Late Triassic porphyritic intrusions and associated volcanic rocks from the Shangri-La region, Yidun Terrane, eastern Tibetan Plateau; adakitic magmatism and porphyry copper mineralization.

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• 责任者：Wang Baiqiu ; Zhou Meifu ; Li Jianwei ; Yan Danping
• 年：2011
• 作者单位：University of Hong Kong, Department of Earth Sciences, Hong Kong, China.
• 刊：Lithos Oslo.
• 期：127,1-2

摘要

Early Mesozoic porphyritic intrusions in the Shangri-La region, southern Yidun terrane, SW China, are spatially associated with andesites and dacites. These intrusions are composed of diorite and quartz diorite, and are closely related to copper mineralization. LA-ICP-MS zircon U-Pb ages of the intrusions range from 230 to 215 Ma. The associated andesites and dacites are interlayered with slates and sandstones and have ages of around 220 Ma. All of the intrusive and extrusive rocks have similar, highly fractionated REE patterns and high La/Yb (13-49) ratios with no prominent Eu anomalies. They display pronounced negative Nb-Ta and Ti anomalies on primitive mantle-normalized spidergrams. Their SiO (sub 2) contents range from 56.6 to 67.1 wt.%, Al (sub 2) O (sub 3) from 14.2 to 17.4 wt.% and MgO from 1.9 to 4.2 wt.%. All the rocks have high Sr (258-1980 ppm), and low Y (13-21 ppm) with high Sr/Y ratios (29-102). These features suggest that both the volcanic rocks and porphyritic intrusions were derived from adakitic magmas. They have similar initial (super 87) Sr/ (super 86) Sr ratios (0.7058 to 0.7077) and epsilon (sub Nd) (-1.88 to -4.93) values, but belong to high silica (HSA) and low silica adakitic rocks (LSA). The HSA represent an early stage of magmatism (230 to 215 Ma) and were derived from oceanic slab melts with limited interaction with the overlying mantle wedge during ascent. At 215 Ma, more extensive interaction produced the LSA. We propose that the early adakitic magmas (HSA) formed by flat subduction leading to melting of oceanic slab, whereas subsequent slab break-off caused the significant interaction between slab melts and the mantle wedge and thus the generation of the later adakitic magmas (LSA).