苏鲁超高压地体部分熔融时间的厘定:荣成花岗质片麻岩中浅色条带的锆石U-Pb定年、微量元素和Lu-Hf同位素证据

Partial melting time of the Sulu UHP terrane: Constraints from zircon U-Pb age,trace element and Lu-Hf isotope composition of leucosome in Rongcheng granitic gneiss

威海-荣成地区混合岩化花岗质片麻岩为研究超高压地体折返过程中的部分熔融提供了理想的窗口。本文通过对荣成地区鲍村花岗质片麻岩中浅色条带野外观察、岩相学以及锆石的CL图像、U-Pb定年、微量元素和Lu-Hf同位素的系统研究表明,鲍村花岗质片麻岩中的浅色条带为部分熔融的产物,其主要矿物组成为石英+斜长石+钾长石+黑云母。锆石CL图像显示清楚的核-边结构:继承岩浆核和新生岩浆边。继承核的206Pb/238U协和年龄为620±8～784±7Ma(Mean=701±33Ma);具有典型的岩浆锆石的微量元素特征,如重稀土(HREE)和Y富集,强烈的正Ce异常和负Eu异常,以及极低的(Gd/Lu)N和Hf/Y比值;εHf(t=700Ma)值为-13.0～-8.0(Mean=-10.9±0.8),相应的tDM2为2120～2429Ma(Mean=2303±51Ma)。新生边的206Pb/238U协和年龄为211±6～244±5Ma(226±10Ma);虽然微量元素的整体含量低于继承核,但也具有岩浆锆石的微量元素特征,如稀土元素配分模式显示HREE明显上翘的特征,正Ce异常和负Eu异常,以及极低的(Gd/Lu)N比值;εHf(t=225Ma)值为-17.3～-14.2(Mean=-15.2±0.6),相应的tDM2为2152～2347Ma(Mean=2214±33Ma)。上述特征表明,部分熔融的原岩是由古元古代(2.2～2.3Ga)的地壳物质在新元古代(701±33Ma)重熔而成,部分熔融可能发生在超高压地体折返早期的热折返阶段(榴辉岩相-高压麻粒岩相条件下),时代应为226±10Ma。威海-荣成混合岩化片麻岩内广泛发育的富含钾长石和石英的酸性岩脉(约220～210Ma),可能为折返晚期的角闪岩相冷凝阶段,部分熔融形成的熔体经历了结晶分异作用的产物。

In order to decipher the partial melting time of the Sulu ultrahigh pressure （UHP） terrane during exhumation, we carried out a combined study of zircon internal structure, U-Pb age, trace element and Lu-Hf isotopic compositions coupled with fieldwork on leucosome of the Baocun grantic gneiss in Rongcheng region. Our works suggest that the leucosome, mainly composed of quartz, plagioclase, K-feldspar, and biotite, was the product of partial melting. Zircon from the leueosome display core-rim structure in cathodoluminescence （CL） images： inherited magmatic core and new growth magmatic rim. Most age data of the inherited cores are concordant with apparent ^（206）pb/^（238）U ages ranging from 620± 8Ma to 784 ± 7Ma with a weighted mean age of 701 ±33Ma. The inherited cores have the characteristics of typical magmatic zircons, e. g. , high HREE and Y contents, strong HREE enrichment on chondrite- normalized REE plot, very large positive Ce and negative Eu anomalies, and very low （GclfLu）N and Hf/Y ratios. The inherited cores have ooltf（t =700Ma） values of - 13.0 to -8.0 （mean = - 10. 9 ±0. 8） and tDM2 of 2120Ma to 2429Ma （mean =2303 ±51Ma）. The new growth rims have concordant ^（206）pb/238U ages ranging from 211 ± 6Ma to 244 ± 5Ma with a weighted mean age of 226 ± 10Ma. Although the new growth rims are greatly different from the inherited cores in trace element compositions, they still display features of the magmatic zircon, e. g. , strong HREE enrichment on chondrite-normalized REE plot, large positive Ce and negative Eu anomalies, and very low （Gd/Lu） N and HffY ratios. The new growth rims have εHf（ t = 225Ma） values of - 17. 3 to - 14. 2 （ mean = - 15.2 ± 0. 6） and tDM2 between 2152Ma and 2347Ma （mean = 2214 ± 33Ma）. These results suggest that protolith of the leucosome are Mid- Neoproterozoic （701 ± 33Ma） magmatie rocks which were from Paleoproterozoic （2. 2 -2. 3Ga） continental crust. The partial melting is expected under eclogite-facies to granulite-facies conditions during the early stage hot exhumation of the Rongeheng UHP felsic gneisses, and the partial melting time might be 226 ± 10Ma. The K-feldspar- and quartz-rich pegmatitic veins （ca. 220 -210Ma） in the Weihai-Rongcheng region might be from fractional crystallization of the partial melts during late stage cool exhumation under amphibolite facies conditions.