Yangtze Block In China Research Articles

Enrichment of Platinum Group Elements in Lower Cambrian Polymetallic Black Shale, SE Yangtze Block, China

Platinum group elements (PGEs) occur mainly in basic–ultrabasic igneous rocks and are concentrated by the differentiation and crystallization of magma. Thin polymetallic layers including Ni, Mo, V, PGEs, and rare-earth elements are widely developed in lower Cambrian black shales in southern China. The PGE contents in such layers may not be economically significant but are still unusually enriched. PGE enrichment mechanisms have not been widely studied, but here the PGE compositions of polymetallic layers in the Shuidong (Nayong) and Niuchang (Weng’an) sections of the SE Yangtze block in China were determined, and results compared with published data for the region. Results indicate that PGEs are enriched in the polymetallic layers, whereas the surrounding country rocks are barren. The ΣPGE contents in the Shuidong Ni–Mo deposits are generally lower than in the Niuchang V deposits. PGE distribution patterns in the polymetallic layers are similar to those in basic–ultrabasic rocks, indicating that such rocks played a role in the PGE enrichment. Oceanic hypoxia during the Sinian–Cambrian transition resulted in the production of large amounts of organic matter and H2S in the ocean. When high-salinity brine reached the ocean bottom, rapid changes in Eh–pH conditions caused enrichment of metals at the sediment–seawater interface, and this enrichment was later enhanced during diagenesis.

Geochemical constraints on the provenance and depositional setting of Neoproterozoic volcaniclastic rocks on the northern margin of the Yangtze Block, China: Implications for the tectonic evolution of the northern margin of the Yangtze Block

Neoproterozoic volcanic and sedimentary rocks, cropping out along the Chengkou–Fengxi Fault on the northern margin of the Yangtze Block in China, consist primarily of volcaniclastic deposits and sandstone–conglomerate successions. The volcaniclastic deposits (termed the Longtanhe Formation) are composed of volcanogenic sedimentary rocks with several primary volcaniclastic rock interlayers. The Longtanhe Formation provides a good opportunity to investigate the Neoproterozoic tectonic evolution of the northern margin of the Yangtze Block. In this paper, geochemistry and zircon U–Pb geochronology are used to infer the provenance and tectonic setting of the Longtanhe Formation volcaniclastic rocks. Two volcanic sandstone samples, collected from the base and top part of the Longtanhe Formation respectively, and one tuff sample were studied for zircon U–Pb dating. They show three consistent age populations with peaks at ∼825Ma, ∼770Ma and 703Ma, indicating that the Longtanhe Formation was deposited after 703Ma and there were no changes in the sources during the period of sedimentation. Chondrite-normalized REE patterns are uniform with slight LREE enrichment (LaN/YbN, average 3.8) and flat HREE patterns without a distinct Eu anomaly (Eu/Eu*, average 0.99), indicating a magmatic arc source. Sc–V enrichment, high La/Th (average 9.4), Cr/Ni (average 2.1) and low Th/Sc (commonly <0.1) suggest that the volcaniclastic rocks were mainly derived from basic to intermediate rocks. Large volumes of basaltic to andesitic clasts in volcaniclastic rocks also suggests that the source rocks were predominantly mafic rocks. A few samples have higher Th/Sc (0.12–0.24) ratios and Hf (4.70–4.90) contents implying minor felsic rocks in the source area. A small amount of quartz clasts in matrix for these six samples also supports a minor contribution from felsic rocks. Not only the lapilli in tuff but also the pebbles and lithic grains in volcanic sandstones are characterized by poor rounding, indicating that the Longtanhe Formation was deposited near-source. Both major- and trace-element discrimination diagrams suggest an oceanic island arc-related setting for the Longtanhe Formation. All these lines of evidence suggest that a contemporaneous (703Ma) magmatic arc, primarily consisting of basalt to andesite, was the main source for the Longtanhe Formation volcaniclastic rocks. The older and minor felsic components were probably derived from mafic to felsic rocks with an arc signature that cropped out widely in the Hannan Complex (840–746Ma) and/or Yaolinghe Group (845730 Ma). Based on its arc-related geochemical affinity, the Longtanhe Formation is proposed to have been deposited in an oceanic arc-related setting, indicating that oceanic crust subduction beneath the northern margin of the Yangtze Block had not ended before 703Ma.

The Late Mesozoic tectonic evolution and magmatic history of west Zhejiang, SE China: implications for regional metallogeny

The granitoids and related polymetallic mineralization in the Zhejiang Province at the southeast margin of the Yangtze Block in China provide an important window to evaluate metallogeny associated with convergent margin magmatism. Here, we present geochronological, geochemical, and isotopic data from the granitic rocks of west Zhejiang, to constrain the timing of transformation of the tectonic setting of this region from volcanic arc to intra-plate during Late Mesozoic and its bearing on regional metallogeny. The granitic rocks in west Zhejiang can be geochemically subdivided into two groups. The first group is characterized by relatively steep rare earth element (REE) patterns with slight Eu anomalies, high Sr, low Yb, and negative Nb–Ta–Ti (NTT) anomalies, indicating a volcanic arc environment with a thickened crust in a convergent setting. The second group is featured by flat REE patterns with prominent negative Eu anomalies, low Sr, high Yb, and weak NTT anomalies, suggesting an intra-plate extensional environment with a thin crust. The geochronology of granitic rocks in west Zhejiang, combined with ages of regional tectonic basins and nappe structures, constrains the timing of the tectonic transformation to be in the range from 150 to 140 Ma. Sr–Nd isotopic data and a positive correlation displayed by oxygen fugacity (fO2), and La/Sm and Ba/Th ratios (proxies of subducted sediments and slab dehydration fluids) suggest that the high oxygen fugacity is probably related to the melting of subducted sediments and slab dehydration. From 180 to 80 Ma, due to the increasing dip angle of the subducted Izanagi Plate, the volcanic arc belt migrated oceanward, leaving most of the interior of Zhejiang Province under an intra-plate environment where insufficient subducted components and upwelling mantle generated reduced magmas which were not favorable for Cu–Mo mineralization. Our model provides a plausible explanation for the absence of Cu–Mo porphyry deposits in the adjacent region of Zhejiang, Jiangxi, and Anhui provinces (Zhe-Gan-Wan region) after 140 Ma.

Continental flood basalts of the Huashan Group, northern margin of the Yangtze block – implications for the breakup of Rodinia

It is generally accepted that Neoproterozoic extension and dispersal of the supercontinent Rodinia was associated with mantle plume or superplume activities. However, plume-generated contemporaneous continental flood basalts (CFBs) have rarely been identified. In this study, we present geochronological and geochemical evidence for the basalts from the Liufangzui Formation of the Huashan Group in the Dahongshan region of east-central China. A representative sample yields a SHRIMP U–Pb zircon age of 824 ± 9 Ma, interpreted as the crystallization age of the rocks. Geochemically, these basalts belong to the subalkaline tholeiite series and display slight enrichments in light rare earth elements (LREE) and varying degrees of deficiency of high field strength elements (HFSE) such as Nb, Ta, and Ti. This pattern is very similar to that of CFBs from the Bikou Group and Tiechuanshan Formation in the northwestern Yangtze block in China and Siberia in Russia. The basaltic magmas underwent partially-fractional crystallization during ascent, but were not intensely influenced by crustal contamination. The characteristic element ratios and negative Hf isotopic analyses (ϵHf(t) = −6.6–2.6) in zircons indicate that the parental magmas of the basalts might have been derived from an enriched lithospheric mantle rather than from the depleted mantle such as normal mid-ocean ridge basalts (N-MORBs). The geochemical signatures and regional geological characteristics show that these basalts were formed along intraplate continental rifts rather than in island arcs or ocean basins. Considering the coeval basic volcanic rocks in South China, we propose that these Huashan Group basalts represent the remnants of plume-generated CFBs and have close spatiotemporal ties with a coeval basic igneous province in Australia. Our results support the Neoproterozoic location of the South China block adjacent to southeastern Australia in the reconstruction model of the supercontinent Rodinia.

Neoproterozoic plutonic rocks from the western Gyeonggi massif, South Korea: Implications for the amalgamation and break-up of the Rodinia supercontinent

We report sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon ages, and whole-rock chemical and isotopic (Sr–Nd) compositions from tonalite–trondhjemite–granodiorite (TTG) and alkaline plutons in and around the “Hongseong-Imjingang suture” along the western margin of the Gyeonggi massif, South Korea. The Neoproterozoic TTG and the alkali plutons from the Hongseong area of the Gyeonggi massif yield zircon U–Pb ages of ca. 841–822 Ma and ca. 751–746 Ma, respectively. Based on the geochemical and Nd–Sr isotope data on these plutons, together with the previously reported SHRIMP age of ca. 900 Ma from a tonalite, TTG plutons of ca. 900–820 Ma are interpreted as the products of arc-related magmatism. Their geochemical signatures include enrichment in LREE, negative Nb, Ta and Ti anomalies and relatively low ɛNd(T) values between −0.5 and +5.0. The TTG plutons are coeval with those recently identified in and around the Yangtze block in China. In contrast, the alkali plutons from the Hongseong area, together with the previously reported ones around the Imjingang belt, represent anorogenic-type magmas that can be attributed to the disruption of Rodinia. They display significant negative Eu anomalies, and are strongly depleted in Sr, P and Ti. This transition from arc setting at ca. 900–820 Ma to continental rift setting at ca. 750–740 Ma suggests a regional tectonic link for the amalgamation and disruption of Rodinia in Northeast Asia.