Late Paleozoic adakites and Nb-enriched basalts from northern Xinjiang, northwest China: evidence for the southward subduction of the Paleo-Asian Oceanic Plate

Abstract Volcanic rocks consisting of adakite and Nb-enriched basalt are found in the early Devonian Tuoranggekuduke Group in the northern margin of the KazakhstanJunggar Plate, northern Xinjiang, northwest China. The geochemical characteristics of the andesitic and dacitic rocks in this area resemble that of adakites. The relatively high Al 2 O 3 , Na 2 O and MgO content and Mg # values indicate that the adakites were generated in relation to oceanic slab subduction rather than the partial melting of basaltic crust. A slightly higher SrI and a lower e Nd (t = 375 Ma) compared to adakites of mid-oceanic ridge basalt (MORB) imply that slab sediments were incorporated into these adakites during slab melting. The Nb-enriched basalt lavas, which are intercalated in adakite lava suite, are silica saturated and are distinguished from the typical arc basalts by their higher Nb and Ti content (high field strength element enrichment). They are derived from the partial melting of the slab melt-metasomatized mantle wedge peridotite. Apparently, positive Sr anomalies and a slightly higher heavy rare earth element content in these adakites compared to their Cenozoic counterparts indicate that the geothermal gradient in the PaleoAsian Oceanic subduction zone and the depth of the Paleo-Asian Oceanic slab melting are between those of their Archean and Cenozoic counterparts. The distribution of the adakites and Nb-enriched basalts in the northern margin of the Kazakhstan-Junggar Plate, northern Xinjiang, indicates that the Paleo-Asian Oceanic Plate subducted southward beneath the Kazakhstan-Junggar Plate in the early Devonian period

Petrogenesis of the Late Archean (similar to 2.5 Ga) Na-and K-rich granitoids in the Zhongtiao-Wangwu region and its tectonic significance for the crustal evolution of the North China Craton

Petrogenesis of coexisting Late Archean (similar to 2.5 Ga) Na-and K-rich granitoids in the Zhongtiao-Wangwu region is significant for understanding the crustal evolution of the North China Craton (NCC). In this paper, we present new zircon U-Pb age, and whole-rock geochemical and isotope data on a similar to 2.5 Ga Na-and K-rich granitoid suite from the Zhongtiao-Wangwu region, southern NCC. The Na-rich granitoids (TTG) and K-rich granitoids (A-type granites) are zircon U-Pb dated to be 2.45 Ga and 2.49 Ga, respectively. Integrated geochemical data indicate that the TTG were formed by partial melting of the mafic lower crust, while the A-type granites were formed by partial melting of a TTG-like source in the shallower crust. The petrogenesis likely occurred under lithospheric delamination and the subsequent asthenospheric upwelling and mafic magma underplating. We proposed that the coeval occurrence of Na-and K-rich granitoids in the southern NCC is probably ascribed to the partial melting of continental crust at different depths, which could be an important way to induce the Archean crustal reworking and compositional changes at similar to 2.5 Ga

Multi-stage Cu remobilization of the Huping metamorphic-hydrothermal deposit in the southern North China Craton

The Zhongtiao region is a well-endowed Cu metallogenic province in the southern North China Craton and contains a large number of early Precambrian Cu deposits of different styles, most of which have been subjected to the ca. 1.85 Ga regional metamorphism. However, whether there was any role for the Proterozoic metamorphism on the extensive Cu mineralization is yet to be well understood. This paper presents an integrated investigation on the field geology, mineral chemistry, fluid inclusion and C-H-O-S isotopes of the Huping metamorphic-hydrothermal deposit in the Neoarchean-Paleoproterozoic Zhongtiao metamorphic terranes, and reveals multi-stage Cu remobilization that formed and upgraded the Cu deposits in the region. We identified three stages of syn- to post-metamorphic Cu remobilization/transportation in the Huping Cu deposit, including (I) early mechanical transport, (II) intermediate metamorphic-hydrothermal remobilization and (III) late meteoric fluid incursion. In stage I, sulfides were remobilized and transported internally under lower-greenschist facies metamorphism, forming the veinlet-disseminated ores and chalcopyrite (avg. delta S-34 = 2.1 parts per thousand, similar to the chlorite-amphibole schist wall rocks). In stage II, the immiscibility of metamorphic fluids (avg. delta O-18 = 4.6 parts per thousand and SD = -56.6 parts per thousand) from the metamorphic terranes triggered the precipitation of higher delta S-34 chalcopyrite (avg. 8.0 parts per thousand), led by the incorporation of extra sulfur during the fluid migration. In stage III, incursion of meteoric fluids into the waning metamorphic-hydrothermal system formed the barren quartz-calcite veins (avg. delta O-18 = 0.09 parts per thousand and delta C-13 = -0.88 parts per thousand). During these Cu remobilization processes, the earlier-formed sulfides were chemically stable during the mechanical transfer by regional metamorphism and deformation. Subsequently, the immiscibility of metamorphic fluids induced significant Cu remobilization and precipitation in the metamorphic-hydrothermal system. We thus propose a model of multi-stage mechanical and chemical Cu remobilization for the Huping Cu deposit, which highlights the crucial impacts of metamorphic-remobilization processes on the regional Cu mineralization in the Zhongtiao region

Trace element characteristics of partial melts produced by melting of metabasalts at high pressures: Constraints on the formation condition of adakitic melts

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Rare earth element enrichment in sedimentary phosphorites formed during the Precambrian–Cambrian transition, Southwest China

Numerous sedimentary phosphorites in Southwest China were formed around the Precambrian–Cambrian transition (PC–C), including the upper Ediacaran Doushantuo Formation and lower Cambrian Gezhongwu Formation. The Gezhongwu phosphorites in Zhijin exhibit marked rare earth element (REE) enrichment (>1000 ppm), and may represent new REE resources. Although the main characteristics of the Gezhongwu phosphorites have been well constrained, the REE enrichment mechanisms remain unclear. We undertook a comparative study of three typical sedimentary phosphorites with variable REE contents formed at the PC–C transition in central Guizhou Province, Southwest China. These include sections A and B of the Doushantuo phosphorites (560 ± 8 Ma) from the Weng’an area (i.e., WA-A and WA-B), and the Gezhongwu phosphorites (527 ± 24 Ma) from the Zhijin area (ZJ). The phosphorites were investigated with state-of-the-art macroscale to nanoscale analytical techniques. In contrast to the extraordinary REE enrichment in the ZJ phosphorites (average ΣREE = 1157 ppm), the phosphorites in WA-A (average ΣREE = 234 ppm) and WA-B (average ΣREE = 114 ppm) are REE-poor. Elemental mapping by laser ablation–inductively coupled plasma–mass spectrometry, along with transmission electron microscopy analyses, showed the REEs in the studied phosphorites are hosted in nanoscale francolites. The 87Sr/86Sr and Y/Ho ratios of the francolite grains indicate that greater terrigenous input may have led to more REE enrichment in the WA-A than WA-B phosphorites, but this cannot explain the extraordinary REE enrichment in the ZJ phosphorites. The F/P2O5 values of the francolite grains in the ZJ phosphorites (∼0.097) are higher than those in the WA-A (∼0.084) and WA-B (∼0.084) phosphorites, and the grain size of the francolite in the ZJ phosphorites (∼89.9 nm) is larger than those in the WA-A (∼56.6 nm) and WA-B (∼57.4 nm) phosphorites, indicative of more intense reworking of the ZJ than WA phosphorites during early diagenesis. A plot of Nd concentration versus Ce/Ce* reveals that lower sedimentation rates characterized the ZJ phosphorites. Therefore, intense sedimentary reworking during early diagenesis resulted in more REEs being sequestered by the marine phosphates from seawater and pore waters at a lower sedimentation rate, which resulted in the extraordinary REE enrichment in the ZJ phosphorites. Our findings highlight the multiple factors that controlled formation of sedimentary phosphorites around the PC–C transition (especially the intense reworking and redox conditions of the overlying seawater), and provide further insights into REE enrichment in sedimentary phosphorites worldwide