北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代

超基性岩是苏鲁超高压变质地体中一类特殊且十分重要的岩石类型,它们通常呈规模不一的块状、条带状或不规则透镜状 (体) 赋存于区域大面积出露的花岗质片麻岩中。锆石中矿物包体激光拉曼测试、阴极发光图像分析和不同性质锆石LA-ICP-MS U-Pb定年等综合研究结果表明,北苏鲁威海地区含橄榄石辉石岩 (样品W1和W2) 中锆石的成因十分复杂,可进一步划分3种不同类型锆石。其中第一类锆石呈自形-半自形晶,阴极发光图像显示清晰的岩浆结晶环带,矿物包体主要为Ol+Cpx+Ap, 记录的²⁰⁷Pb/²⁰⁶Pb年龄为1835~1845Ma,应代表含橄榄石辉石岩的原岩形成时代;第二类为变质重结晶锆石,呈半自形-他形晶,阴极发光图像显示模糊的岩浆结晶环带,矿物包体与第一类完全一致,记录的²⁰⁶Pb/²³⁸U年龄变化范围大,为250~784Ma之间,表明部分继承性岩浆结晶锆石明显受到后期岩浆-变质热事件的影响而发生不完全重结晶和Pb丢失,进而使其记录的年龄相对偏新;第三类锆石呈他形晶,为典型的变质锆石,阴极发光图像十分均匀,矿物包体相对少见,主要为Grt+Cpx,记录的²⁰⁶Pb/²³⁸U年龄为230~234Ma, 且与苏鲁地体榴辉岩及其围岩中含柯石英锆石微区记录的超高压变质年龄 (225~235Ma) 十分一致,应代表含橄榄石辉石岩的峰期超高压变质时代。超基性岩中超高压变质锆石的准确识别表明苏鲁地体在峰期超高压变质阶段的确存在流体,流体的存在对超高压变质锆石的形成起着至关重要的作用。该项研究不仅准确厘定北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代,而且对于深入探讨苏鲁-大别超高压地体流体行为、演化规律及其水-岩相互作用机理具有重要的科学意义。

Protolith and ultranhigh-pressure (UHP) metamorphic ages of ultramafic rocks in Weihai area, North Sulu UHP terrane.

Ultramafic rocks as irregular lenses and blocks are commonly identified within gneissic rocks in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. A combined study of mineral inclusions, cathodoluminescence (CL) images, and in situ U-Pb LA-ICP-MS dates of zircons provided insight into the protolith and UHP metamorphic ages of these rocks. Zircon grains separated from olivine-bearing pyroxenite (samples W1 and W2) in Weihai area, North Sulu can be subdivided into three types: (1) inherited (magmatic) zircons with clear magmatic CL images and Ol+Cpx+Ap inclusions, which record Paleoproterozoic protolith ages of 1835~1845Ma; (2) some inherited zircons with blurry CL images and Ol+Cpx+Ap inclusions give a younger age group of 250~784Ma related to partial recrystallization and Pb loss during late magmatic and metamorphic events; and (3) metamorphic zircons with homogeneous CL images and Grt+Cpx inclusions that record UHP metamorphism at 230~234Ma, which are consistent with the UHP ages (235~225Ma) recorded by the coesite-bearing zircons from eclogite and its country-rocks in Sulu UHP terrane. Existence of UHP metamorphic zircons in ultramafic rocks indicates that the fluid may play an important role in the UHP metamorphic process of the Sulu terrane. The results from this study not only exactly constrain the protolith and UHP metamorphic ages of ultramafic rocks, but also show important significance for an in-depth insight into of the fluid property and evolution feature, fluid-rock interaction mechanism in the Sulu-Dabie UHP terrane.