Meso-Neoproterozoic Magmatic Events In The Shennongjia Region:Implications For The Amalgamation Evolution Of The South China Craton

The South China Craton (SCC) was formed during the Neoproterozoic through collision and welding of the Yangtze and Cathysia continental blocks along the Jiangnan orogenic belt. However, items of orogenic timing and its duration remain controversial, and documented works on pre-collision histories of the two blocks are also relatively scarce. Forming and evolution of the SCC has been thought to be connected with the assembly and breakup of Rodinia supercontinent, but due to diverse viewpoints, competing reconstruction schemes of Rodinia have been proposed for positioning the SCC. Among the works, most subjects are～860-750Ma igneous rocks formed within extensive setting. Thus, studies of pre-collision history of the individual continental blocks are critical to understand the forming and evolution of the SCC and its correlation with Rodinia.The Shennongjia Group is located in the northwestern Yangtze block. It consists mainly of carbonates interbedded with minor siliciclastic and volcanic strata. Though the Shennongjia Group has long been regarded as typical Mesoproterozoic strata, highly precision geochronological data are absent. This thesis is broadly divided into two parts. The firstpart reports a comprehensive study of zircon U-Pb geochronology, elemental and Sr-Nd isotope geochemistry of the Zhengjiaya volcanic suite, upper Shennongjia strata, and accordingly its activity timing, petrogenesis and tectonic setting are discussed; In the second part, comparison studies are made between detrital zircon U-Pb age spectra of the Shennongjia clastic rocks and geochronological frame of the Kongling high-grade crystalline basement rocks, and geochemical features of lithospheric mantle source rocks of the Neoproterozoic mafic dykes occurring in the Shennongjia and Kongling areas. Accordingly, tectonic evolution of the western Yangtze block during the late Meso-to early Neoproterozoic and its correlation with the final forming of the SCC are discussed. The main points of the thesis are listed below:1. The Zhengjiaya volcanic succession, top strata of the Shennongjia Group, comprises alkali-, calc-alkaline basalts and tholeiitic andesites, which is dated at1103±8Ma by the U-Pb zircon method. The alkali basalts have high TiO2(2.62-4.48wt%) and relatively low Mg#(42-55), and display OIB-like incompatible elemental patterns. They have initial Sr isotopic ratios of0.7021-0.7056and εNd(t) values of+2.4to+4.0. By contrast, the calc-alkaline basalts have high Mg#(57-68), Cr (67.1-211ppm) and Ni (62.8-121ppm), and exhibit a pronounced depletion in high field strength elements (HFSE)(e.g. Nb, Ta, P and Ti) and positive Pb anomalies, displaying lAB-like trace elemental features. Compared to the alkali basalts, they possess higher initial Sr isotopic ratios of0.7086-0.7120and large negative εNd(t) values of-15.3to-14.2. The andesitic rocks are characterized by negative Eu anomalies (Eu/Eu*=0.62-0.65) and depleted Nb, Ta, P and Ti anomalies along with small negative εNd(t) values (-2.7to-2.1) and low Sr isotopic ratios (0.7028-0.7042). It is indicated that the alkali basalts were derived from a depleted asthenospheric mantle source, the calc-alkaline basalts from previously metasomatised subcontinental lithospheric mantle rocks, whereas the andesitic rocks were derived from mafic lower crustal anatexis induced by basaltic underplating and/or asthenospheric ascent. Based on the geochemical features, the Shennongjia Grenvillian volcanic sequence is proposed to have developed within a continental-arc setting.2. The Taoyuan granitic gneiss is located in the northwestern Kongling High-grade Terrain. It has long been regarded as a Mesoproterozoic intrusion, but an U-Pb zircon age of2998±22Ma is obtained by this study using LA-ICPMS method. The Taoyuan gneiss suite is trondhemitic, and have high SiO2(67.80-74.93wt.%), Na2O (5.11-5.81wt.%) contents with Na2O/K2O ratios greater than unity, and low Yb (0.32-0.82ppm) and Y (4.48-11.5ppm) contents. It shows large variable La/Yb and Sr/Y ratios and pronounced depletion in Nb, Ta and Ti in the primitive mantle-normalized spiderdiagram. The gneiss suite displays two-stage Nd model ages close to its crystalline timing with corresponding εNd(t) values of-2.5to+3.5. It is thus suggested that the Taoyuan gneiss, in fact, is part of the Archean Kongling basement rocks.3. Based on documented works, a comparison study of geochronological framework and zircon Hf-O isotopes of the Kongling basement and detrital zircon U-Pb age spectra of the Shennongjia Group is carried out. The zircons from the Shennongjia strata show a strike population at～1.6Ga, and their Hf and O isotopes indicate a large scale mantle-input, which has not been recognized in the Kongling terrain. Coupled with elemental geochemistry and Nd isotope stratigrsaphy of the Shennongjia fine-grained sedimentary rocks, it is suggested that, during the Mesoproterozoic, provenance of the Shennongjia sedimentary deposits were not derived from the Kongling terrain through weathering and erosion. This thus reveals that the sedimentary basin of the Shennongjia strata was not develop along the western margin of the Yangtze nucleus during the late Mesoproterozoic.4. A comprehensive study of elemental and Sr-Nd isotope geochemistry is carried out on the Neoproterozoic diabase dyke intruded into the Taizi Formation of the Shennongjia Group. The dyke has a tholeiitic composition, featured by high MgO (7.92-9.57%), FeOr (13.01-15.12%), CaO (7.64-10.07%) and moderate Ni content (65.7-94.2ppm) and Mg#value (53-60). The samples from the dyke have relatively flat REE-and MORB-like trace element patterns with negligible HFSE negative anomalies. They possess highly radiogenic initial Sr isotopic ratios of0.7113-0.7140and εNd(t) values of+3.5to+5.7, which is likely indicative of carbonate contamination of the Shennongjia wall-rocks by AFC process. These geochemical features suggest that the diabase dyke formed at an intracontinental extension setting, and were derived from a moderately depleted source rocks of subcontinental lithospheric mantle. By contrast, former works of the Neoproterozoic mafic dykes occurring in the Kongling-Huangling area infer that they have large negative εNd(t=800Ma) values ranging from-11.2to-4.7, superchondritic Nb/Ta ratios (17.8-26.5), and IAB-like elemental patterns. They are accordingly suggested to have be derived from previously metasomatised mantle rocks of subcontinental lithosphere. The distinct geochemical features thus suggest that the Shennongjia and Kongling areas belonged to two divided subordinate continental block with diverse lithospheric mantle during the early Neoproterozoic.5. The above lines of evidence suggest that the Shennongjia area was a microcontinent independent of the continental nucleus of the Yangtze block during the late Mesoproterozoic to early Neoproterozoic. Accordingly, a three-stage model of tectonic evolution for the Shennongjia and Kongling continental segments is proposed as following:(1)>1100Ma, continental margin basin of the Shennongjia region received deposits with detritus derived from some unknown provenance, resulted in carbonate-dominated thich strata. The Shennongjia microblock was independent of the Kongling area, continental nucleus of the Yangtze block. By the end of the period, regional uplifting induced formation of the Zhengjiaya flysch sequence;(2)～1100Ma, the Zhengjiaya volcanic succession was produced by continental marginal arc volcanism;(3)-1000-900Ma, the Shennongjia microcontinent collided and welded with the Kongling high-grade terrain, indicated by of the contemporary ophiolite suite and molasse formation along the western Kongling margin. During the late period, orogenic collapse resulted in extensional tectonic setting and mafic igneous activities derived from lithospheric mantle in both areas.6. It is suggested that the amalgamation evolution of the SCC is more complex than the traditional Yangtze-Cathysia collision model. During the late Mesoproterozoic to early Neoproterozoic, the northwestern-and western segments of the Yangtze block likely comprised a collage of microcontinents and underwent westward lateral continental growth by subduction accretion and microcontinent growth by subduction-related collision.7. Given the～1.6Ga peak population of Shennongjia detrital zircons and its juvenile-crust signature for original host rocks, a linkage of provenance of the Shennongjia detrital sediments with the igneous activities in southern and mid-southern Australia is discussed. A western Yangtze-South Australia connection during Rodinia assembly is suggested. It thus provides a new insight into the position of the SCC in the supercontinent.