A mafic intrusion of 'arc affinity' in a post-orogenic extensional setting: A case study from Ganluogou gabbro in the northern Yidun Arc Belt, eastern Tibetan Plateau

摘要

Generally, arc-related or subduction-related mafic magmas are formed during or slightly postdate sub-duction, and characterized by depletion in high field strength elements (HFSEs) relative to the large ion lithophile elements (LILEs) and light rare-earth elements (LREEs). Combining with mineral chemistry and Sr-Nd isotopes, these geochemical characteristics were usually used to define an arc setting, especially for some ancient arcs that had been strongly modified by later tectonic activities. However, we report an exceptional case from the northern part of the Triassic Yidun Arc Belt, eastern Tibetan Plateau. The Ganluogou gabbro (～152 Ma) occurs as several intrusive bodies. Its mineral assemblage is olivine (chrysolite), plagioclase (anorthite), clinopyroxene (diopside), amphibole (edenite and pargasite) and phlogopite. Whole rock geochemistry shows low SiO_2 (42.87-46.99 wt.%), total rear earth elements (∑REE = 22.8-28.4 ppm), Na_2O + K_2O (0.92-1.34 wt.%), and high Al_2O_3, MgO and FeO contents. It has small variations of initial ~(87)Sr/~(86)Sr ((~(87)Sr/~(86)Sr)_i = 0.7053-0.7055) and ε_(Nd)(t) values (-4.8 to -1.8). All the samples exhibit enrichment in LILEs including Th and U, but strongly depleted in HFSEs, including Nb, Ta, Zr and Hf. For the mineral chemistry, there are two type amphiboles. Amp(Ⅰ) show higher V, Sc, Cr, Sr, Nb and Zr contents, but lower Th and U contents than those of Amp(Ⅱ). Their REE patterns range from convex shape without Eu anomaly to LREE-enriched pattern with weak positive Eu anomaly. We suggest that Amp(Ⅰ) was crystallized from a liquid that was mainly buffered by olivine, clinopyroxene and plagioclase, while Amp(Ⅱ) crystallized from later melt that was mainly buffered by olivine. Based on clinopyroxene chemistry, compositions of coexisting olivine and plagioclase, and whole rock Sr-Nd isotopes, the parental magma of the Ganluogou gabbro is interpreted as a tholeiitic arc-affinity magma, which might be derived from an N-MORB mantle that had been metasomatised by slab-derived melts in the late Triassic (237-206 Ma). Thus, the Ganluogou gabbro provides an example that magmas exhibiting arc-affinity could in fact be formed in a post-orogenic extensional setting.