苏鲁超高压岩石部分熔融时间的准确限定:来自含黑云母花岗岩中锆石U-Pb定年、REE和Lu-Hf同位素的证据

Partial melting time of ultrahigh-pressure metamorphic rocks in the Sulu UHP terrane: Constrained by zircon U-Pb ages, trace elements and Lu-Hf isotope compositions of biotite-bearing granite

在北苏鲁超高压变质带中,广泛分布强变形的新三叠纪含黑云母花岗岩和伟晶岩脉。锆石中矿物包体激光拉曼测试、阴极发光图像分析、不同性质锆石微区LA-(MC)-ICP-MS和SHRIMP U-Pb定年、REE及Lu-Hf同位素测试等综合研究结果表明,北苏鲁威海地区超高压正片麻岩在构造折返的(高压)麻粒岩相升温减压阶段,发生部分熔融(深熔)作用形成了花岗质岩浆,并在临近角闪岩相退变质作用之前结晶结束形成了含黑云母的花岗岩。该类含黑云母花岗岩中的锆石成因复杂,可划分为三类锆石微区。第一类为强发光效应(白色)继承性岩浆锆石(微区Ⅰ),具有典型的岩浆结晶环带,矿物包体为Otz+Kfs+Ap,记录的^(206)Pb/^(238)U年龄为790～782Ma;第二类为新生锆石微区(微区Ⅱ),发光强度相对较弱(灰色-灰白色),也具有较明显的岩浆结晶环带,矿物包体为Qtz+Kfs+Ab+Ap,记录的^(206)Pb/^(238)U年龄为222～217Ma,加权平均年龄为219±2Ma,表明苏鲁超高压地体的部分熔融(深熔)作用发生在新三叠纪,这组年龄比苏鲁地体超高压变质时代(235～225Ma)明显偏新,指示部分熔融(深熔)作用的时代要晚于苏鲁地体的超高压变质时代;第三类锆石微区(微区Ⅲ)围绕第二类锆石微区分布,发光强度最弱(黑色),也具有典型的岩浆结晶环带,矿物包体十分少见,为Qtz+Ap,记录的^(206)Pb/^(238)U年龄集中于216～209Ma之间,加权平均年龄为214±2Ma,应代表部分熔融(深熔)而成的岩浆结晶结束的年龄,这组年龄比苏鲁地体构造折返晚期角闪岩相退变质时代(210～200Ma)偏老,表明新生岩浆结晶结束的时间要早于角闪岩相退变质时代。继承性岩浆结晶锆石(微区Ⅰ)^(176)Hf/^(177)Hf(t)=0.281975～0.281984,^(176)Lu/^(177)Hf=0.00196～0.00221,ε_(Hf)(t)=-11.6～-11.8,相应的t_(DM2)=2160～2170Ma,与研究区周围新元古代(795～730Ma)正片麻岩继承性岩浆结晶锆石的Lu-Hf同位素特征十分相似,表明新元古代正片麻岩是新三叠纪部分熔融(深熔)成因的含黑云母花岗岩的母岩。新三叠纪新生岩浆结晶锆石的核部(微区Ⅱ)和边部(微区Ⅲ)具有类似的^(176)Hf/^(177)Hf(t)、^(176)Lu/^(177)Hf比值和ε_(Hf)(t)值,^(176)Hf/^(177)Hf(t)=0.282110～0.282199,^(176)Lu/^(177)Hf=0.00041～0.00183,ε_(Hf)(t)=-15.8～-19.1,t_(DM2)=1980～2130Ma,表明新三叠纪由超高压正片麻岩部分熔融而成的岩浆自结晶开始到结束是在一个相对封闭体系条件下完成的。晚期角闪岩相退变质作用对其Lu-Hf同位素体系也未造成破坏。

Strongly deformed biotite-bearing granite and pegmatitic dikes are widely distributed in the northern part of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. In this study we link mineral inclusion data, cathodoluminescence (CL) imaging, trace element analyses, U-Pb age and Lu-Hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin of biotie-bearing granite formed by partial melting of Neoproterozoic orthogneiss during Late Triassic high-pressure (HP) granulite-faices stage. The zircons separated from biotite-bearing granite can be subdivided into three distinct zircon domains. The inherited magmatic zircon cores (domain I) are white-luminescent in CL image and contain mineral inclusions of Qtz + Kfs + Ap. In contrast, the newly magmatic zircon cores and mantles (domain II) are grey-luminescent in CL image and preserve a mineral inclusion assemblage of Qtz + Kfs + Ab + Ap. the newly magmatic zircon rims (domain III) are dark-luminescent in CL images, retain a few mineral inclusions of Qtz + Ap. LA-(MC)-ICP-MS and SHRIMP U-Pb dating on these zoned zircons identify three discrete (206)Pb/(238)U age groups: (1) 790 similar to 782Ma is recorded in the inherited magmatic zircon cores (domain I) derived from Neoproterozoic granitic protolith; (2) the newly magmatic zircon cores and mantles (domain II) record the starting time for partial melting of Proterozoic granitic protolith at 222 similar to 217Ma with a weighted mean of 219 +/- 2Ma, which are uniformly younger than UHP metamorphic ages of 235 similar to 225Ma as dated by the SHRIMP U-Pb method for coesite-bearing domains of zircons from various UHP metamorphic rocks in the Sulu UHP terrane; and (3) the crystallization for the granitic magma overprint in the newly zircon outermost rims(domain III) ended at 216 similar to 209Ma with a weighted mean of 214 +/- 2Ma, suggesting biotite-bearing granite crystallization prior to the regional amphibolite-facies retrogression (210 similar to 200 Ma). The inherited magmatic zircon cores (domain I) show (176)Lu/(177)Hf ratios of 0.00196 similar to 0.00221, (176)Hf/(177)Hf(t) of 0.281975 similar to 0.281984, corresponding to epsilon(Hf)(t) of -11.6 to -11.8 and t(DM2) ages of 2.16 to 2.17Ga, which are consistent with the Lu-Hf isotopic characteristics of the inherited magmatic zircons from UHP orthogneiss in the neighboring area, indicating that Late Triassic biotite-hearing granite in north Sulu UHP terrane was derived from the partial melting of Neoproterozoic orthogneiss. In contrast, the different zircon domains (domains II and III) from the newly magmatic zircons record similar Lu-Hf isotopes with (176)Lu/(177)Hf ratios of 0.00041 similar to 0.00183, (176)Hf/(177)Hf(t) of 0.282110 similar to 0.282199, corresponding to epsilon(Hf)(t) of -15.8 to -19.1 and t(DM2) ages of 1.98 to 2.13 Ga. The similarity between domain II and III indicates that late amphibolite-facies retrogression did not significantly modify chemical and isotopic composition of the partial melting system.