of South China in the Mesozoic-Cenozoic era [48]. The structure has six strata from inside to outside, namely, Baota Formation, Nanjinguan Formation (including Honghuayuan Formation, Dawan Formation, Guniutan Formation, etc.), Longmaxi Formation of Ordovician, Shamao Formation, Luoraping Formation of Silurian, and Paomagang Formation of Cretaceous. ...
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Geological maps have wide coverage with low acquisition difficulty. When other geological survey data are scarce, they are a valuable source of geological structure information for geological modeling. However, for structures with large deformation, geological map information has difficulty meeting the requirement of its 3D geological modeling. Therefore, this paper takes the dome structure as an example to explore a 3D modeling method based on geological maps, DEM and related geological knowledge. The method includes: (1) adaptively calculating the attitude of points on the stratigraphic boundaries; (2) inferring and generating the bottom boundary of the model from the attitude data of the boundary points; (3) generating the model interface constrained by Bézier curves based on the bottom boundary; (4) generating the top and bottom surfaces of the stratum; and (5) stitching each surface of the geological body to generate the final dome model. Case studies of the dome in Wulongshan in China and the Richat structure in Mauritania show that this method can build a solid model of the dome based only on geological maps and DEM data, whose morphological features are basically consistent with those embodied in the section view or the model generated by traditional methods.
... H. Zhao et al., 2013, 2011, andreference therein). Recently, Peng et al. (2010) interpreted Mesoproterozoic ultramafic-mafic rock association consisting of serpentinized dunite and harzburgite, ductile basalt, massive gabbro and diabase and associated metasedimentary rocks as an oceanic relict, and labeled it Miaowan ophiolite ( Figure 1b). Further studies (Deng et al., 2017;Jiang et al., 2016;Peng et al., 2012) proposed that ca. ...
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The Kongling terrane (Huangling dome) outcrops Precambrian metamorphic basement of the Yangtze craton. However, the Precambrian tectonic evolution of the southern Kongling terrane remains poorly constrained. In this contribution, we report zircon geochronology of the metasedimentary rocks from southern Kongling terrane of the Yangtze craton. Zircon U‐Pb dating results yielded magmatic age clusters at ca. 3,000 Ma, ca. 1,520 Ma, ca. 1,150 Ma, ca. 860 Ma, 900–920 Ma, and 980 Ma, and metamorphic age at ca. 2,900 Ma. These data display that the southern Kongling terrane witnessed Mesoarchean magmatism and metamorphic event, Mesoproterozoic and Neoproterozoic magmatism. In combination with previous studies, it is proposed that the Precambrian metamorphic basement of the Yangtze craton was amalgamated by the multi‐stage subduction‐collision processes at ca. 3,000–2,900 Ma, ca. 2,000–1,800 Ma, and ca. 1,100–940 Ma. In addition, the Proterozoic processes were related to the assembly and breakup of Columbia and Rodinia supercontinents.
... However, a few researchers paid attention to trace element characteristics of chromites in mantle peridotites from Precambrian ophiolites (Rui et al., 2019;Yu et al., 2019). The Proterozoic Miaowan ophiolitic complex (MOC) is located in the northern margin of Yangtze Craton of southern China (Deng et al., 2017;Jiang et al., 2016;Peng et al., 2010). Previous studies (Deng et al., 2017;Huang et al., 2017;Peng et al., 2012) have revealed that chromites in mantle peridotites of the MOC have similar characteristics to those of podiform chromites from ophiolites worldwide (Thayer, 1964). ...
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The chrome spinel (chromite) in mantle peridotites from ophiolites can shed light on the formation and evolution process of ophiolites. Podiform chromites were found in the Late Proterozoic Miaowan ophiolitic complex (MOC), Yangtze Craton. Due to the metamorphism and intense deformation, most chromite grains in the MOC peridotites show typical chemical zoning (core-rim texture). The values of major and trace elements largely vary from core to rim within chromite grains, indicating that the chromites have undergone strong alteration and element mobility. Major and trace elements in the core parts of chromites are used to infer the tectonic origins and evolution of mantle peridotites in the MOC. The chromites from the MOC peridotites have higher Cr# values and lower Ni and Ga contents with respect to those from Phanero-zoic mantle peridotites, indicating a higher degree of depletion. In-situ major and trace elements (e.g., Ga) characteristics of podiform chromites in the MOC show that chromites from both harzburgites and dunites have strong subduction-related signatures, indicating that the MOC has formed in a supra-subduction setting which is consistent with the geological and geochemical data presented in previous studies.
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Mid-Neoproterozoic (850–720 Ma) magmatism around the periphery of the Yangtze Block records an intense extensional event related to the breakup of Rodinia. However, the detailed geodynamic mechanisms of this extensional tectonism are still poorly constrained. To address this issue, we undertook a whole-rock geochemical and Sr–Nd isotopic, and zircon U–Pb–Hf–O isotopic study of mid-Neoproterozoic silicic meta-igneous rocks from the Feidong Complex and Zhangbaling Group in the northeastern Yangtze Block. These rocks can be classified into two groups: Group A (granitoids) and Group B (volcanic rocks). Zircon U–Pb dating revealed that Group A (808–786 Ma) pre-dates Group B (762–750 Ma). All these rocks are characterized by A-type granite features, with high heavy REE, Zr, and Hf contents, (K2O + Na2O)/CaO ratios, Fe index values, and zircon saturation temperatures, and formed during a mid-Neoproterozoic extensional event. However, the extremely enriched whole-rock Sr–Nd and zircon Hf isotopic compositions (ISr = 0.7024–0.7068, εNd(t) = −19.3 to −14.1, and εHf(t) = −25.7 to −11.8) of Group A suggest derivation by reworking of ancient crustal basement. Group B is characterized by relatively depleted Sr–Nd–Hf isotopic compositions (ISr = 0.7015–0.7063, εNd(t) = −1.6 to +3.6, and εHf(t) = −4.8 to +4.1), indicating a significant contribution from juvenile-crust-derived material. Group A contains zircons with δ¹⁸O = 4.1‰–7.1‰, whereas Group B contains zircons with δ¹⁸O = 1.6‰–3.1‰. These chemical and isotopic data, along with the gradual change in residual mineralogy from garnet to plagioclase (Group A to B), indicate that magmatism transitioned from an ancient lower crustal source to a juvenile middle–upper crustal source that had experienced intense high-temperature meteoric water–rock interactions. Our results provide new insights into the tectonic evolution from early extension to peak rifting driven by upwelling asthenosphere, due to rifting of the Yangtze Block from Rodinia during the mid-Neoproterozoic.
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Some metamorphosed pelites (commonly called as kondalites) expose over the Archean crystal basement in the Kongling terrane of the Yangtze Block. These sedimentary rocks mainly contain biotite – plagioclase gneisses, garnet bearing biotite – plagioclase – silimanite gneisses, garnet bearing biotite – plagioclase – silimanite – cordierite gneisses, graphite schists, and some marbles and quartzites. Amphibolites and mafic granulites always occur as lenses or layers in these metasedimentary rocks. In this study, three kinds of metasedimentary rocks, which are Grt-Bi-Pl gneiss, Grt-Sil-Crd-Bi-Pl gneiss and Grt-Sil-Bi-Pl gneiss, are chosen for petrographic, zircon U-Pb, and Lu-Hf isotopic analysis. The mineral assemblages of these three rocks are Grt + Bi + Pl + Q, Grt + Sil + Crd + Bi + Pl + Q + Spl + Kf and Grt + Sil + Bi + Pl + Q respectively, which imply granulite facies metamorphism. In addition, garnet porphyroblasts in both of these metasedimentary rocks and the mafic granulites which are enclaves in these meta-sedimentary rocks develop decompressional coronas, indicating isothermal decompression (ITD) processes during retrograde metamorphism. Core-Rim structures in zircons can be observed in all these three samples based on cathodoluminescence images. Morphological characteristics, Th/U ratios and trace element compositions indicate that zircon cores are of magmatic origin while zircon rims and some single grain zircons are of metamorphic origin. Ages of magmatic zircon cores in Grt-Bi-Pl gneiss are between 3532 ± 20 Ma to 2286 ± 47 Ma while those of metamorphic zircons are 2022 ± 22 Ma. Most zircons in the Grt-Sil-Crd-Bi-Pl gneiss are of metamorphic origin and they have a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2017 ± 15 Ma. Magmatic zircon cores in Grt-Sil-Bi-Pl gneiss have a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2160 ± 31 Ma and metamorphic zircons have a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2028 ± 43 Ma. One zircon core (3199 ± 25 Ma) in Grt-Bi-Pl gneiss possesses the oldest two-stage Hf model age of 3676 ± 85 Ma. Two-stage Hf model (TDM2) ages of all the other analyzed zircon cores in this sample range from 3452 ± 40 to 3277 ± 43 Ma, and the metamorphic zircons give a weighted mean TDM2 age of 3471 ± 14 Ma. Metamorphic zircons in the Grt-Sil-Crd-Bi-Pl gneiss have a weighted mean two-stage Hf model (TDM2) age of 3468 ± 16 Ma. These TDM2 ages indicate that the sources of these metasedimentary rocks are obviously older than those documented by previous studies and they also suggest that protolith of these metasedimentary rocks belong to Archean crustal materials of the Kongling terrane, but with obviously different sedimentary sources. The widespread decompressional textures around garnet in various samples in the study area suggest that the Kongling area underwent a high-pressure granulite facies metamorphism and an isothermal decompression process at ∼2.0 Ga, corresponding to the assemblage of the Paleoproterozoic Nuna Supercontinent.
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Chromian-spinel (chromite) is a common mineral in ophiolitic rocks and the study of chromite from the mantle sections of ophiolites can help shed light on their petrogenetic origin and tectonic setting for formation. The Proterozoic Miaowan Ophiolite Complex (MOC) in the Yangtze Craton contains disseminated chromite grains in mantle harzburgites and podiform chromitites associated with serpentinised dunites. Most chromite grains display compositional zoning due to alteration but the fresh cores preserve primary igneous compositions. Podiform chromitites in the MOC dunites and harzburgites are compositionally similar to typical ophiolitic chromitites elsewhere. The chromite grains contain numerous inclusions of clinopyroxene and amphibole with minor amounts of olivine, chlorite, base-metal sulphides (BMS) and platinum-group minerals (PGM). The abundant hydrous mineral inclusions within the chromite grains suggest a hydrous mantle source. Core compositions of chromite grains indicate that the parental melts of the chromitites were similar to typical boninitic melts in a forearc settings. However, chromite grains in the harzburgites show mixed MORB and arc signatures. Thus, the mineralogy and geochemistry of the MOC peridotites suggest that the chromitites in the MOC formed in a forearc setting during reaction between boninitic melts and MORB-type harzburgite in a supra-subduction zone (SSZ) mantle wedge.
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To understand the connection between continental cratonization and global tectonothermal event is essential for recognizing the formation and evolution of continental crust. Paleoproterozoic is an important era with occurrence of megascale tectonomagmatism in the world, but it has been intriguing whether they also influenced the oldest continent in South China. In order to decipher the nature of Paleoproterozoic event in South China, a combined study of zircon U-Pb dating, Hf and O isotope analyses was carried out for metasediments and amphibolite from the Kongling terrane, the only Archean microcontinent outcropped in South China. U-Pb ages of 1.97±0.03Ga were obtained with low Th/U ratios of 0.01–0.14, indicating that the ages are a record of Paleoproterozoic metamorphic event. δ18O values of ∼11‰ and ∼8‰ were measured for quartz from the metasediments and garnet from the amphibolite, respectively, suggesting that their sources experienced supracrustal recycling. ɛHf(t) values of about −6.5 and model Hf ages of about 3.0Ga were acquired for zircons from the metapelites, suggesting an Archean source. Thus a response to the Paleoproterozoic global tectonothermal event in South China is reworking of Archean continental nucleus. Compared with Archean rocks at Kongling, abrupt changes in K2O/Na2O, REE and other trace elements are observed in the Paleoproterozoic metasedimentary rocks. This is interpreted to reflect a change in upper crustal composition at the Archean–Proterozoic boundary. A survey of Paleoproterozoic ages throughout the Yangtze Block suggests that metamorphic event and subsequent magmatic activity occurred in the north, but only magmatic activity in the south. Both metamorphic and magmatic activities are associated with formation of a unified basement responsible for cratonization of the Yangtze Block. This provides a geodynamic connection between the formation of this craton and the global tectonomagmatism in the Paleoproterozoic, marking continental accretion by arc-continent collision orogeny during assembly of the supercontinent Columbia.
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Mafic volcanic rocks incorporated within the Churchill-Superior Boundary Zone at Ospwagan Lake, Moak Lake, Assean Lake and the Fox River Belt constitute part of the Circum-Superior Belt. Major and trace element data have been obtained from mafic and ultramafic volcanic rocks from four localities. The data provide a means of comparing and contrasting the geochemistry of the rocks from the portion of the Circum-Superior Belt surrounding the Archean Superior Province in Manitoba with other units from elsewhere in the belt, and with rocks from the adjacent Proterozoic Churchill Province.Major element geochemical data are consistent with the rocks having originally formed in a oceanic environment. Trace element and rare earth element data suggest that the environment was a marginal basin. Al2O3/TiO2 ratios indicate that the magmas were derived from a mantle source by small degrees of partial melting. High field strength element patterns show that the mantle source region giving rise to the rocks was relatively uniform in composition.Trace element, mid-ocean ridge basalt (MORB) and chondrite-normalized patterns indicate similarities in alkali element enrichment for the rocks at Asean and Ospwagan Lakes. Virtually all the rocks have Ti, Zr, Nb, Y and P levels < MORB, which is consistent with the mantle source region having undergone a small degree of partial melting. Zr/P ratios for the rocks from the Fox River Belt are consistently lower than those from the other localities, and the Assean Lake rocks show slight enrichment in light rare earth elements, which may suggest local compositional variability in the mantle source regions supplying the magmas to the Fox River Belt and the Ospwagan, Assean and Moak Lake areas.
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Major- and trace- element analyses of samples from the upper and lower members of the Crescent Formation demonstrate that these volcanic rocks are dominated by tholeiitic compositions, with minor amounts of alkali basalt also present. Basalt flows from both members have undergone little differentiation, and are compositionally similar. Differences in TiO2 and REE contents demonstrate, however, that these members are geochemically distinguishable. Lower-member tholeiites are identical to ridge tholeiites and are interpreted as representing a slice of oceanic upper crust. The upper member is similar to oceanic island basalts, but its primary tectonic significance is uncertain. It is postulated that the lower member was generated at a mid-ocean ridge and was subsequently subducted to a shallow level. Miocene uplift then juxtaposed the upper and lower members.
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A revision of previous models of Proterozoic crustal accretion in the southwestern U.S. is presented, based on the more extensive data now available. Five Early Proterozoic predominantly volcanic supracrustal terranes are recognized, with ages of 1760-1800 Ma, 1730-1740 Ma, 1720 Ma, 1680-1700 Ma, and 1650 Ma. In some areas they are overlain by a sixth, comprised chiefly of quartzite-pelite. In the oldest three terranes mafic volcanic rocks of largely submarine origin are more abundant than felsic. In the 1680-1700 Ma and 1650 Ma terranes felsic rocks exceed mafics, volcanics are mixed submarine and subaerial, and non-volcanogenic sediments are important. Least mobile incompatible element distributions in basalts from the 1760-1800 Ma and 1720 Ma terranes are similar to those in basalts from evolved oceanic arcs and associated back-arc basins. Basalts from the other terranes resemble those from continental-margin arcs and associated back-arc basins at early stages of opening. Incompatible element ratios in...
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The incomplete Evros ophiolites in NE Greece form a NE−SW-oriented discontinuous belt in the Alpine orogen of the north Aegean. Field data, petrology and geochemistry are presented here for the intrusive section and associated mafic dykes of these ophiolites. Bodies of high-level isotropic gabbro and plagiogranite in the ophiolite suite are cross-cut by NE–SW-trending boninitic and tholeiitic–boninitic affinity dykes, respectively. The dykes fill tensile fractures or faults, which implies dyke emplacement in an extensional tectonic regime. The tholeiitic–transitional boninitic gabbro is REE-and HFS-depleted relative to N-MORB, indicating derivation from melting of a refractory mantle peridotite source. Associated boninitic dykes are slightly LREE-enriched, showing mineral and whole-rock geochemistry similar to the gabbro. The plagiogranite is a strongly REE-enriched high-silica trondhjemite, with textures and composition typical for an oceanic crust differentiate. Plagiogranite-hosted tholeiitic and transitional boninitic dykes are variably REE-enriched. Geochemical modelling indicates origin of the plagiogranite by up to 75 % fractional crystallization of basaltic magma similar to that producing the associated tholeiitic dykes. All mafic rocks have high LILE/HFSE ratios and negative Ta–Nb–Ti and Ce anomalies, typical for subduction zone-related settings. The mafic rocks show a similar trace-element character to the mafic lavas of an extrusive section in Bulgaria, suggesting they both form genetically related intrusive and extrusive suites of the Evros ophiolites. The field occurrence, the structural context, the petrology and geochemical signature of the studied magmatic assemblage provide evidence for its origin in a proto-arc (fore-arc) tectonic setting, thus tracing the early stages of the tectono-magmatic evolution of Jurassic arc-marginal basin system that has generated the supra-subduction type Evros ophiolites.
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The abundances of the REE in Alpine peridotites, ultramafic rocks associated with ophiolites, ocean-floor peridotites, and ultramafic inclusions in basalts and kimberlites are each discussed in some detail, and the implications of REE data for upper mantle composition and upper mantle processes are examined.-R.A.H.
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Ion microprobe (SHRIMP II) U-Pb zircon analyses reveal trondhjemitic magmatism at 2.90 2.95 Ga in the Kongling area of the Yangtze craton, south China, about 150 km south of the Permian-Triassic Qinling-Dabie-Sulu orogenic belt. Detrital zircons from nearby Archean metapelites are 2.87 3.28 Ga, and the rocks have Sm-Nd depleted mantle model ages of 3.07 3.21 Ga. The new data reveal, for the first time, >3.2 Ga sialic crust in the Yangtze craton, part of which predates that of the adjacent southern North China craton. Both trondhjemites and metapelites contain ca. 2.75 Ga high-grade metamorphic zircons, and ca. 1.9 Ga zircons, related to intrusion of the Quanqitang K-feldspar granite into the Archean basement. Many zircons also underwent Pb loss ca. 1.0 Ga during the Jinning orogeny, when the Cathysian block accreted to the Yangtze craton. The new data support correlation of part of the Korean Peninsula with the Yangtze craton along the eastern extension of the Qinling-Dabie-Sulu orogenic belt.
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The Paleozoic Trinity terrane is a massif of ultramafic rock and mafic intrusive complexes that forms the basement of eugeoclinal rocks in the eastern Klamath Mountains. The origin of the Trinity terrane has been particularly enigmatic and several mutually exclusive origin models have been proposed; (1) mantle diapir; (2) slice of obducted monogenetic oceanic lithosphere; (3) monogenetic backarc-basin lithosphere; (4) forearc lithosphere; and (5) volcanic arc basement. We have compiled new and existing field, petrograpic, geochronologic, isotopic, and whole-rock chemical data for the Trinity terrane to evaluate these models and conclude that the Trinity terrane is an supra-subduction zone ophiolite whose mafic intrusive complexes record subduction-related magmatism and forearc extension that occurred during the inception of intraoceanic subduction. According to this extensional forearc model, all four terranes in the eastern Klamath Mountains evolved principally during the inception and evolution of a single, E-dipping, mid-Paleozoic convergent margin analogous to the Eocene Izu-Bonin-Marianas arc system. Conventional wisdom has been that the Devonian volcanic succession in the Eastern Klamath terrane records the inception of Paleozoic subduction-related magmatism in California. New U-Pb zircon age data for gabbros in mafic intrusive complexes of the Trinity terrane range from Early Silurian to Early Devonian (431 to 404 Ma), indicating that subduction-related magmatism began earlier than previously believed. Silurian and Devonian igneous rocks of the Trinity and Eastern Klamath terranes are coeval, broadly cogenetic, and represent different levels of exposure of the same ophiolite. This interpretation provides the first integrated explanation of the origin of seemingly disparate Paleozoic lithotectonic elements in the eastern Klamath Mountains.
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The nearly parallel Dinaridic ophiolite belt and the Vardar zone western belt are assumed to be the remnants of two distinct oceanic basins, constituting different parts of the Tethys Ocean that separated the Gondwana and Eurasia continents during Mesozoic time. These belts comprise numerous large peridotite massifs and small bodies whose petrology was poorly known. This paper presents a large set of internally consistent analytical data for peridotites, including primary mineralogy, major-element chemistry and clinopyroxene geochemistry for massifs of both the ophiolitic belts. We propose, discuss and apply a set of mineralogical, geochemical and petrologic criteria that allow a recognition of the probable geodynamic setting of formation of the ultramafic massifs.