Late Devonian–Early Permian A-type granites in the southern Altay Range, Northwest China: Petrogenesis and implications for tectonic setting of “A 2-type” granites

Abstract

The Altay Range is an important part of the Central Asian Orogenic Belt (CAOB) that contains A-type granites that have been attributed to a post-collisional setting during the Late Carboniferous–Permian. However, our new LA-ICP-MS zircon U–Pb age data demonstrates that there were two episodes of A-type granite magmatism in the southern Altay Range. A Late Carboniferous to Early Permian (308–291 Ma) suite does occur in the vicinity of Qiakuerte but a distinct Late Devonian (382–367 Ma) suite is present in the Kouan area. The early Kouan A-type granites are mainly composed of alkali-feldspar granites, which are approximately synchronous with adakites, boninites, high-Ti basalts, picrites, ophiolitic rocks, and high temperature-low pressure metamorphic rocks, whereas the late Qiakuerte A-type granites mainly consist of arfvedsonite and aegirine-bearing granites, which are approximately coeval with some mafic–ultramafic rocks in the southern Altay Range. A-type granites are commonly classified as A 1 and A 2 sub-types, which are considered to be generated in anorogenic (rift or plume-related) or post-collisional settings, respectively. Both suites of the southern Altay Range are geochemically similar to typical A 2-type granites, e.g., high K 2O + Na 2O, FeO/MgO, and Ga/Al, and Y/Nb values, and low CaO, Ba, Sr, and Eu contents. The Kouan A-type granites have relatively low (La/Yb) N (2.7–5.9), high ε Nd( t) (+6.7 to +7.7) and Nb/La (0.59–1.67), and variable ε Hf( t) (+5.0 to +14.1) values, whereas the Qiakuerte A-type granites have comparatively high (La/Yb) N (4.2–9.2), low ε Nd( t) (+5.3 to +6.0) and Nb/La (0.33–0.74), and variable ε Hf( t) (+10.0 to +18.7) values. The early Kouan A-type granites may have been generated by partial melting of a mafic source containing more depleted mantle-derived components than the late Qiakuerte A-type granites. We suggest that the Late Carboniferous to Early Permian Qiakuerte granites were formed in a post-collisional extensional setting, as is typical of A 2-type granites. However, the Late Devonian Kouan A-type granites were more plausibly generated in an extensional setting as a result of slab window caused by a ridge subduction. In this case, the upwelling of asthenosphere through the slab window provided the source of parental magmas, or the heat for the generation of Devonian magmas. Therefore, our results suggest that A 2-type granites can also form in a ridge subduction-related extensional setting.