塔里木东北缘坡北基性-超基性岩体形成时代及岩浆演化

坡北基性-超基性岩体位于塔里木板块东北缘，出露面积约180 km2，岩浆分异充分，主要岩性包括橄榄岩、橄榄辉长岩、辉长苏长岩、辉长岩等。锆石U-Pb测年结果显示，辉长岩结晶年龄为277.1±2.2 Ma，辉长苏长岩结晶年龄为273.5±2.9 Ma，橄榄辉长岩结晶年龄为275.5±1.5 Ma，含斜长石橄榄岩结晶年龄为280±2 Ma，不同类型岩石形成年龄集中在270~280 Ma，与塔里木地幔柱在时间和空间上具有一定耦合性。
矿物地球化学数据显示，纯橄榄岩中橄榄石Fo(镁铁比值)值最高，达到90.9%；单斜辉石成分在不同类型的岩石中相似，MgO和CaO含量连续变化，且呈负相关关系，指示其具有相似的原始岩浆成分；不同类型岩石的微量元素蛛网图与稀土元素配分图变化一致，表明其为同源岩浆演化；Sr-Nd同位素地球化学图解显示，坡北岩体源自亏损地幔，经历了4%~15%下地壳物质的混染。综合研究认为，坡北基性-超基性杂岩体不同岩相具有过渡或侵入接触关系，形成时代相近，具有同源岩浆演化特征，但是岩浆就位顺序不同，地壳混染程度存在差异。

Magmatic evolution and formation age of the Pobei mafic-ultramafic intrusion on the northeastern margin of the Tarim plate

The Pobei mafic-ultramafic intrusion is located on the northeastern margin of the Tarim plate and has an outcrop area of 180 km2. The magma differentiation was sufficient with the main lithologies consisting of such rocks as peridotite, olivine gabbro, gabbro norite, and gabbro, during the formation of the Pobei intrusion. Zircon U-Pb geochronology indicates that crystallization ages of gabbro, gabbro norite, olivine gabbro and plagioclase-bearing peridotite are 277.1±2.2 Ma, 273.5±2.9 Ma, 275.5±1.
5 Ma and 280±2 Ma, respectively. The ages for the different types of rocks are concentrated at 270~280 Ma, which are temporally and spatially consistent with the age of the Tarim mantle plume. Mineral geochemical data suggest that olivine from the dunite has the highest Fo values, with maximum at 90.9%; clinopyroxene has similar concentrations in different types of rocks, with consistent negative relations between MgO and CaO, indicating similar primitive magma components. Moreover, these different rocks have similar variations for their trace and rare earth elements in spider pattern diagrams, suggesting that these rocks evolved from the same parental magma. Sr-Nd isotopic data indicate that the Pobei intrusion was sourced from depleted mantle and contaminated by 4%~15% lower crustal materials during magma ascent. Combined with published data and data obtained from this study, the authors hold that different lithofacies of the Pobei mafic-ultramafic intrusive complex have transitional or intrusive contact relation, similar formation ages and co-magmatic evolution characteristics, but they also have different emplacement sequences and degrees of crustal contamination.