Petrogenesis of the Sanshilipu gabbro complex in the Shangdan Suture: New constraints on evolution of the Qinling Orogen

Abstract

The Sanshilipu gabbro complex is located along the Shangdan Suture, which is important to understanding the evolution of the Qinling Orogen, Central China. The complex is mainly composed of norite, gabbro, and gabbro–diorite. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon dating yields a weighted mean 206Pb/238U age of 414 ± 2.8 Ma, interpreted to represent the crystallization age of the gabbro complex. The complex has SiO2 contents ranging from 48.6 to 51.3 wt% with enrichment of Na relative to K, showing calc-alkaline feature. The studied samples are characterized by an enrichment of large-ion lithophile elements (Ba, Sr, and Pb) and depletion of HFSE (Nb, Ta, Zr, Hf, and Ti) and a large range of Nb/Ta ratios (3.7 to 11.8). They also have early flattened-LREE distribution patterns. These geochemical characteristics indicate that the Sanshilipu gabbro complex was formed in an island-arc environment. They have slightly enriched Sr–Nd isotope compositions with (87Sr/86Sr)i = 0.7047–0.7063 and ɛNd(t) from −0.44 to −4.63, indicating that the formation of the complex was involved in some subducted sediments and oceanic crust and fluid released from the subducted slab. The identification of island-arc-originated mafic rocks suggests that the onset time of the collision between the Yangtze Craton and the North China Craton should be younger than 415 Ma.

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