Petrogenesis of the mineralized granitoids from the Kounrad and Borly porphyry Cu deposits and the East Kounrad porphyry Mo deposit in Kazakhstan: Implication for tectonic evolution and mineralization of the western part of the Central Asian Orogenic Belt

摘要

The Kounrad region, located in the western part of the Central Asian Orogenic Belt, hosts the Kounrad porphyry Cu, the Borly porphyry Cu-Mo, and the East Kounrad porphyry Mo deposits. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating indicates that the mineralized granitoids from the Kounrad, the Body and the East Kounrad deposits emplaced at 331.7 +/- 2.2 Ma, 311.6 +/- 2.6 Ma, and 295.4 +/- 2.9 Ma, respectively. The mineralized granodiorite porphyries at Kounrad show a geochemical affinity to adakitic rocks with high Sr (357-670 ppm), Sr/Y (40-68) and Mg numbers (Mg# = molar Mg/(Mg + Fe2+)) from 0.43 to 0.51, low Yb (0.97-1.1 ppm) and Y (8.3-11.1 ppm). They have variable Sr-Nd-Hf-O isotopic compositions ( (Sr-87/Sr-88)(i) = 0.7046 to 0.7051, epsilon Nd(t) = -0.1 to +1.1, epsilon Hf(t) = +5.2 to +9.0, delta O-18 = +5.7 to +6.8). These features indicate that the Kounrad adakitic magmas derived from the MASH (melting, assimilation, storage, homogenization) zone at depth of similar to 40 km with 5-15% ancient basement rocks contamination. The mineralized granodiorite porphyries at Body have a normal arc magma geochemical signature (e.g., enrichment of light rare earth elements (LREE) and depletion of heavy REE, Nb and Ti) and experienced fractional crystallization. They also have variable Sr-Nd-Hf-O isotopic compositions ((Sr-87/Sr-88)(i) = 0.7047 to 0.7053, epsilon Nd(t) = 0 to -1.3, epsilon Hf(t) = -0.6 to +7.4, delta O-18 = +5.5 to +6.7) and Mg# (0.45 to 0.51), indicating that they were generated by melting of juvenile basaltic lower crust with normal thickness, followed by 10-30% ancient crustal contamination. The East Kounrad mineralized intrusions, consisting of granite and leucogranite, have experienced advanced degrees of fractional crystallization and have similar Sr-Nd-Hf-O isotopic compositions ( (Sr-87/Sr-88)(i) = 0.7048, epsilon Nd (t) = +1.0 to +2.8, epsilon Hf(t) = +4.8 to +8.9, delta O-18 = +5.6 to +6.4) and low Mg# (0.18-0.37), indicating a juvenile lower crust source at depths of <27 km with 10-20% ancient crustal contamination.