Geochemistry of the rare metal-bearing pegmatite No. 3 vein and related granites in the Keketuohai region, Altay Mountains, northwest China

Abstract

The chemical composition of a barren biotite granite, a two-mica granite, and a rare metal-bearing pegmatite (the No. 3 vein in Keketuohai, Altay Mountains, northwest China), along with mineral separates of apatite and mica, were analyzed and compared in the present work. Bulk chemistries of biotite granite, two-mica granite, and pegmatite rim are consistent with fractional crystallization trend from granites toward pegmatite. Changes in pegmatite composition can be explained mainly by fractional crystallization of biotite. Rb–Sr isochrons yield 248.8±7.5 Ma for the biotite granite, 247.8±6.3 Ma for the two-mica granite, and 218.4±5.8 Ma for the pegmatite rim. Similar geochemical characteristics, including initial ε Nd(T) values (ranging from −0.76 to −3.04) for apatites, similar initial ε Nd(T) values for whole-rock samples of studied granites and pegmatite (ranged from −1.40 to −3.21) and for biotites from granites (ranged from −2.75 to −3.15) suggest that these rocks were derived from a common magma source. Old continental crust may have played a significant role in magma generation. Based on these data, we interpret the evolution of the Keketuohai granite–pegmatite by magma differentiation from a common source. Biotite granite was the first to crystallize, followed by two-mica granites. The residual melt was probably stored in a granitic batholith. The pegmatite veins were injected ∼30 M.y. later during another episodic tectono-magmatic event in the Altay Mountains.