Neoproterozoic I-type and highly fractionated A-type granites in the Yili Block, Central Asian Orogenic Belt: Petrogenesis and tectonic implications

Abstract

The Yili Block in NW China and NE Kazakhstan is a continental fragment within the Central Asian Orogenic Belt (CAOB). We present a systematic study of whole-rock geochemistry, Sr–Nd–Hf isotopic compositions, and U–Pb geochronology of newly identified Neoproterozoic granitic plutons from the southern Yili Block to further constrain the Proterozoic evolution of microcontinents constituting the CAOB. Gneissic, augen, and mylonitized granites yield intrusion 206Pb/238U ages of 947 ± 4 Ma, 889 ± 5 Ma, and 892 ± 5 Ma, respectively. The gneissic granites display affinities to calc-alkaline, weakly peraluminous, magnesian I-type granites (Mg# = 33–34; FeOT/MgO = 3.49–3.59). The augen and mylonitized granites lie on the ferroan, calc-alkaline, highly fractionated A-type granite trend (Mg# = 11–21, FeOT/MgO = 6.52–14.58, SiO2 > 74 wt%, 10000*Ga/Al = 2.80–3.26). The markedly enriched Sr–Nd–Hf isotopic compositions of the ca. 947 Ma magnesian I-type granites suggest a derivation from ca. 2.0 Ga MgO-rich basement rocks. The varied initial 87Sr/86Sr ratios (0.716530–0.720543), chondrite-like εHf(t) values (−2.11 to 0.72), and differentiated incompatible elements of the ca. 890 Ma A-type granites suggest a derivation from partial melting of ca. 1.8 Ga crustal sources, followed by strong fractional crystallization. The Yili Block probably constituted part of an exterior orogen that developed along the margin of the Rodinia supercontinent during the early Neoproterozoic, undergoing a tectonic transition from syn-collisional to post-collisional extension at ca. 890 Ma. This study reveals that crustal reworking played a key role in Neoproterozoic crustal evolution in the Yili Block and that this block has a tectonic affinity to the Central Tianshan Block but is distinct from the Tarim Craton.