Petrology and geochemistry of garnet-bearing dioritic prophyry of east Kunlun: Implications for genesis of adakitic rocks

Abstract

This paper reports a garnet-bearing dioritic porphyry, which was first recognized in east Kunlun, NW China. The rocks formed in the late Triassic and consisted mainly of Ca-rich(CaO >5 wt%), Mn-poor (MnO < 3 wt%) garnet, Al-rich hornblende (> 15.9 wt%), andesitic plagioclase, quartz phenocrysts and matrix. The rock possesses intermediate SiO 2 (61.1 ̃ 62.2 wt%), low MgO(<2.0 wt%), K 2O(<1.3 wt%) and relatively high Al 2O 3(>17 wt%), showing metaluminous to slightly peraluminous characteristics(ACNK = 0.89 ̃ 1.05). On the aspect of trace elements, the rock is enriched in large ion lithosphile elements and light rare element elements and depleted in Nb-Ta-Ti, displaying typical features of subduction-related magma. The extremely low HREE (Yb < 0.8 × 10 -6) and relatively high Sr/Y ratios (̃ 38) indicate garnet was a residual phase, while relatively high Al 2O 3, mostly positive Eu-anomalies suggest accumulation of plagioclase in late stage of magma due to depressed crystallization by high water content. Nd-Sr isotopic composition(ε Nd(t)= -2.33 ̃ 1.38 ; 87Sr/ 86Sr = 0.706 5 ̃ 0.706 7) and reversed zoning of plagioclase imply an important role of magma mixing during the genesis of the rock. Garnet phenocrysts and ilmenite inclusions enclosed in garnet contain relatively low MgO, and hornblende enclosing garnet formed under relatively high pressure(8 ̃ 10 kb), suggesting that these minerals most likely crystallized from a silicic magma probably around the crust-mantle transition zone. Although the rock is very similar to adakites on various aspects, such as Al-rich, HREE-poor and high Sr/Y ratios, its intermediate Sr contents(<260 × 10 -6) and La/Yb ratios(16 ̃ 21) are strikingly lower than those of typical adakites and Archean TTG. Considering high partitioning parameters of Sr and LREE of apatite and the common occurrence in garnet, we propose that crystallization of apatite may effectively scavenge Sr and LREE from melt. This suggests that, even under high water fugacity, substantial crystallization of apatite during early stage of magmatic evolution is able to prevent some arc magmatism from evolving into adakitic rocks.