新疆布鲁斯台辉长岩岩石学特征及对南天山洋俯冲时限的制约

Petrological characteristics of Bulusitai gabbro and its constraint to the time of South Tianshan ocean subduction

布鲁斯台辉长岩位于中天山南缘缝合带中段古洛沟蛇绿岩以北30km,是该带上出露的为数不多的较大的基性侵入岩体。辉长岩主要由斜长石(35%~45%)、单斜辉石(20%~25%)和角闪石(30%~35%)组成,角闪石主要为交代辉石后的产物。岩石化学成分显示,辉长岩具有高Al2O3(15.43%~23.20%)和CaO(12.96%~14.48%),低碱K2O(0.13%~0.19%)、Na2O(1.47%~3.00%)、P2O5(0.03%~0.07%)和TiO2(0.34%~0.61%)的特征。辉长岩稀土元素球粒陨石标准化配分模式与E-MORB相似,但ΣREE低(12.91×10-6~23.14×10-6),微量元素具有Ba、U、K、Sr等LILE富集和Nb、Ta、Zr、Hf、Th等HFSE亏损的特点。锆石原位εHf(t)值为+9.7^+15.0。上述特征反映辉长岩形成的构造背景为活动大陆边缘,源区为受俯冲流体影响的亏损地幔橄榄岩,且在上升的过程中未遭受地壳的明显混染。La、Sm、Yb判别图解显示岩石可能是亏损的尖晶石二辉橄榄岩发生5%~10%的部分熔融的产物。锆石LA-ICP-MS测年表明,辉长岩形成年龄为316.8±2.1Ma(MSWD=3.4,n=36)。结合区域地质、年代学和地球化学的资料,我们提出至少在晚石炭世,南天山洋还存在洋内和洋陆俯冲事件,南天山洋闭合的时间可能在更晚的早二叠世,而不是志留纪或者早泥盆世。

The Bulusitai gabbro, which is located at the middle of southern margin of central Tianshan, is about 30km away from Guluogu ophiolite to the south, and is a relatively large and rare mafic intrusion outcropping at the suture. The gabbro consists mainly of plagioclase （35% ～ 45% ）, clinopyroxene （20% ～ 25% ）, amphibole （ 30% ～ 35% ） from metasomatised clinopyroxene. The gabbrc is characterized by its high content of A12 03 （ 15.43% ～ 23.20% ） and CaO （2. 96% ～ 14.48% ）, low content of alkali （ K20 =0. 13% ～ 0. 19%, Na20 = 1.47% ～ 3.00% ）, P205 （0. 03% ～ 0. 07% ） and TiO2 （0. 34% ～ 0. 61% ）. The REE distribution patterns are quite similar to that of the E-MORB with low ∑REE content （ 12. 91 × 10^-6 ～ 23.14 × 10^-6 ）. The LILE such as Ba, U, K, Sr is enriched, while the HFSE such as Nb, Ta, Zr, Hf, Th is depleted, and zircon 8af （t） = ＋ 9. 7 ～ ＋ 15. 0. All of these indicate that the parental magma of gabbro derived probably from depleted peridotite influenced by subduction fluid with the tectonic environment of active continental margin, and with no or negligible crustal contamination. The La, Sm, Yb diagram shows that gabbro perhaps originated from 5% ～ 10% partial melting of depleted spinel lherzolite influenced by subduction fluid. U-Pb dating of zircons from the gabbro yielded ages of 316. 8 ± 2. 1 （MSWD = 3.4, n = 36）. Combining with regional geology, geochronological and geochemical data, we propose that both intra-ocean subduction and ocean-continent subduction still existed in the South Tianshan ocean at Late Carboniferous and the closing of South Tianshan ocean occurred at Early Permian, which is apparently different from most viewpoints that closing of South Tianshan ocean occurred at Silurian or Devonian.