矿床地球化学国家重点实验室知识库

Available cores of porphyritic granite and aplitic granite from the Diyanqinamu porphyry Mo deposit in the north central Great Xing’an Range presented an opportunity to examine and analyze Mesozoic igneous rocks far from the Paleo-Pacific subduction zone. The Diyanqinamu granites are highly fractionated I-type, distinguished from the M-, A- or S-type granite by: high SiO₂, and Rb; low Zr, Nb, Y, and Ce; low Fe₂O₃^total/MgO and (K₂O + Na₂O)/CaO ratios; low alumina saturation index (<1.1); low initial I_Sr ratios (0.70137–0.70451); positive εNd(t) values (2.37–3.77); and negative correlation between P₂O₅ and SiO₂. The aplitic granites were generated by fractional crystallization of the porphyritic granite, as evidenced by: spatial proximity; consistent zircon U–Pb ages (156 Ma) within error; correlations between other oxides and SiO₂ in Haker diagrams; low Ba, Sr, Nb, P, Ti, Eu; linear relationship in both (La/Yb)_N vs. La and Sr vs. Ba diagrams; and, decreasing LREE and ∑REE with increasing SiO₂. The Diyanqinamu granites have young depleted-mantle two-stage model ages (avg. T_DM2 = 660 Ma) similar to those of most Mesozoic voluminous felsic magmas in northeastern China, and were likely sourced from pre-existent crustal components both “old” and juvenile that had been juxtaposed during the tectonic evolution of the Paleo-Asian Ocean. These granites project in the transitional field from syn-collision to post-collision tectonic settings on tectonic discrimination diagrams, implying emplacement in an extensional environment. Extensional volcanism and basin formation in the Great Xing’an Range region in Late Jurassic is coeval with the Diyanqinamu granites, demonstrating that post-orogenic lithospheric extension related to the closure of the Mongol-Okhotsk Ocean was the main driving force for Late Jurassic magmatism in this region.