Garnet-bearing tonalitic porphyry from East Kunlun, Northeast Tibetan Plateau: Implications for adakite and magmas from the MASH Zone

Abstract

A garnet-bearing tonalitic porphyry from the Achiq Kol area, northeast Tibetan Plateau has been dated by SHRIMP U-Pb zircon techniques and gives a Late Triassic age of 213 ± 3 Ma. The porphyry contains phenocrysts of Ca-rich, Mn-poor garnet (CaO > 5 wt%; MnO < wt%), Al-rich hornblende (Al 2O 3 ∼ 15.9 wt%), plagioclase and quartz, and pressure estimates for hornblende enclosing the garnet phenocrysts yield values of 8 - 10 kbar, indicating a minimum pressure for the garnet. The rock has SiO 2 of 60 - 63 wt%, low MgO (<2.0 wt%), K 2O (<1.3 wt%), but high Al 2O 3 (>17 wt%) contents, and is metaluminous to slightly peraluminous (ACNK = 0.89 - 1.05). The rock samples are enriched in LILE and LREE but depleted in Nb and Ti, showing typical features of subduction-related magmas. The relatively high Sr/Y (∼38) ratios and low HREE (Yb < 0.8 ppm) contents suggest that garnet is a residual phase, while suppressed crystallization of plagioclase and lack of negative Eu anomalies indicate a high water fugacity in the magma. Nd - Sr isotope compositions of the rock (εNd T = -1.38 to -2.33; 87Sr/ 86Sr i = 0.7065 - 0.7067) suggest that both mantle- and crust-derived materials were involved in the petrogenesis, which is consistent with the reverse compositional zoning of plagioclase, interpreted to indicate magma mixing. Both garnet phenocrysts and their ilmenite inclusions contain low MgO contents which, in combination with the oxygen isotope composition of garnet separates (+6.23‰), suggests that these minerals formed in a lower crust-derived felsic melt probably in the MASH zone. Although the rock samples are similar to adakitic rocks in many aspects, their moderate Sr contents (<260 ppm) and La/Yb ratios (mostly 16 - 21) are significantly lower than those of adakitic rocks. Because of high partition coefficients for Sr and LREE, fractionation of apatite at an early stage in the evolution of the magma may have effectively decreased both Sr and LREE in the residual melt. It is suggested that extensive crystallization of apatite as an early phase may prevent some arc magmas from evolving into adakitic rocks even under high water fugacity. © Springer-Verlag 2008.