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Abstract
Machihe country and Shahewan granitoid rock masses are located in the south of Shang-Dan fault. Zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used for U-Pb chronologic analysis to determine crystallization ages for Machihe country granite and Shahewan quartz monzonite magma of 490.8±2.9 Ma (Early Ordovician), and (240.6±1.5)-(228.2±1.5) Ma (Middle Triassic), respectively. The results of zircon in-situ Lu-Hf isotopic analysis indicate that the two-stage Hf model ages (tDM2) of zircon are 759-1096 Ma and 891-1516 Ma, respectively. The source-rock material of Machihe country granite is Neoproterozoic and Mesoproterozoic depleted mantle material. Shahewan quartz monzonite is the mixing product of crustal with mantle materials at different levels in the Mesoproterozoic and Neoproterozoic time. In the northward subduction of the QinLing oceanic crust beneath North Qinling, the oceanic crust and the upper mantle interacted to produce the hybridized liquid and formed Machihe country granite by crystallization differentiation in the Late Cambrian to the Early Ordovician. Shahewan rock mass was formed during at least two geological events with large time spans between (240.6±1.5)-(228.2±1.5) Ma and 214-197 Ma, respectively. After the closure of the Mian-Lüe oceanic basin at about 250 Ma, a collision event occurred between the Yangtze and the North China plates in the Qinling orogenic belt. This collision resulted in the subduction of the continental Yangtze plate beneath South Qinling, in which a small part of the crust melted to form the Early-Middle Triassic granitoids at (240.6±1.5)-(228.2±1.5) Ma. After this collision, approximately at 220 Ma, the detachment of the slab induced the upwelling of the asthenosphere material. The subducted continental crust began to uplift, and under the conditions of mantle heat and tectonic stress, widespread partial melting of the subducted continental crust and the upper lithosphere mantle occurred to form granitoids with various degrees of adakite characteristics in the Late Triassic (214-197 Ma) and associated with mafic inclusions.
... NASC-normalized REE distribution pattern for chert from the Wenxian Sanhekou Group in the Mianlue suture zone (after Gromet et al., 1984). (Zeng et al., 2008;Wang et al., 2009aWang et al., , 2009bWang et al., , 2010Lei, 2010;Zhang et al., 2010;Nong et al., 2012;Liu et al., 2013aLiu et al., , 2013b (Fig. 11). Thus, the detritus from the Palaeozoic magmatic suite in the South China Block appears to have been the major contributor of the continental detritus that was mixed with the chert of the Mianlue suture zone. ...
... Histogram of U-Pb ages for the zircons from chert in the Wenxian Sanhekou Group in the Mianlue suture zone and for the magmatic zircons from Qinling Block, South and North China Block. Data source: South China Block Zheng et al. (2007), Liu et al. (2008), Zeng et al. (2008), Sun et al. (2009), Wang et al. (2010, 2012), Zhang et al. (2010), Duan et al. (2011), Nong et al. (2012) and Ping et al. (2014); North China Block fromXie et al. (2012) andWu (2014) and references in; Qinling Block fromWang et al. (2009b) andLiu et al. (2013bLiu et al. ( , 2014. ...
... Hf(t) versus U-Pb age diagram for zircons from chert in the Wenxian Sanhekou Group in the Mianlue suture zone. Data source: South China Block from Zheng et al. (2007), Liu et al. (2008), Zeng et al. (2008), Sun et al. (2009), Wang et al. (2010), Zhang et al. (2010), Duan et al. (2011), Nong et al. (2012),Ping et al. (2014) andWang et al. (2012); North China Block fromGeng et al. (2012),Wu (2014) and references in; Qinling Block fromWang et al. (2009b) andLiu et al. (2013bLiu et al. ( , 2014. ...
The Mianlue suture marks the southern boundary of the Qinling−Dabie orogen and divides the South China and North China continental blocks. Here we present in situ U-Pb geochronology and Lu-Hf isotopic data on detrital zircons in chert beds intercalated with continental detritus from the Sanhekou Group in Wenxian along this suture. Our data suggest that deposition of these sediments in the Mianlue suture zone occurred later than ca. 412 Ma. The data define four main age peaks with more than 25% of the detrital zircons yielding ages between 801 to 999 Ma with εHf(t) ranging from −26.9 to 12.2. The age data are comparable with those zircons in the Neoproterozoic granites and volcanic rocks in the Bikou Block which has an affinity to the South China Block. Zircons with ages from 2310 to 2595 Ma comprise 14.1% of the detrital population, and were mainly sourced from the South China Block as inferred from their mostly negative εHf(t) values compared to the North China Block. Zircons with ages ranging from 412 to 587 Ma constitute 12.6% and show markedly negative εHf(t) values ranging from −0.6 to −25.5 (the majority ranging from −4.4 to −19.6), correlating with Palaeozoic magmatic intrusions in the South China Block. Another age peak of 1548 to 1686 Ma with εHf(t) values varying between −6.5 and −14.8 is comparable to the zircons in magmatic intrusions of the South China Block. Our results suggest that the major source of zircons in the continental detritus mixed with chert within the Mianlue suture zone is Proterozoic and Palaeozoic magmatic intrusions in the South China Block. Combined with the geochemical features of the chert in Wenxian and other outcrops in the Mianlue suture zone, we infer that these sediments were deposited in the Mianlue sea basin adjacent to the continental margin, and that the opening of the Mianlue suture was initiated in the eastern section of the northern margin of South China Block, with subsequent migration to the western part, in a zipper-type opening of the continental rift from the east to the west of the Mianlue suture zone.
Rapakivi granite is characterized by its unique structure, which has important implications for tectonic settings, magmatic processes, and crust–mantle interactions. In this study, we conducted a combined analysis of the petrography, mineral chemistry, geochemistry, and zircon U–Pb dating and Lu–Hf isotopic compositions of the Niujiaoshan Early Paleozoic rapakivi-textured granite from the North Qinling Belt. Zircon U–Pb dating yielded a crystallization age of 447 ± 7 Ma, which is younger than the ultra-high-pressure (UHP) metamorphic age (~ 500 Ma) but similar to the granulite facies retrograde age (~ 450 Ma) of UHP eclogites and felsic gneisses in the North Qinling Belt. The rapakivi feldspar phenocrysts have ovoid K-feldspar cores, which are rich in mineral inclusions, such as amphibole, biotite, quartz, and plagioclase, indicating early crystallization. The ovoid K-feldspar cores are mantled by oligoclase, whreras the matrix comprises biotite, amphibole, and coarse-grained plagioclase. The amphibole and biotite in the granite are rich in Mg and are indicative of a crust–mantle origin. The εHf (t) values of the zircons range from − 2.04 to + 3.63, suggesting formation via crust–mantle interactions. The rapakivi-textured granite displays high-K meta-aluminous I-type granite affinity, with high SiO2, K2O, and Na2O contents. Based on the geological background and results of this study, we propose that the Niujiaoshan rapakivi-textured granite was formed via the mixing of crustal materials induced by upper mantle magma during the exhumation of the North Qinling UHP metamorphic terrane, which occurred in a post-orogenic setting.
The petrogenesis and tectonic setting of the Triassic granitoids, and their relationship with the coeval Au and Pb–Zn polymetallic mineralization in the Western Qinling Orogen (WQO) in central China remain debated. The Fengxian–Taibai Ore Cluster (FTOC) in the eastern part of the WQO hosts large-scale Triassic granitoids and numerous polymetallic deposits, providing an excellent opportunity to constrain the controversies mentioned above. In this work, we choose the Huahongshuping granodiorite pluton, Guji biotite monzogranite pluton, Taoerling biotite monzogranite dike, Dongtangzi granite porphyry dike and Dagou dioritic dikes in the FTOC for a combined study on the zircon U–Pb geochronology, whole-rock major, trace elements and Sr–Nd isotopes. LA–ICP–MS zircon U–Pb dating yields ages of ∼225 Ma, ∼224 Ma and 227–217 Ma for the Huahongshuping pluton, Guji pluton and granitoid dikes, respectively. All these granitoids have metaluminous and high-K calc-alkaline series affinities, and are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE) and depleted in high field-strength elements (HFSE) with a marked LREE/HREE fractionation ((La/Yb)N = 10.66–32.42), fractionation of LREE ((La/Sm)N = 2.72–6.60), and negative Eu anomalies (δEu = 0.45–0.99). They have initial Sr isotopic ratios of 0.7062 to 0.7082, and negative εNd(t) values of –9.42 to –8.09. The above features indicate that the FTOC granitoids were mainly derived from the partial melting of the Neoproterozoic meta-volcanic rocks induced by the decrease in pressure and the addition of water dehydrated from biotite, with direct contribution of mantle materials. Combined with previous data on sedimentation, deformation and metamorphism, and tectonics, we suggest that the WQO underwent the Early Mesozoic tectonic evolution from the subduction at 248–235 Ma, through syn-collision at 235–225 Ma, to post-collision at 225–195 Ma. Integrating with the ore geology, geochronology, mineralogy, and quartz and carbonate H–O, and in-situ S isotope data of sulfides from different mineralization stages at representative Au and Pb–Zn deposits in literatures, we proposed a conceptive model that the post-collision process and granitic magmatic activity played an important role in the polymetallic mineralization within the FTOC.
The Fushui mafic intrusion in the Qinling orogenic belt (QOB) is composed of meta-gabbro, meta-gabbro-diorite, diorite, and syenite. Most of these rocks are metamorphosed under the upper greenschist facies to lower amphibolite facies metamorphism. Zircon separates from eight samples have LA-ICP-MS U-Pb ages of 497–501 Ma which are taken to be the emplacement age of magmas that formed the Fushui intrusion. Most of the zircon grains exhibit negative εHf values, correspond to TDM2 model ages of late Paleoproterozoic-early Mesoproterozoic or Neoproterozoic and suggest that the mafic rocks were most probably derived from mafic melts produced by partial melting of a previously metasomatized lithospheric mantle.
The intrusion is not extensively contaminated by crustal materials and most chemical compositions of rocks are not modified during the greenschist to amphibolite-facies metamorhism. Rocks from the intrusion have primitive mantle-normalized trace element patterns with significant enrichment in light-REE and large ion lithophile elements (LILE) and depletion in high field-strength elements (HFSE). On the basis of the trace element contents, the Fushui intrusion was derived from parental magmas generated by <10% partial melting of both phlogopite-lherzolite and garnet-lherzolite mantle sources. These sources are best interpreted to be in a subduction-related arc environment and have been modified by fluids released from a subducting slab. The formation of the Fushui intrusion was related to the subduction of the Paleotethyan Shangdan oceanic lithosphere at ∼500 Ma.
Devonian quartzite occurs as blocks within a phyllite matrix in Puziba area of the Mianlue Suture Zone (MLSZ) in central China. The depositional time of the quartzite is younger than 425 Ma (mainly Early Devonian), constrained by the zircon U–Pb geochronology data from the quartzite, cross-cutting relationships with granite, and palaeontology evidence. The detrital zircons in the quartzite show typical magmatic features with four main age peaks at: 2676–2420 Ma (11.6% of the population), 1791–1606 Ma (4.8%), 997–817 Ma (26.5%), and 597–425 Ma (17.5%). In combination with the zircon εHf(t) values, we propose that the quartzite in the MLSZ was sourced from Neoproterozoic and Palaeozoic magmatic and sedimentary rocks in the South Qinling Block and the South China Block (particularly from the Bikou Terrane), with minor contributions from Archaean and Palaeoproterozoic magmatic units from both of the South and North China blocks. The blocks of quartzite, slate, marble, metasandstone, and chert blocks in the phyllite matrix in the Puziba area show a typical block-in-matrix texture in a tectonic mélange, and provide significant evidence from sedimentary rock blocks rather than ophiolite or volcanic rock for the existence of the MLSZ.
The large Yu'erya gold deposit (65 t of contained gold averaging 2.3 g/t Au) in the eastern part of the Hebei Province of China is spatially associated with the Yu'erya Granite, and a group of NE- and NNE-trending faults. The alteration associated with mineralization is characterized by the assemblage pyrite, quartz, sericite, albite, and carbonate. Four stages of mineralization, in chronological order, are (1) quartz and medium- to coarse-grained pyrite; (2) quartz, fine-grained pyrite, and gold; (3) quartz, polymetallic sulfide, tellurobismuthite, and gold; and (4) quartz, pyrite, and carbonate. Most of the gold was deposited during the second and third stages of alteration from mesothermal fluids. These fluids were relatively rich in H2O, CO2, K+, Ca2 +, Cl, and S, and low salinity. The H-O and sulfur isotope ratios determined for the mineralized samples indicate a magmatic source, and the Pb isotope data indicate that the Au mineralization originated from the mantle and lower crustal materials. Geochronological data indicate that the gold mineralization event was restricted to 200-163 Ma whereas the associated magmatism occurred between 200 and 150 Ma. This Mesozoic gold mineralization is related to the subduction of the Mongolia-Okhotsk and Paleo-Pacific oceans along the edges of the North China Craton.
This article is an introductory chapter for the proceedings of the last, seventh, meeting of the International Geological Correlation Programme (IGCP) Project 315 “Correlation of Rapakivi Granites and Related Rocks on a Global Scale”, 1991–1996. Based on current knowledge, progress made during the six years of the project, and the seven papers published in this special issue, a synthesis of the age distribution, geotectonic environment, petrogenesis, geochemistry, and metallogeny of the rapakivi granites and related rocks is presented