Rongguo Zheng

Timing, petrogenesis, and setting of granites from the southern Beishan late Palaeozoic granitic belt, Northwest China and implications for their tectonic evolution

Late Palaeozoic granites are widely distributed in the southeastern Beishan area, which is located in the central part of the southern Central Asian Orogenic Belt (CAOB). U–Pb zircon dates of five late Palaeozoic granitic plutons from the southeastern Beishan area yield Permian ages: 285 ± 2 Ma (Shuwojing and Western Shuwojing plutons), 269 ± 3 Ma (Jianquanzi and Jiuquandihongshan plutons), and 260 ± 1 Ma (Jiujing pluton). The early Permian Shuwojing pluton, an unfractionated calc-alkaline biotite monzogranite with slightly positive εNd(t) (+0.7 and +0.6) and low initial 87Sr/86Sr (0.70722 and 0.70712), and the early Permian Western Shuwojing pluton, a high-K calc-alkaline biotite granite with slightly negative εNd(t) (−0.2 and −0.1) and low initial 87Sr/86Sr (0.70390 and 0.70919), are likely derived from a mixture of depleted (juvenile) mantle and upper continental crustal (or sedimentary) material. The mid-Permian Jianquanzi and Jiuquandihongshan monzogranites have highly fractionated potassium-rich calc-alkaline characteristics with negative εNd(t) (−4.3) and very high initial 87Sr/86Sr (0.71949), reflecting a continental crustal component. The compositionally diverse Jiujing pluton and the granodiorite and high-Sr monzogranite phases display adakite-like compositions with relatively low εNd(t) (−0.1 and −2.2) and high initial 87Sr/86Sr (0.70822 and 0.70913). The Jiujing low-Sr monzogranite has higher initial 87Sr/86Sr (0.73464) and lower εNd(t) (−2.8), indicating a significant continental crustal component in its genesis. This work, combined with the regional geology and previous studies, suggest that the early to middle Permian southern Beishan plutons formed in a post-collisional environment, probably an intracontinental rift environment linked to asthenospheric upwelling in response to the break-off of a subducted slab. In the late Permian, the southern Beishan area was in a compressive tectonic regime and thickening of the continental crust resulted in the formation of the Jiujing adakite-like granite.

Discovery of a Neoproterozoic granite in the Northern Alxa region, NW China: its age, petrogenesis and tectonic significance

A Neoproterozoic granite (Western Huhetaoergai granite) from the Northern Alxa region, southern Central Asia Orogenic Belt (CAOB) is first recognized by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon dating (889±8 Ma). It is a highly fractionated potassium-rich calc-alkaline pluton with low e Nd ( t ) (−2.6 to −1.1) and high ( 87 Sr/ 86 Sr) t (0.727305–0.735626), and is probably derived from a mantle source and assimilated crustal rocks with very high 87 Sr/ 86 Sr. Regional geology implies that it may reflect the existence of a microcontinent, and the formation of the Western Huhetaoergai granite is related to the assembly of Rodinia.

Age and petrogenesis of Mingshui-Shuangjingzi granites from the northern Beishan area, northwest China, and their implications for tectonic evolution

The Mingshui and Shuangjingzi granitic plutons are exposed in the northern Beishan area, in the southern Central Asia Orogenic Belt. U-Pb zircon dating indicates that Mingshui secondary intrusive lithosfacies and Shuangjingzi quartz monzodiorite formed at 328 ± 2 Ma and 272 ± 2 Ma, respectively. The former is composed of grey white monzogranites and potassic altered flesh red monzogranites. Geochemistry indicates they are calc-alkaline I-type granites. Shuangjingzi granites are mainly composed of unfractionated calc-alkaline I-type quartz monzodiorite, granodiorite and monzogranite. The Mingshui grey white monzogranites with negative initial εNd (–2.0 to–1.7) and moderate initial 87Sr/86Sr values (0.706740 to 0.710092) were probably derived from mixing of the depleted mantle (juvenile) component and the Palaeoproterozoic–Archean crust (or sedimentary) component and represent volcanic arc granites. The Shuangjingzi quartz monzodiorites are most likely derived from mafic to intermediate igneous protolith and represent post-collisional granites. This work, combined with the regional geology, suggests the Palaeo-Asian Ocean closed between 328 Ma and 272 Ma in the northern Beishan area.