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Abstract
The geochemistry and detrital zircon characteristics of the sediments play an important role in the study of sedimentary provenance and their tectonic settings. This paper presents petrography and geochemical study of the Early Carboniferous Baogutu Group and Tailegula Group clastic rocks in Karamay area, western Junggar, NW Xinjiang. Geochemical data show that the sedimentary rocks have SiO2 contents (54.64%-73.14%), relatively high Al2O3 contents (11.89%-19.36%) and high (TFe2O3+MgO) contents (3.97%-9.99%), indicating low chemical maturity. Their total rare earth element contents vary between 83×10-6 and 252×10-6, characterized by no Ce anomaly and slightly negative Eu anomalies, which is similar to those of sandstones formed at oceanic island arc setting. U-Pb dating results of detrital zircons show that Baogutu Group has 340 and 440 Ma two peak values. Detrital zircons of Tailegula Group show only one peak (325 Ma) and suggests that they were sourced from the Early Carboniferous rocks. Discrimination diagrams of the major and trace elements show that the provenance and tectonic settings of Early Carboniferous Baogutu and Tailegula groups sedimentary rocks were formed from the oceanic island arc and continental island arc acidic volcanic rocks, with minor mafic volcanic rocks.
... southern West Junggar) An et al. 2019). (2) The sources of the Baogutu Formation were probably the Middle Devonian and early Carboniferous intermediate-acidic effusive rocks and the middle Silurian Mayile ophiolitic mélange in the Barleik Mountain area (Liao et al. 2015). (3) The clastic source of the Telegula Formation may mainly be from the early Carboniferous intermediate-acidic island arc effusive rocks in the western margin of HLM and Xiemisitai Mountain (Liao et al. 2015;Zhang et al. 2015). ...
... (2) The sources of the Baogutu Formation were probably the Middle Devonian and early Carboniferous intermediate-acidic effusive rocks and the middle Silurian Mayile ophiolitic mélange in the Barleik Mountain area (Liao et al. 2015). (3) The clastic source of the Telegula Formation may mainly be from the early Carboniferous intermediate-acidic island arc effusive rocks in the western margin of HLM and Xiemisitai Mountain (Liao et al. 2015;Zhang et al. 2015). (4) The potential provenances of the Hala'alat Formation were primarily related to the early Carboniferous intermediate-acidic effusive rocks of the BCA, and the secondary provenances were the early Carboniferous intermediate-acidic igneous rocks distributed in the DTMB and the early Carboniferous island arc in the basin. ...
... Considering that the proximal sedimentation is obviously controlled by the regional tectonic pattern, the change in the detrital (Fig. 8). This migration may be related to the regional tectonic activities in the same period: as the residual ocean basin was gradually filled, the Carboniferous seawater in the middle of West Junggar retreated from southwest to northeast to the HLM area, and the depocentre also migrated from south to north and from west to east; these processes are almost consistent with research results reported by Zong et al. (2014), Gong & Zong (2015), Liao et al. (2015) and Zhang et al. (2015). ...
During late Carboniferous time, the residual ocean basin gradually closed in West Junggar and only a small amount of seawater remained in the Hala’alat Mountain area, where discussions of provenance and tectonics are limited. In this study, LA-ICP-MS U–Pb dating and heavy mineral identification are conducted on the upper Carboniferous tuffaceous sandstones from the Hala’alat and Aladeyikesai formations in the Hala’alat Mountain area. The results reveal the low maturity of the clastic sediments, indicating proximal deposition. The Hala’alat Formation detrital zircons present a single peak ( c . 330 Ma). Speculatively, the primary provenance is the Boshchekul–Chingiz Arc, and the secondary sources are the Darbut Tectono-Magmatic Belt and island arcs in the basin. The main peak and provenance of the Aladeyikesai Formation are similar to those of the Hala’alat Formation. Moreover, several age groups, namely, 370–344 Ma, 427–404 Ma and 478–476 Ma, potentially correspond to provenances of the Darbut Tectono-Magmatic Belt, the Boshchekul–Chingiz Arc and the Kujibai–Hongguleleng Ophiolitic Mélange Belt. In addition, the maximum depositional ages of the Hala’alat and Aladeyikesai formations calculated are 314.6 ± 1.54 Ma and 330.8 ± 0.61 Ma, respectively. Comprehensive analysis shows a relatively singular provenance of the Hala’alat Formation. While the provenance of the Aladeyikesai Formation shows inheritance, the provenance area extends northwards to the Kujibai–Hongguleleng Ophiolitic Mélange Belt. Furthermore, the closure of the Junggar Ocean during Carboniferous time caused the potential source region of the Hala’alat Mountain area to migrate northeastwards from Barleik Mountain to Xiemisitai Mountain. This study provides a basis for the analysis of regional geological evolution.
... Detrital zircon geochronology is an important tool to constrain provenance and the depositional time of sedimentary sequences (Dickinson and Gehrels 2010). In addition to our new data, we review nine published detrital zircon U-Pb datasets from sandstone samples from the Carboniferous of the West Junggar block (Zhang et al. 2011a;Choulet et al. 2012a, b;Chen et al. 2013;Liao et al. 2015;Zhang et al. 2015a, b, c) and the basal Permian of the western Junggar Basin (Lu 2018;Tang et al. 2021b). Since the dated zircons appear to be of proximal igneous origin, comparison of the detrital zircon U-Pb age peaks with the regional magmatism is a powerful tool to define provenance. ...
... Samples N1008-3 and N1052-3 from Zhang et al. (2015a, b, c). Sample N1060-1 from Liao et al. (2015). Sample 08YY-02 from Zhang et al. (2011a, b). ...
The final closure of oceans in accretionary orogens is a transformative process for continental crustal growth. The geodynamic mechanism is still controversial. This study synthesizes the Permo-Carboniferous tectono-stratigraphy, detrital zircon U–Pb dating and magmatism in the Junggar terranes to determine their interlinkage during ocean closure. The tectono-stratigraphy shows a transition from a late Mississippian regressive to an early Pennsylvanian transgressive and late Pennsylvanian transgressive–regressive sequences controlled by the basin-bounding transtensional faults. Detrital zircon U–Pb chronology displays a change of the provenance area from distant in the late Mississippian to more proximal in the Pennsylvanian. The magmatism indicates that the late Mississippian island-arc volcanic successions, related to subduction, progressively gave way to early Pennsylvanian magmatic activity indicative of regional asthenosphere upwelling, which suggests back-arc basins opened since ~ 319 Ma. The termination of the subduction is constrained by A-type granite plutons (~ 307 Ma). The Junggar terranes changed from an active back-arc basin behind an active subduction zone (319–307 Ma), to a post-subduction back-arc basin that continued to extend in 307–290 Ma. This back-arc basin extension is synchronous with voluminous Pennsylvanian–early Permian granite emplacement in both West Junggar and East Junggar blocks. Asthenospheric upwelling in the back-arc area was triggered by subduction, but continued after subduction had ceased. Transfer of juvenile magma to the crust was enhanced by crustal thinning, high thermal gradient, decompression melting and magma pathways along strike-slip faults. The prolonged back-arc setting provided a mechanism for rapid continental crust generation in the Paleozoic southwestern part of Central Asian Orogenic Belt.
... corals, crinoid stems, brachiopods, trilobites) to brachiopod, fusulinid, radiolarian, and deep-water trace fossils and heterochthonous buried plant debris to radiolarians as well as deep-water trace fossils (Zong et al. 2014;Weng et al. 2021), indicating regional sedimentary facies changes from a neritic facies to a semi-deep-water facies to a deepwater facies. Therefore, an extensional setting during the deposition of the clastic rocks in the Carboniferous was proposed (Liao et al. 2015;Weng et al. 2021). Moreover, the sedimentary palaeogeography reveals thethat seawater in the WJT retreated completely at the end of the Carboniferous and entered a unified stage of intracontinental orogenic evolution, and the continental fossils were also found in the strata (Zong et al. 2014), further supporting the hypothesis of postcollisional extension regime in the late Carboniferous (Zong et al. 2014). ...
The West Junggar terrain (WJT), as a crucial part of the Central Asian Orogenic Belt (CAOB), is distributed with numerous igneous rocks, which provide critical information for crustal growth. However, the closure of the Junggar Ocean (JO) and the beginning of the postcollisional tectonic stage of the WJT have been controversial. This study delimited the regional lithologic units based on remote sensing geological mapping and recognized a series of bimodal volcanic rocks (BVR) in Hala’alate Mountain of the WJT. LA-ICP‒MS zircon U‒Pb geochronology and geochemistry were used to discuss the petrogenesis of the BVR and determine the stage of regional tectonic evolution. Geochronological results yield crystallization ages of 302 ± 4 Ma, 298 ± 2 Ma, 304 ± 1 Ma, and 303 Ma± 2 Ma for the basaltic andesite, basalt, and two rhyolitic samples, respectively. Basalts and basaltic andesites are calc-alkaline, and display enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). Notably, basaltic andesites in this area were once misjudged as sanukitoids owing to their low contents of Mg#, Ni, Cr, and other characteristics that are inconsistent with the typical definition of sanukitoids. The rhyolites are A2-type granitoids with high SiO2 contents and are depleted in Nb, Ta, P, Ti, and Sr, showing enriched LREE patterns with negative Eu anomalies. These features indicate that the magma of the mafic end-member of the BVR is derived from the partial melting of the depleted lithospheric mantle, whereas the felsic end-member magma can be associated with the remelting of the lower crust due to the upwelling and underplating of mafic magma. In combination with previous studies of simultaneous basic dikes and felsic rocks, a postcollisional tectonic stage was proposed for the WTJ during the late Carboniferous, suggesting that the JO was closed.
The Carboniferous tectonic evolution of Western Junggar is crucial to understanding the subduction-accretion process of the Central Asian Orogenic Belt (CAOB), but the nature of this setting is still controversial. In this work, composite mineralogical, geochemical and detrital zircon U-Pb geochronological investigations have been conducted on Carboniferous clastic rocks in the Darbut region. The chemical compositions and sedimentary features show low sediment maturity and limited recycling, suggesting short-distance transportation and rapid accumulation. The samples contain igneous rock debris, mainly andesite and small amounts of basalt and granite, and a heavy mineral assemblage of Zr + Ap + Aug + Hbl + iron-bearing minerals (Hem-Lm, Ilm, Mag, and Py). The samples feature moderate ratios of Zr/Sc (average 15.47) and Th/Sc (average 0.61), and low ratios of La/Sc, Co/Th, and La/Th, as well as low Hf contents, suggesting intermediate to felsic arc-related igneous provenances. Detrital zircon grains from the clastic rocks show prominent age peaks in the Devonian and Carboniferous with positive ε Hf (t) values, indicating a consistent provenance associated with the Tiechanggou–Halaalate island arc. Combining the petrology, geochemistry and geochronology of the sedimentary and magmatic rocks, we conclude that the Darbut Carboniferous volcanic-sedimentary strata were deposited in a back-arc basin during ∼327-311Ma.
Supplementary material: https://doi.org/10.6084/m9.figshare.c.5357293
Carboniferous magmatism in the Darbut region is critical for understanding the evolutionary history of the accretionary orogenesis of the West Junggar area in the Central Asian Orogenic Belt. In this paper, zircon U–Pb geochronological, whole‐rock geochemical and Sr–Nd–Pb–Hf isotopic analyses of the basalts, basaltic andesite, and dacite from the Carboniferous Tailegula Formation, Darbut region, have been performed. The magmatic zircons from basalt, basaltic andesite, and dacite yield concordia U–Pb isotope ages of 344 ± 4, 347 ± 3, and 340 ± 5 Ma, respectively, which are interpreted as the crystallization ages of these rocks. The basalts and basaltic andesite are characterized by an alkali composition; high Nb/Yb, Th/Yb, Ta/Yb, La/Sm, and Sm/Yb trace element ratios and Pb isotopic ratios; moderate (87Sr/86Sr)i ratios; low (143Nb/144Nb)i ratios; and positive εNd(t) (+0.66 − +4.73) and εHf(t) (+1.7 − +15.2) values. These characteristics indicate that the magmas were derived from mixed mantle sources, that is, enriched mantle sources and high U/Pb ratio mantle (HIMU) component sources with ≤10% partial melting of garnet and spinel lherzolite. The samples display pronounced light rare earth elements (LREEs) and incompatible element enrichment patterns with positive Nb–Ta anomalies and a high Pb content and Nb/La ratios but low (Th/Nb)N ratios and a lower degree of partial melting, implying that these rocks were formed in a continental extensional setting. Based on our results and that of previous studies, we conclude that the early Carboniferous volcanic rocks in the Darbut region formed by mantle plume or regional‐scale mantle upwelling in a continental extensional setting after closure of the Junggar Ocean.
Carboniferous volcanic rocks are widely distributed in the Junggar Basin and may provide a unique insight into the tectonic history of this basin. In this paper, we focus on the tectonic evolution of the northern Junggar Basin (NJB) using geochronological and geochemical data to establish the diachronous closure model for the Keramaili oceanic basin. The Lower Carboniferous strata in the eastern part (well WC‐1) and the Upper Carboniferous strata in the western part (well Y‐2) of the NJB are composed of a mix of compositional and rock‐type forms. These volcanic rocks are mostly high‐K assemblages characterized by high K (1.73–6.48%), high alkali (Na2O + K2O = 5.04–7.45%), and low TiO2 (0.57–1.06%). They also exhibit various REE concentrations (68.44–168.19 ppm), an enrichment of LREE (LaN/YbN = 4.54–8.09), and small negative Eu anomalies (δEu = 0.74–0.90). The depletion of HFSE and relevant tectonic setting discrimination diagrams demonstrate that these two volcanic associations were formed in a subduction‐related environment. The data also indicate that the high‐K volcanic rocks are derived from a mix of melted subducted sediments and mantle metasomatized by silica‐rich fluids. Although these two sets of volcanic rocks formed in Early and Late Carboniferous, the similar petrographic and geochemical natures suggest similar magma source and tectonic setting. Along with different regional tectonic settings in the NJB during the Carboniferous, we propose an asynchronous subduction of Keramaili oceanic basin, resulting in the inconsistent temporal and spatial distribution of Upper Carboniferous volcanic rocks in the NJB.
Xibeikulasi, Baogutu and Tailegula Formations have been controversies over their ages and stratigraphic relationships for almost half a century since they were established in southern western Junggar; it is caused by the unbalanced view of the lithologic characteristics of these three formations, and by misusing paleontological or chronological evidences of other geological units to define the ages of these three formations, thus resulting in a variety of opinions on their sequential arrangement and ages. On the basis of the 1:250000 regional geological survey, we re-measured their stratotype sections in detail, and combined with a large number of auxiliary sections and geological survey routes, the definitions, characteristics and distribution of three formations were determined. The scouring structures and graded beddings near the boundaries between the three formations support the stratal sequences of original assignment, namely the Xibeikulasi, Baogutu and Tailegula Formations in ascending order. The 'fossil chaos' of the three formations is due to mistaking fossils of other stratigraphic units for fossils of these three formations. After revision, only the Early Carboniferous fossils were considered reliable, and combined with the newly found plant fossils, the Xibeikulasi, Baogutu and Tailegula Formations were assigned to the early Visean, late Visean, and latest Visean to Serpukhovian ages, respectively. According to the spatial distribution of the Xibeikulasi, Baogutu and Tailegula Formations, the direction of turbidity currents of the Early Carboniferous Visean was NE in southern western Junggar, and changed into the opposite direction since the latest Visean. Based on the above, the Carboniferous palaeogeographical evolution of the southern western Junggar has been discussed. The Late Paleozoic remnant oceanic basin was filled in the Early Carboniferous. It began with the coarse clastic rocks occurring in the Xibeikulasi Formation, and it was nearly filled when the emergence of the coarse clastic rocks in the upper part of the Tailegula Formation. The closure of the western Junggar remnant oceanic basin was marked by the emergence of the molasse and continental volcanic rocks of the Jiamuhe Formation in the late Late Carboniferous.
As an important part of the Paleozoic orogonic belt in northern Xinjiang, the Junggar is characterized by intense and extensive development of Late Paleozoic granitoid plutons together with minor mafic plutons, and it is one of the representative regions for the Phanerozoic continental crustal growth in the Central Asian Orogenic Belt. On the basis of new SHRIMP zircon U-Pb dating on the plutons in the East and West Junggar, in association with the zircon U-Pb ages from the literature, an attempt is made in this study to establish the timing of the late Paleozoic post-collisional plutonism in the region. It is concluded that the late Paleozoic post-collisional plutonism occurred from middle-late Visean of Carboniferous to the end of Early Permian in accordance to the Geological Time Scale (Gradstein et al., 2004). Temporally, the plutons in the East and West Junggar formed in a period of 65Ma, from 330 to 265Ma and from 340 to 275Ma, respectively. However, the plutonism in the East Junggar showed two peaks of 330 to 310Ma and 305 to 280Ma, whereas most plutons in the West Junggar were emplaced from 310 to 295Ma. Spatially, the plutonism occurred in all tectonic units separated by ophiolite belts and even some granitoid plutons intruded in ophiolites. Furthermore, the post-collisional plutons can be found not only in the Junggar, but also in the Altai orogenic belt to the north and in the Tianshan orogenic belt to the south.
The Yidun Group extends from the Shangri-La region to the south and the Changtai region to the north, and is an important component of the Triassic Yidun arc in the eastern Tibetan plateau. It is composed of the Lieyi, Qugasi, Tumugou and Lanashan Formations from the base upward. Both the Lieyi and Lanashan Formations consist dominantly of black or gray slate and sandstone, whereas the Qugasi and Tumugou Formations have variable amounts of mafic to felsic volcanic rocks and tuffs accompanied with gray slate and sandstone.
Sandstone from the Yidun Group has variable CIA values from 55 to 76, indicative of mild to moderate weathering condition for the source rocks. All the sandstones define a general weathering trend nearly parallel to the A-CN boundary in the A-CN-K triangular diagram, implying limited effect of diagenetic and post-depositional K-metasomatism. Dominant detrital quartz and feldspar grains of the sandstones suggest predominantly felsic sources. Relatively high Y/Ni and low Cr/V ratios of sandstones from the Yidun Group indicate more contribution from felsic than mafic sources. Similarly, the Yidun sandstones have Co/Th and La/Sc ratios generally similar to upper continental crust (UCC) and cluster between UCC and felsic sources, indicating felsic rocks as primary sources. Granodiorite represents the average chemical composition of sources as evaluated by extending the predicted weathering trend back to the feldspar join in A-CN-K diagram.
Prominently high Zr/Sc ratio or Hf concentration and Paleoproterozoic Nd modal ages (1.94–2.21 Ga) point to input of recycling components derived from old sedimentary source in a relatively stable tectonic setting.
Lutites are commonly metasomatized during diagenesis, but the analysis presented here accounts for most postdepositional change. Potassium metasomatism is particularly common, and typically involves the conversion of kaolin (residual weathering product) to illite by reaction with K+-bearing pore waters. Sandstones also undergo K metasomatism, which involves the replacement of plagioclase by potassium feldspar. These changes can be identified petrographically and are quantitatively accounted for by techniques discussed herein. Bulk chemical analyses and ternary diagrams are used to determine the amount of K addition, premetasomatized sediment composition, and composition of provenance areas. The premetasomatized mineralogy of paleosols can be compared with the mineralogy of recent soil profiles and thus, climate and topographic conditions determined for past weathering events. Some weathering indices lead to erroneous conclusions because, by excluding K2O from consideration, correction cannot be made for metasomatic effects.
Granitic intrusions emplaced within Laurentia during the Grenville Orogeny (1.15–1.05 Ga) are exceptionally Zr-rich, primarily a function of high modal zircon, relative to Paleozoic granitoids emplaced within eastern Laurentia during phases of Appalachian orogenesis. Erosion of Grenville source rocks would generate disproportionately large volumes of detrital zircon compared to less zircon-fertile source regions. The latter sources are difficult to detect by standard in situ U–Pb dating methods of detrital zircon (SHRIMP or LA-ICP-MS U–Pb analysis of centers of > 100 μm grains), or could be overlooked during sampling. Grenvillian zircon fertility biased the Neoproterozoic to Recent sedimentary record as a result of two factors: (1) zircon durability and insolubility led to recycling during repeated orogenesis; (2) inertness of zircon below the onset of anatexis means dominantly metamorphosed sedimentary terranes failed to generate significant new zircon corresponding in age to the time of accretion of those terranes to Laurentia. Zircon growth under anatectic conditions generates new zircon, commonly as overgrowths on preexisting zircon, which may be too narrow to easily analyze by laser or ion beam techniques. Accretion of metasedimentary terranes therefore could be rendered all but undetectable via standard U–Pb detrital zircon dating. Grenville age modes dominate detrital zircon age distributions for Laurentian Neoproterozoic rift basins, Appalachian Paleozoic synorogenic clastic sequences, Appalachian metasedimentary terranes, and modern rivers draining these terranes. This is an artifact of the Grenville zircon signal that echoed throughout all Paleozoic orogenies on the eastern Laurentian margin. The natural Grenville bias in the detrital zircon record is further amplified by standard sampling biases (such that large (e.g. 100–300 μm) crystals are preferentially chosen) and analytical biases (e.g. cores are much more frequently analyzed).
West Junggar, as a crucial part of the Central Asian Orogenic Belt (CAOB), has been paid a lot of attention by numerous scholars. Six ophiolitic melange belts, numerous granitoids and mafic-intermediate dyke swarms outcrop in West Junggar, indicating how complex the Paleozoic tectonic evolution of West Junggar is. Although a great of improvements has been made, there are still many controversies, such as the formation time, rock assemblages and petrogenesis of diverse ophiolitic melanges, the petrogenesis, tectonic setting and thermal mechanism of the I-tvpe and A-tvpe granitoids, the geochronology, petrogenesis, tectonic background and paleo-stress regime of mafic-intermediate dyke swarms, the Paleozoic geochonological structure, the tectonic setting, the Phanerozoic crustal growth, the basement and tectonic evolution of West Junggar, and so on. The author collects the reported data and summarizes on the ophiolitic melange. Paleozoic granitoids and intermediate-mafic dyke swarms in West Junggar. Combining with the geological survey and geochemical work, we put forward several cognitions; ( 1 ) Daerbute and Karamay ophiolitic melanges that are derived from high degree partial melting of spinel-bearing Iherzolite, are formed in back-arc tectonic setting related with subduction. (2) C1 granitoids are the outcomes of subduction, C2-P1 are generated in post-collisional background, while P2 granitoids are formed in within-plate setting. (3) The petrogenesis of I-tvpe granitoids are subduction-related, while the A-tvpe granitoids and mafic- intermediate dyke swarms are post-collisional background-related. (4) The A-tvpe granitoids are derived from the high degree of fractionated crystallization after the lower crust partial melting induced by upwelling mantle. Geochemically, the mafic-intermediate dyke swarms are featured as adakitic rocks or sanukite, which are most possibly from partial melting of remnant oceanic slab metasomatised by fluids or melts. (5 ) The mafic-intermediate dyke swarms are formed a little later than that of the host granitoid, both of which are formed in post-collisional background in C2-P1 , suggesting a Paleozoic stress regime with near North-South direction extension. (6) The Paleozoic tectonic settings of West Junggar are oceanic basin system in Devonian, subdiction in Early Carboniferous, post-collisional setting in Late Carboniferous-Early Permian and within-plate setting in Middle-Late Permian.
There are two ophiolite belts, Armantai and Kelamaili ophiolite belts, in northeastern border of the Junggar basin, Xinjiang. Ages of the ophiolite and geological time of their emplacement of these two ophiolite belts are still not clear though many research works have be done since 1981. According to new data of isotopic age determination, microfossils in the ophiolite suite and regional tectonic analysis, this paper get following new knowledge: geologic time of development of these two palaeo-oceans representing by two ophiolite belts were close, and it might be from late Neo-proterozoic to early Silurian. The nascent oceans were formed in late Neo-proterozoic. The oceans were in a period of great prosperity from Cambrian to early-middle Ordovician. And they were withered and closed from late Ordovician to early Silurian. This new knowledge is important to reconstruction of Paleo-Asian Ocean and understand continental crust evolution of Northern Xinjiang.
Several previous studies have shown that sandstones from different tectonic settings possess characteristic chemistry, particularly SiO2 content and K2O/Na2O ratio. Systematic variation of these variables with change in grain size in New Zealand greywackes shows that such discrimination can be extended to the finer-grained members (siltstones, mudstones) of sedimentary sequences. Compilation of published data from ancient sedimentary suites defines passive margin (PM), active continental margin (ACM), and oceanic island arc (ARC) tectonic setting fields on a simple bivariate K2O/Na2O-SiO2 plot. These fields are confirmed by published analyses of modern sands and muds of known tectonic setting, which also reveal distinctive sand-mud trends in sediments from the forearc environment of island arcs, where muds show greater ratios and SiO2 than the sands. Results gained from New Zealand terranes are in good agreement with published paleoenvironmental interpretations. -from Authors
Sandstones of the Palaeozoic turbidite sequences of eastern Australia show a large variation in their major element geochemistry, reflecting the distinct sedimentary provenance and tectonic setting of each suite. A close correlation exists between the geochemical composition of sandstones and tectonic settings of sedimentary basins. The nature of the continental margin and oceanic basins can be deciphered on the basis of the major element composition of sandstones. The most discriminating parameters are described.-from Author
The Quruqtagh area is situated in the linkage of South Tianshan and Tarim basin where well preserved sedimentary records. The Beiyixi. Formation in the Quruqtagh area is a volcanic-sedimentary succession which consists of igneous rocks and sandstones. The Yukengou Formation is characterized by sandstones and siltstones formed by turbidity current. Detrital zircon U-Pb ages of two samples from the sandstones were measured by the LA-ICP-MS method. The sample 09DPL25 and 09DPL41 respectively records 93 and 71 age data. Detrital zircons of sample 09DPL25 from Beiyixi Formation whose peak age is 821Ma and subordinate peak age is 861 Ma are prevalent. And the majority of detrital zircons of sample 09DPL41 from Yukengou Formation is weighted-age 768Ma (subordinate 800Ma and 741Ma). Based on CL images, there occurred a few magmatic events about 741Ma, 768Ma, 821Ma and 861.Ma. Together with the literature data, it strongly suggests that the basement rocks in the Quruqtagh region were formed in the Mesolo Neo-Archean period. Detrital zircons indicate that the provenances of Beiyixi Formation are 761 ∼ 847Ma, 851 ∼972Ma and 1808 ∼ 2498Ma rocks and main provenance ages of Yukengou Formation are 734 ∼ 845Ma, 858 ∼963Ma and 1708 ∼2486Ma.
The modern Junggar Basin was located in the Central Asian Orogenic Belt (CAOB) during the Devonian and Carboniferous. Few data concerning the weathering, provenance and tectonic setting of the Carboniferous sedimentary rocks in the Junggar Basin have been published. The recent discovery of an oil–gas field in the East Junggar Basin provides the impetus for this study. Twenty-eight sediment samples were recovered from eight boreholes, and all were analysed for major and trace element geochemistry. Two samples of sedimentary rocks were selected for zircon U–Pb dating. Low chemical index of alteration (CIA) values (44–71) and relatively high index of compositional variability (ICV) values (0.72–1.83) suggested weak chemical weathering and an immature source for these samples. The relatively low Th/U (2.16–10.55) and Rb/Sr (0.03–5.44) ratios of the drill core samples reflect a simple recycling history. Two samples indicated upper Carboniferous ages (314.6 Ma) and a lower-middle Devonian age (397.9 Ma). Geochemistry and detrital zircon geochronology from the East Junggar Basin reveal a distinct difference in the composition and source rock ages of clastic rocks from the lower and the upper Carboniferous. Major and trace element compositions of the lower Carboniferous sedimentary rocks (LCRs) were derived from typical mafic rocks and an intermediate ocean island arc of early to middle Devonian age, whereas the upper Carboniferous sedimentary rocks (UCRs) were derived from a continental island arc or active continental margin setting with a dominantly felsic to intermediate late Carboniferous age. The current data provide little support for tectonic models that show a passive continental margin of a Precambrian crystalline basement; our study suggests that a volcanic arc accretion or archipelago (early Carboniferous to early-middle Devonian) model for the East Junggar Basin is more viable.
The Portscatho Formation, within the allochthonous unit of the Middle and Upper Devonian Gramscatho Group, is a thick sequence of deep-water sandstones and interbedded slates deposited by southerly-derived turbidity currents into the Gramscatho basin of S Cornwall. Throughout an approximately 3.5 km thick sequence, the Portscatho Formation is petrographically and chemically coherent, except that the upper section shows a higher proportion of metamorphic clasts, high, but variable Cr, and low, uniform Zr abundances. Complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition, similar to average upper continental crust, but with an admixture of minor intermediate/basic material. Flysch deposition took place in a fore-arc setting.-from Authors
A significant change in the composition of turbidites deposited at active tectonic settings is seen at the Archean/post-Archean transition. Early crustal growth is consistent with a hotter Archean earth, but discontinuous aspects of crustal evolution are not so readily understood for a continually cooling earth. It may be possible to reconcile these features by considering the effects of supercontinental cycles, beginning in the later Archean, superimposed on a framework of secular earth cooling. -from Authors
Integrated U–Pb dating, Hf-isotope and trace-element analysis of detrital zircons from the Fanjingshan and Xiajiang sediments in the southeastern part of the Yangtze Block has been used to identify ancient crustal remnants and the provenance of clastic sediments, and to provide an overview of crustal evolution in the now-covered parts of the Yangtze Block. The zircon dating indicates that the Fanjingshan Group, which previously was regarded as 870Ma old, actually was deposited in a ∼800Ma rift basin similar to other contemporaneous basins in the Yangtze Block; the regional unconformity separating the Fanjingshan Group from the overlying Xiajing Group is dated to 800–740Ma. Differences in clast compositions and the trace-element and Hf-isotope signatures of zircons between the Fanjingshan Group and the Xiajiang Group suggest changes in the source areas within the Yangtze Block. The combined ages and Hf-isotope data for each group show different histories of crustal evolution in their source areas. In the Yangtze Block, a previously unrecognized source (∼4.3Ga) is suggested by the Hf model ages of the oldest zircons. Both recycling of ancient crustal materials and minor addition of juvenile material took place in the time intervals 2.5–2.4Ga, 2.0–1.7Ga and 0.85–0.8Ga. The most important generation of juvenile crust appears to have occurred at 1.6–1.4Ga in the source area of the Fanjingshan Group, and at 0.95–0.85Ga and 0.78–0.74Ga in the source area of the Xiajiang Group. This is the first documentation of early Mesoproterozoic (1.6–1.4Ga) juvenile crust in South China.
The geochemical composition of the Middle-Upper Proterozoic turbidites in Hunan, South China is related to provenance and tectonic setting of the sedimentary basin. The turbidites are characterized by moderate SiO 2 contents (Al 2O 3/SiO 2 typically 0.1-0.3) and moderate K 2O/Na 2O ratios (generally 0.1-0.3) but relatively high Fe 2O 3* + MgO contents (5-10 wt %) and unusually low contents of CaO (typically < 1 wt %). Abundances of ferromagnesian trace elements and incompatible elements are moderate. The REE patterns are uniform and similar to the upper continental crust and typical post-Archean shales, with LREE enrichment, flat HREE, and significant negative Eu-anomalies. In general, the slates show higher contents of Fe 2O 3* + MgO and ferromagnesian trace elements, and lower ratios of La/Sc and Th/Sc than the associated graywackes, suggesting that more mafic materials were incorporated into the clay-size fraction. Compared to the Upper Proterozoic Banxi Group, the Middle Proterozoic Lengjiaxi Group has higher concentrations for most ferromagnesian trace elements and lower La/Sc ratios, implying more mafic components in the provenance of the latter. The relationship among alkali and alkaline earth elements, the Chemical Index of Alteration (CIA), and the Th/U ratios indicate that the source area was affected by a moderate weathering history. The main source was old continental crust (the Yangtze craton) dominated by felsic rocks of magmatic origin, with a variable admixture of mafic components from continental island arcs (e.g., the Fangjingshan-Dayong and the Sibao-Yiyang island arc chains). The flysch deposition took place in a back arc basin, rather than a failed intracontinental rift as proposed by some workers. We suggest that suturing between the Yangtze and Cathaysia blocks did not occur until late Proterozoic time and that the final assembly of the supercontinent Rodinia in South China was probably marked by the Jingning orogeny at ∼0.8 Ga.
Whole-rock major and trace element compositions of quartzofeldspathic Permian–Cretaceous
sandstones of the Torlesse terrane, New Zealand, display progressive changes which compare
well with published petrographic data. Chemical indices in three Permian–early Late Triassic
Rakaia subterrane petrofacies show small contrasts, as do the modes, reflecting original source compositions.
A Late Triassic Rakaia petrofacies displays sharp increases in SiO2/Al2O3,
K2O/Na2O, Th/Sc, La/Sc, Ce/Sc,
LaN/YN and decreases in Ti/Zr and V/La, consistent with partial cannibalistic
recycling and a small influx of lithic volcanic detritus. In the post-collisional Late Jurassic–Early
Cretaceous Pahau subterrane, SiO2/Al2O3 remains high, but other indices decrease to levels seen in the
Permian, due to recycling of the earlier Rakaia petrofacies and influx of 10–30% mafic-intermediate
volcaniclastic detritus derived from inboard volcanogenic terranes. The results are confirmed by additional
suites, including mudstones, which cover a larger area, but also show that some spatial variation
occurs within the terrane as a whole. Whole-rock data and Chemical Index of Alteration indices
indicate that the Torlesse was derived from a relatively unweathered source with granodioritic bulk
composition. Comparisons with data from a proposed Rakaia source in the New England and
Hodgkinson orogens of eastern Australia suggest that the observed chemical and modal compositions
cannot be simply derived from that area as presently exposed.
Terra Nova, 24, 189–198, 2012
In situ U–Pb geochronology of detrital zircons from various Palaeozoic sedimentary rocks of West Junggar accretionary complexes (Central Asia) suggests two distinct episodes of arc magmatism, and an evolution in three steps: (i) Ordovician-Silurian subduction generating juvenile arc crust; (ii) Late Silurian subduction jamming, erosion and intraplate magmatism; (iii) development of two new opposed Devonian-Carboniferous subductions recycling the Early Palaeozoic crust. Zircon Hf isotopes document three pre-Permian episodes of mantle-derived magmatic input into the crust: (1) Neoproterozoic (850–550 Ma), (2) Early Palaeozoic (530–450 Ma) and (3) Late Palaeozoic (380–320 Ma). Zircons also record the recycling of Neoproterozoic and Early Palaeozoic juvenile crusts during the Early and Late Palaeozoic. These data support a model of episodic continental crust growth in Central Asia.
The graywackes of Paleozoic turbidite sequences of eastern Australia show a large variation in their trace element characteristics, which reflect distinct provenance types and tectonic settings for various suites. The tectonic settings recognised are oceanic island arc, continental island arc, active continental margin, and passive margins. Immobile trace elements, e.g. La, Ce, Nd, Th, Zr, Nb, Y, Sc and Co are very useful in tectonic setting discrimination. In general, there is a systematic increase in light rare earth elements (La, Ce, Nd), Th, Nb and the Ba/Sr, Rb/Sr, La/Y and Ni/Co ratios and a decrease in V, Sc and the Ba/Rb, K/Th and K/U ratios in graywackes from oceanic island arc to continental island arc to active continental margin to passive margin settings. On the basis of graywacke geochemistry, the optimum discrimination of the tectonic settings of sedimentary basins is achieved by La-Th, La-Th-Sc, Ti/Zr-La/Sc, La/Y-Sc/Cr, Th-Sc-Zr/10 and Th-Co-Zr/10 plots. The analysed oceanic island arc graywackes are characterised by extremely low abundances of La, Th, U, Zr, Nb; low Th/U and high La/Sc, La/Th, Ti/Zr, Zr/Th ratios. The studied graywackes of the continental island arc type setting are characterised by increased abundances of La, Th, U, Zr and Nb, and can be identified by the La-Th-Sc and La/Sc versus Ti/Zr plots. Active continental margin and passive margin graywackes are discriminated by the Th-Sc-Zr/10 and Th-Co-Zr/10 plots and associated parameters (e.g. Th/Zr, Th/Sc). The most important characteristic of the analysed passive margin type graywackes is the increased abundance of Zr, high Zr/Th and lower Ba, Rb, Sr and Ti/Zr ratio compared to the active continental margin graywackes.
In the Paleozoic graywackes of eastern Australia there is an increase in the total abundance of rare earth elements (REE), a light to heavy REE ratio and a decrease in the chondrite normalized Eu anomaly with the increase in SiO2/Al2O3 and K2O/Na2O ratios. This is due to the change in the dominant source rocks from andesite to dacite to granite-gneiss and sedimentary rocks. Similarly, an increase in total REE and a decrease in Eu anomaly is also seen in the associated mudrocks with the increase in total clay content. This correlation in Paleozoic sedimentary rocks suggests that the REE characteristics of terrigenous sedimentary rocks reflect the depositional tectonic setting of the basin. The oceanic island-arc-type graywackes are characterised by their lower total REE abundance, only slight enrichment of LREE over HREE, and the absence of a negative Eu anomaly on chondrite-normalised plots. The continental island arc type graywackes are discriminated by their higher REE abundance and La/Yb ratio, the presence of a small negative Eu anomaly on the chondrite normalised plots and the significant enrichment of Eu compared to PAAS (Post-Archean Average Australian Shale). The sedimentary rocks deposited on Andean-type active continental margins, passive margins, platform and cratonic basins are all characterised by the high enrichment of LREE over HREE and the presence of a pronounced negative Eu anomaly on chondrite-normalised plots and can not be discriminated from each other by REE patterns alone.
Sandstones and argillites from selected New Zealand greywacke terranes represent four differing provenance groups: P1 (mafic) — first-cycle basaltic and lesser andesitic detritus, represented by the Maitai terrane and part of the Caples terrane; P2 (intermediate) — dominantly andesitic detritus (Waipapa and Pelorus terranes); P3 (felsic) — acid plutonic and volcanic detritus (Torlesse terrane); and P4 (recycled) — mature polycyclic quartzose detritus (Greenland Group). Contrasts in major-element composition occur between the groups due to differing provenance. and ratios increase, and Fe2O3T + MgO decreases from P1 to P4, as a result of petrologic evolution (P1-P3) and sedimentary maturation (P4). Some parts of oxide or ratio variation diagrams are distinctive for each group, but considerable overlap occurs due to bulk compositional variation with decrease in grain size. This overlap is almost eliminated by discriminant function analysis using Al2O3, TiO2, Fe2O3T, MgO, CaO, Na2O and K2O, and a plot of the first two functions gives effective separation between the four groups.Tests of the functions with analyses of volcanic and plutonic rocks from New Zealand and the Lau volcanic arc, additional sedimentary terranes of New Zealand, and published data of sedimentary suites from Australia and the Pacific margin give good results, suggesting that the method is a viable technique for provenance determination which is largely independent of grain-size effects. The functions are applicable only to rocks which lack significant biogenic fractions, or to those where analyses can be corrected for these inputs. A second set of functions using ratios are designed for samples influenced by biogenic sedimentation. Although classification and test results are adequate, results are mixed for suites of Tertiary and modern sediments with substantial calcareous or siliceous components from New Zealand, the Japan Trench and the Solomon Islands.