苏鲁仰口超高压岩石SHRIMP锆石U/Pb定年与部分熔融时限

在大型碰撞造山带中,在陆壳物质深俯冲或快速折返早期,在超高压-高压条件下,易熔组分可能发生水致或脱水部分熔融,形成花岗质熔体。在超高压-高压条件下,苏鲁超高压岩石发生过部分熔融作用,形成长英质多晶体包裹体和不同尺度的花岗质岩石, 导致可观的地球化学效应。为确定苏鲁超高压岩石部分熔融的时限,对山东仰口超高压副片麻岩和其中平行片麻理的同构造钾质花岗岩脉进行了SHRIMP锆石U/Pb地质年代学、全岩地球化学和锆石内矿物包裹体的研究。副片麻岩的锆石具有典型的核-幔-边结构。核部锆石为碎屑锆石,²⁰⁶Pb/²³⁸U年龄大于282Ma,可能反映了副片麻岩的原岩包含不同成因的物质;幔部和边部的Th/U比都小于0.1,分别给出233±3Ma和214±4Ma的²⁰⁶Pb/²³⁸U 年龄,分别对应于超高压变质和角闪岩相退变质年龄。同构造花岗岩脉是富钾过铝质花岗岩(A/CNK=1.2),锆石也具有核-幔-边结构;核部锆石年龄与副片麻岩的核部锆石年龄相当,反映了该花岗岩脉的源区可能是变沉积岩;除幔部锆石的一个点具有²⁰⁶Pb/²³⁸U年龄为234.6±3.9Ma之外,其它幔部锆石位于谐和线附近,给出²⁰⁶Pb/²³⁸U年龄为220.8±2.9Ma, 该年龄代表着该花岗岩脉的形成年龄。上述数据表明,在仰口地区,超高压岩石的部分熔融作用早于角闪岩相退变质作用。

SHRIMP zircon U/Pb dating on ultrahigh pressure rocks from Yangkou: Implications for the timing of partial melting in the Sulu UHP metamorphic belt.

Fertile components in UHP rocks could undergo partial melting at both water-present or -absent conditions during deep subduction as well as early stages of rapid exhumation of continental materials. Petrographic and geochemical observations had shown that the Sulu UHP rocks experienced partial melting at UHP-HP conditions, which led to the formation of quartzo-feldspathic inclusions, granitic veins or plutons, and substantial geochemical effects. To further constrain the timing of these partial melting events, we have carried out SHRIMP zircon U/Pb dating on UHP metapelite as well as concordant K-rich granitic dike from Yangkou, Shandong Province. Zircon separates from both samples have core-mantle-rim structure. Zircon cores from metapelite yield ²⁰⁶Pb/²³⁸U ages greater than 282Ma, which represents different crustal components incorporated into the protiolith of metapelite. The mantle and rim yield ²⁰⁶Pb/²³⁸U ages of 233±3Ma and 214±4Ma, representing the timing of UHP and amphibolite facies retrograde reactions, respectively. Zircon grains from the synkinematic peraluminous granitic dike (A/CNK=1.2) yield similarly wide range ²⁰⁶Pb/²³⁸U ages (483~800Ma) from the core, implying that it might be derived from metasediments like sample SYK08-08. Except for one spot from the mantle yields ²⁰⁶Pb/²³⁸U age of 234.6±3.9Ma, all the other spots cluster around a concordant ²⁰⁶Pb/²³⁸U age of 220.8±2.9Ma. These mantle domains also contain mineral inclusions of Pl+Kfs+Qtz+Ap, typical of granitic composition. Such an association indicates that the Sulu UHP metasediments underwent partial melting at ~221Ma and prior to the widespread amphibolite facies retrograde metamorphism.