Late Mesozoic time constraints on tectonic changes of the Luanchuan Mo belt, East Qinling orogen, Central China

Two late Mesozoic granitoids in the Luanchuan area of the East Qinling orogen are considered; ore-bearing rocks are granite porphyries and granodiorite, with K2O>Na2O, appearing in the form of stocks. The Laojunshan rocks contains dominantly monzonitic granite, with K2O approximate to Na2O, in the form of a batholith. Both the ore-bearing rocks and the Laojunshan rocks are highly siliceous and shoshonitic, high-K calc-alkaline, similar to some l-type granites. Light rare earth elements (LREEs) are enriched in both rock suites, although the Luanchuan ore-bearing granitoids have higher concentrations, with (La/Vb)(N) ratios twice that of the barren Laojunshan granite suite. Ore-bearing rocks have, therefore, undergone greater fractionation of heavy rare earth elements. All Laojunshan rocks have negative Eu anomalies, indicating plagioclase fractionation. delta Eu values are different in both rock suites, the values in the ore-bearing granites, ranging from 0.52 to 1.04, which are much higher than that of Laojunshan batholith, ranging from 0.4 to 0.65. (La/Sm)(N) values of ore-bearing granites are 5.32-8.28, while that of Laojunshan batholith are 3.75-5.77, confirming the observation that the ore-bearing granites have undergone a higher degree of strong differentiation than that of Lanjunshan batholith. Major and trace element data, and REE data, combined with isotope data from previous work and the close relationships between the tectonic settings of the barren and ore-bearing rocks indicate that both groups of rocks were derived from the lower crust. At similar to 157 Ma, with the tectonic regime in transition from a syn-collisional to a post-collisional setting, highly fractionated granites ascended from their storage area via faults; at similar to 145 Ma, ore-bearing plutons, which are triggered by slab melts, formed at the junctions of fault planes trending WNW-ESE and NE-SW. At similar to 115 Ma, the tectonic regime changed from compression to extension; in this environment, the barren Laojunshan batholith was emplaced, representing the end of the collisional event. (C) 2012 Elsevier Ltd. All rights reserved.