Fuquan Yang

The 401–409 Ma Xiaodonggou granitic intrusion: implications for understanding the Devonian Tectonics of the Northwest China Altai orogen

Abstract Palaeozoic granitoids in the Chinese Altai are important for understanding the evolution of the Central Asian Orogenic Belt (CAOB). The Xiaodonggou granitic intrusion, situated in the Chinese Altai (southern CAOB), is composed of two intrusive phases, medium-grained granite intruded by porphyritic granite. Zircon LA-ICP-MS U–Pb analyses of medium-grained granite and porphyritic granite yield ages of 409 ± 2 Ma and 400 ± 1 Ma, respectively, indicating that these formed in Early Devonian time. Medium-grained granite and porphyritic granite have similar geochemical features and Nd–Hf isotopic compositions. Arc-like geochemical characteristics (e.g. enrichment of LILEs and negative anomalies of Nb, Ta, Ti, and P) show that both phases are volcanic arc granites (VAGs). Geochemical and isotopic characteristics suggest that these magmas originated from melting older crust. Based on their near-zero or negative εNd(t) values (−1.4to 0) and positive εHf(t) values (+1.4 to +7.8), together with Nd model ages of 1.15–1.26 Ga and zircon Hf model ages of 0.90–1.30 Ga, we suggest that the Xiaodonggou granites were derived from a mixture of juvenile and old crustal components. Some other Devonian granitic intrusions were recently identified in the Chinese Altai with ages between 416 and 375 Ma. These Devonian granites have similar geochemical characteristics and petrogenesis as Xiaodonggou granites. The formation of these Devonian granites was in response to subduction processes, suggesting that Chinese Altai was an active continental margin in Early Devonian time.

Geology, genesis, and geodynamic setting of Cihai: an Early Permian diabase-hosted skarn iron deposit in the eastern Tianshan, Northwest China

ABSTRACT Extensive Permian mafic–ultramafic intrusions crop out within the eastern Tianshan, southern part of Central Asian Orogenic Belt (CAOB). Most of these mafic–ultramafic complexes are associated with Cu-Ni-Co deposits. However, Cihai, located in the southern part of the eastern Tianshan, is a large Fe deposit hosted in the Early Permian mafic rocks. The mafic to intermediate rocks are composed of gabbro, diabase, and monzodiorite. Geological and geochemical characteristics suggest that their parental magmas might have been generated by interaction between the depleted asthenospheric mantle and the metasomatized lithospheric mantle. Iron ores of the Cihai iron deposit are hosted in the diabase, and all Fe–Ti oxides in the ore-hosted diabase are ilmenite, instead of magnetite as previously reported. Chondrite-normalized REE patterns show that the magnetite separates from disseminated, banded, and massive iron ores, which are distinct from those in magmatic Fe-Ti deposits. Geological and chemical features suggest that the main ore bodies in the Cihai iron deposit are of hydrothermal origin, rather than magmatic as previously suggested. Numerous other Early Permian mafic rocks were recently identified in the Tarim basin and the eastern Tianshan with ages between 301 and 269 Ma. The mafic rocks in the Tarim basin exhibit characteristics of Oceanic Island Basalt (OIB), whereas the mafic rocks in the eastern Tianshan show island arc basalt (IAB) affinity. In addition, the presence of skarn iron deposit instead of Fe–Ti oxide deposit in the eastern Tianshan during the Early Permian time also lends little support for a plume-related environment. These features, together with a lack of verified anomalous high-temperature magmas in the eastern Tianshan, suggest that the Permian Tarim mantle plume may not account for the formation of the mafic rocks in the eastern Tianshan area, and that the Tarim LIP does not extend to the eastern Tianshan area.

Geochemistry and Sr–Nd–Hf isotopes of Middle Devonian igneous rocks of the Sarsuk polymetallic Au deposit: implications for understanding the tectonic evolution of the south Altay Orogenic Belt, Northwest China

ABSTRACT The medium-tonnage Sarsuk polymetallic Au deposit is located in the Devonian volcanic–sedimentary Ashele Basin of the south Altay Orogenic Belt (AOB), Northwest China. Within the deposit, the rhyolite porphyries and diabases are widespread, emplaced into strata. The orebodies are hosted by the rhyolite porphyries. We studied the petrography, geochemistry, and Sr–Nd–Hf isotopes of the rhyolite porphyries and diabases, in order to understand the petrogenesis of these rocks and their tectonic significance. They display typical bimodality in geochemistry compositions. The diabases are characterized by SiO2 contents of 44.84–59.77 wt.%, high Mg# values (43–69), enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in Nb and Ta, low (87Sr/86Sr)i (0.706687–0.707613) values, positive εNd(t) (4.8–6.8) values, and positive and high εHf(t) (7.15–15.19) values, suggesting a depleted lithosphere mantle source that might have been metasomatized by subduction-related components. The rhyolite porphyries show affinity to sanukitoid magmas contents [high SiO2 (78.6–81.82 wt.%) and MgO (3.38–5.94 wt.%, one sample at 0.61 wt.%), and enrichments in LILE and LREE], they were derived from the equilibrium reactions between a mantle source and subducted oceanic crust materials. Those characteristics together with the positive εNd(t) (4.1–8.4) and εHf(t) (2.88–15.17) values indicate that the diabases and rhyolite porphyries were generated from the same mantle peridotite source. But the rhyolite porphyries underwent fractional crystallization of Fe–Ti oxides, plagioclase, and apatite due to their negative Eu (δEu = 0.21–0.28) and P anomalies. According to the geochemical and isotopic data, the Sarsuk Middle Devonian igneous rocks are considered to be the products of the juvenile crustal growth in an island arc setting. The Sarsuk polymetallic Au deposit formed slightly later than the Ashele Cu–Zn deposit in the Ashele Basin, but they have the same tectonic setting, belonging to the trench–arc–basin system during extensional process in the south AOB.