Wu-Xian Li

Magmatic and metamorphic events during the early Paleozoic Wuyi-Yunkai orogeny, southeastern South China: New age constraints and pressure-temperature conditions

The early Paleozoic Wuyi-Yunkai orogen in South China is a major orogenic belt in East Asia that formed at a similar time as the classic Caledonian orogeny in Europe. Despite the possibility of its being one of the few examples of intraplate orogenesis in the world, details about the orogen remain poorly defined. In this study, we provide age constraints on metamorphic and magmatic events in the eastern segment of the orogen, and the protoliths of the amphibolite-facies metamorphic rocks found there. By combining previous work with our new metamorphic and petrogenetic analyses, we present the following findings: (1) the Wuyi-Yunkai orogeny occurred between mid-Ordovician (>460 Ma) and earliest Devonian (ca. 415 Ma) time; (2) amphibolite-facies metamorphism in the eastern Wuyi-Yunkai orogen occurred between ca. 460 and 445 Ma, whereas cooling below 500–300 °C occurred by ca. 420 Ma; (3) the orogen exhibits a clockwise pressure-temperature ( P - T ) path and a maximum pressure of >8 kbar, indicating crustal thickening during the orogeny; (4) protoliths of the high-grade metamorphic rocks in the eastern segment of the orogen were dominantly Neoproterozoic (840–720 Ma) volcanic and volcaniclastic rift successions and younger deposits formed in a failed rift, and Paleoproterozoic rocks account for only a small proportion of the outcrops; and (5) the analyzed granites indicate a mixed source of Paleoproterozoic basement and Neoproterozoic continental rift rocks, with elevated melt temperatures of >800 °C, which are interpreted as reflecting dehydration melting of basin sediments taken to below midcrustal levels.

Initiation of the Indosinian Orogeny in South China: Evidence for a Permian Magmatic Arc on Hainan Island

It has been widely accepted that an active continental margin existed along the coast of Southeast China during the Mesozoic time that produced extensive magmatism in the region. However, there is little constraint as to when this active margin was first initiated. Here we present new SHRIMP U‐Pb zircon ages and geochemical and Sr‐Nd isotopic data for syntectonic granites on Hainan Island. Our data demonstrate that these rocks, dated at 267–262 Ma, are typical of calc‐alkaline I‐type granites formed in continental arc environments. The age of this magmatic arc coincides with a sudden change in sedimentary environments in South China during the Permian time, suggesting that the South China Indosinian Orogeny was likely contemporaneous with the onset of continental arc magmatism.

Precise Sm–Nd and U–Pb isotopic dating of the supergiant Shizhuyuan polymetallic deposit and its host granite, SE China

The supergiant Shizhuyuan W-Sn-Bi-Mo deposit is hosted by the Qianlishan granite, a small, highly fractionated granitic pluton (∼ 10 km 2 ) with multiple phases of intrusions within the Early Yanshanian granitoid province of SE China. Strong alteration of skarn and greisen that formed in the contact zone between the first and second phases of granite intrusions and Devonian limestone is responsible for the polymetallic mineralizations. SHRIMP U-Pb zircon analysis indicates that the two early phases of the Qianlishan granite formed contemporaneously at 152 ± 2 Ma. Metasomatic minerals (garnet, fluorite and wolframite) separated from the skarn and greisen yield a Sm-Nd isochron age of 149 ± 2 Ma that is interpreted as the formation age of the Shizhuyuan deposit. Therefore, the mineralization of the supergiant Shizhuyuan polymetallic deposit formed contemporaneously with, or very shortly after, the intrusion of the small, highly fractionated Qianlishan granite.

Jurassic gabbro-granite-syenite suites from Southern Jiangxi province, SE China: Age, origin, and tectonic significance

SHRIMP U-Pb zircon and hornblende 39Ar/40Ar ages, major and trace elements, and Sr-Nd isotopes are reported for the Chebu gabbro, the Zhaibei granite, and the Quannan and Tabei syenites, southern Jiangxi, Southeast China. Two major episodes of Jurassic magmatism were identified, i.e., the ~172 Ma bimodal gabbro-granite and the ~165 Ma syenite suites. The Chebu gabbros show geochemical affinities of intraplate transitional basaltic rocks with restricted Sr and Nd isotopic variations (I Sr = 0.7065 to 0.7067, εNd(T) = +0.55 to -0.82). The Zhaibei granites have variable εNd(T) values (-0.78 to -6.55) and I Sr > 0.710. They share geochemical characteristics with aluminous A2-type granites, and probably were generated by shallow dehydration melting of hornblende-bearing granitoid source rocks heated by contemporaneous intrusion of mantle-derived basaltic magmas. The Quannan and Tabei syenites are high in K2O, with K2O/Na2O ratios of 0.9-1.3, typical of shoshonitic affinities. They have a relatively large range of Sr and Nd isotopic compositions (I Sr = 0.704-0.710, εNd(T) = -3.54 to +3.44). Among them, the least crust-contaminated syenites display OIB-type incompatible trace-element patterns with Nb/La >1 and Nb/Th >14. The geochemical and isotopic data of the gabbros and syenites imply that their mantle sources might have been metasomatized by OIB-type melts in an intraplate regime, contrasting with the subduction-related metasomatism occurring in the mantle wedge overlying a subduction zone. The 172-165 Ma gabbro-A-type granite-syenite suites in southern Jiangxi are coeval with the 175-160 Ma alkaline basalts and syenites in southern Hunan-southeastern Guangxi, suggesting that an extension-to-rifting regime was dominant over much of Southeast China during the Middle to Late Jurassic. This rifted continental margin seems appropriate to account for the widespread Middle to Late Jurassic magmatism in Southeast China.

The Bikou basalts in the northwestern Yangtze block, South China: Remnants of 820-810 Ma continental flood basalts?

National Natural Science foundation of China[40721063]; Chinese Academy of Sciences[KZCX3-SW-141]; Australian Research Council[DP0770228]

Ca. 825 Ma komatiitic basalts in South China: First evidence for >1500 °C mantle melts by a Rodinian mantle plume

Mantle plume or superplume activities have repeatedly been invoked as a cause for the breakup of the Neoproterozoic supercontinent Rodinia, with supportive evidence including radiating dike swarms, globally synchronous anorogenic igneous activity, large-scale lithospheric doming and unroofing, and geochemical signatures similar to recent plume-related magmatism. However, high-temperature magmas such as picrite or komatiite, a requisite product of mantle plume activities, have not previously been identified for those events. We present here geochronological and geochemical data from a suite of pillow lavas in central South China. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of zircons from an evolved member of andesitic composition within the suite indicates that the lavas were erupted at 823 ± 6 Ma. All but a few highly evolved, crust-contaminated basaltic rocks are characteristically high in MgO (10.2%–17.5%), Ni (183–661 ppm), and Cr (677–1672 ppm), but low in TiO2 (0.5%–0.7%), Al2O3 (10.6%–12.7%), and FeOT (total Fe as FeO) (7.4%–10.5%), typical of komatiitic basalts. Our geochemical modeling, which removes the effect of olivine crystallization, suggests that their primary magma has typical komatiitic compositions with MgO ≈ 20%, FeOT ≈ 11%, SiO2 ≈ 47%, TiO2 ≈ 0.48%, Al2O3 ≈ 10%, Ni ≈ 860 ppm, and Cr ≈ 1780 ppm. Such a high MgO content in the primary melts implies a melt temperature of >1500 °C, suggesting that the Yiyang komatiitic basalts were generated by melting of an anomalously hot mantle source with potential temperature ( T p) 260 ± 50 °C higher than the ambient mid-oceanic-ridge basalt (MORB)–source mantle, similar to that of modern mantle plumes. The Yiyang komatiitic basalts are thus the first solid petrological evidence for the proposed ca. 825 Ma mantle plume that played a key role in the breakup of the supercontinent Rodinia.

Formation of high δ18O fayalite-bearing A-type granite by high-temperature melting of granulitic metasedimentary rocks, southern China

The genesis of A-type granites has been controversial. Fayalite granite is a member of the most reduced A-type granites, commonly thought to have been primarily sourced from tholeiitic rocks. In this paper we report petrography, whole-rock geochemistry, Sr-Nd isotope, and in situ zircon Hf-O isotope results for a fayalite-bearing A-type granite suite at Jiuyishan in southern China. High zircon δ 18 O (8.0‰–9.8‰), negative zircon eHf(t) (–6.2 to –2.3), and evolved whole-rock Sr-Nd isotopes ( I Sr = 0.7151–0.7181; eNd(t) = –7.4 to –6.6) indicate the reworking of old supracrustal rocks. Isotopic and geochemical results, particularly downward inflections of Zr and Ba at ∼70 wt% SiO 2 , point to fractional crystallization rather than magma mixing as the controlling process for the evolution of the igneous suite. Integrated petrological, geochemical, and isotopic studies present the first convincing case that reduced A-type granites can also be generated by high-temperature (>960 °C) melting of granulitic metasedimentary rocks, likely related to the upwelling of the asthenosphere and/or underplating and intrusion of basaltic melts. We emphasize that key factors for the genesis of this unique rock type are low oxygen fugacity ( f O 2 ), low f H 2 O , and high temperature.

Ca. 850 ma bimodal volcanic rocks in northeastern Jiangxi province, south China: initial extension during the breakup of rodinia?

Multiple Meso- to Neoproterozoic magmatic events in South China are believed to record the transition in tectonic regime from the assembly of the supercontinent Rodinia to its breakup. We document in this paper the earliest, ca. 850 Ma bimodal volcanism in South China which may indicate the beginning of continental extension and rifting. The Zhenzhushan bimodal volcanic rocks were dated at 849 ± 6 Ma using the secondary ion mass spectrometry (SIMS) U-Pb zircon method. The mafic members of the succession are Si-saturated tholeiitic basalts with moderate enrichment in most incompatible trace elements and high εNd(T) values of 2.1 to 4.8. This suggests a likely derivation of the basalts from a depleted asthenospheric mantle source in an extensional environment, and that the magma experienced low-degree fractional crystallization and crustal contamination during its ascent to the surface. The felsic members are peraluminous and are characterized by negative εNd(T) values of −0.3 to −1.9 with geochemical features similar to those of the adjacent ca. 820 Ma peraluminous granitoids derived from a Mesoproterozoic to earliest Neoproterozoic sedimentary source. In combination with the distribution of 850 to 830 Ma igneous and sedimentary successions in South China, we suggest that continental rifting along the southern margin of the Yangtze Block probably started by 850 Ma. Coeval anorogenic magmatism has also been reported in other parts of the Rodinia supercontinent, and may represent the early stage of the Rodinia superplume in response to a circum-Rodinia mantle avalanche after the complete assembly of the supercontinent.

Middle Neoproterozoic syn-rifting volcanic rocks in Guangfeng, South China: petrogenesis and tectonic significance

MiddleNeoproterozoicigneousrocksarewidespreadinSouthChina,buttheirpetrogenesis andtectonicimplicationsarestillhighlycontroversial.TheGuangfengmiddleNeoproterozoicvolcani- sedimentary succession was developed on a rare Sibaoan metamorphic basement(the Tianli Schists) in the southeastern Yangtze Block, South China. This paper reports geochronological, geochemical and Ndisotopicdataforthevolcanicrocksinthissuccession.Thevolcanicrocksconsistofalkalinebasalts, andesites and peraluminous rhyolites. SHRIMP U-Pb zircon age determinations indicate that they were erupted at 827 ± 14 Ma, coeval with a widespread episode of anorogenic magmatism in South China. Despite showing Nb-Ta depletion relative to La and Th, the alkaline basalts are characterized by highly positive eNd(T) values (+3.1 to +6.0), relatively high TiO2 and Nb contents and high Zr/Y and super-chondritic Nb/Ta ratios, suggesting their derivation from a slab melt-metasomatized subcontinental lithospheric mantle source in an intracontinental rifting setting. The andesites have significantlynegative eNd(T)values(−9.3to −11.1)andawiderangeofSiO2 contents(57.6-65.6 %). They were likely generated by the mixing of fractionated basaltic melts with felsic melts derived from theArchaeanmetasedimentaryrocksinthemiddletolowercrust.Therhyolitesarehighlysiliceousand peraluminous. They are characterized by depletion in Nb, Ta, Sr, P and Ti and relatively high eNd(T) values (−3.0 to −4.8), broadly similar to those of the adjacent c. 820 Ma peraluminous granitoids derived from the Mesoproterozoic to earliest Neoproterozoic sedimentary source at relatively shallow levels. We conclude that the Guangfeng volcanic suite is a magmatic response of variant levels of continental lithosphere (including lithospheric mantle and the lower-middle to upper crust) to the middle Neoproterozoic intracontinental rifting possibly caused by mantle plume activity.

From Rodinia to Gondwanaland: A tale of detrital zircon provenance analyses from the southern Nanhua Basin, South China

The paleogeographic positions of the South China Block (SCB) during the Neoproterozoic and early Paleozoic are important for understanding the transition from the break-up of the supercontinent Rodinia to the formation of Gondwanaland. Integrated in situ U-Pb ages and Hf-O isotope analyses of detrital zircons from Cambrian sedimentary rocks in the southwestern SCB reveal major age populations at 2500 Ma, 1100 to 900 Ma, 850 to 750 Ma and 650 to 500 Ma, with a predominant group at ∼980 Ma that counts for ∼50 percent of all analyses. Zircon Hf-O isotopic results suggest three Precambrian episodes of juvenile crustal growth for the source area(s) (3.0 Ga, 2.5 Ga and 1.0 Ga), with major crustal reworking at 580 to 500 Ma. The source provenance as defined by the U-Pb and Hf analyses is distinctly different from the known tectonomagmatic record of the SCB, or that of western Australia or western Laurentia, but matches well with that of the Ediacaran (latest Neoproterozoic)–Cambrian clastic sedimentary rocks and granitic intrusions in the NW Indian Himalaya. The SCB–NW India provenance linkage appears to have started from the Ediacaran. We propose that after breaking away from central Rodinia, the SCB collided with NW India during the Ediacaran–Ordovician time, causing the “Pan-African” Kurgiakh/Bhimphedian orogeny at the northern margin of India as well as the intraplate Wuyi-Yunkai orogeny (>460 Ma − 415 Ma) in South China. The Ediacaran–lower Paleozoic clastic sedimentary rocks in the Nanhua Basin are therefore interpreted to be foreland deposits formed during the collision of the SCB with Gondwanaland.