Intermediate rocks in the Comei large igneous provinces produced by amphibole crystallization of tholeiitic basaltic magma

Abstract

The origin of intermediate rocks in large igneous provinces (LIPs) is unclear. We investigated this with an integrated petrological, mineralogical, geochronological, and geochemical study of the Chigu Tso tholeiitic intrusive suite (including dolerite, diorite, and tonalitic dykes and sills) in central Comei LIP, SE Tibet. The intermediate rocks (diorites and tonalites) within this suite were dated at c. 133 Ma, coeval with the c. 132 Ma Comei LIP. This suite exhibits a compositional continuum from dolerite through diorite to tonalite (SiO2 = 51.2–62.6 wt%) and similar Nd isotopic compositions, suggesting derivation from a common parental magma. Mass balance calculations of major elements suggest that the tholeiitic intermediate rocks with a high FeOt/MgO ratio (> 3.4) in Chigu Tso can be produced by large degrees (60–85 wt%) of differentiation of primary magmas with a low FeOt/MgO ratio (0.7). The P–T estimates based on clinopyroxene and amphibole compositions suggest a persistent mid-crustal magma chamber (11–19 km deep) beneath Chigu Tso that allowed for cooling (> 350 °C) and extensive magma differentiation (up to 85 wt% fractional crystallization). We attribute the generation of intermediate tholeiitic magmas to the fractional crystallization of a large amount of SiO2-poor and halogen-rich amphiboles at the late stage of differentiation. This process can also explain the generation of “granophyre” in large layered intrusions, based on their similar volatile-rich nature and high FeOt/MgO ratio.