Microgeochemistry Of Eclogites From The Sulu Ultrahigh-pressure Terrane And Its Geodynamic Implications

It has been accepted that microgeochemistry would lead the direction of modern geochemistry. Microanalysis could reveal the spatial variations of trace element and isotope compositons of single mineral grain at the micron scale. Thus, geochemistry and petrology investigations can be integrated at the micron scale to decipher the geodynamic process. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), recently well developed, provide effective tools for in situ, real time, quick and accurate microanalysis of trace element and isotope compositions. In this work, in situ trace element and isotope compositions of mineral in eclogites from the Sulu ultrahigh-pressure (UHP) terrane were analyzed by LA-ICP-MS and LA-MC-ICP-MS. Zircon U-Pb ages, trace element and Lu-Hf isotope compositions and trace element distribution and diffusion-based model calculation were combined to deciper the element mobility, fluid flow and thermal evolution during exhumation of the subducted continental crust, as summaried as following:(1) Trace element partitioning between metamorphic minerals is more sensitive than major elements to chemical and thermal changes of the eclogite, and trace element zoning of minerals could record complex metamorphic history. Trace element compositions of garnet, omphacite and apatite in UHP eclogites from the main hole of the Chinese Continental Scientific Drilling (CCSD-MH) project were in situ analyzed by LA-ICP-MS. Although both garnet and omphacite have homogeneous major element compositions, their trace elements show zonations from the core to the rim in rare earth elements (REE). In particular, middle rare earth elements (MREE) in garnet, heavy rare earth elements (HREE) in apatite and all REE in omphacite increase from the core to the rim, respectively. Combination of dependence of trace element partition and diffusion coefficients on temperature and pressure and REE zonations in these minerals indicates that the trace element zoning in these minerals could record a short-lived heating event during exhumation. Such hot exhumation could have contributed significantly to the overprint of granulite facies metamorphism on the eclogites and the widespread partial melting in the Sulu UHP terrane.(2) Sr isotope and trace elment zoning of apatite could be a well recorder of the different UHP metamosphic stage of eclogite. Trace element and Sr isotope compositions of an apatite grain (-2mm×6.5mm) in a strongly retrograded eclogite from the CCSD-MH were analyzed by LA-ICP-MS and LA-MC-ICP-MS. Both line-scanning and single spot analyses reveal that apatite is heterogeneous in trace element and Sr isotope compositions. Na, Sr, LREE, MREE, U, Th and Pb gradually decrease from the core to the rim. Remarkable secondary changes of Na, Sr, LREE and MREE occur in some local parts of the apatite. HREE are generally homogeneous in the main part of the apatite, but enriched in the outmost rim (-400 mm) close to garnet. The outmost rim (<200 mm) has the highest 87Sr/86Sr ratios. Combining the trace element partition and diffusion, and crystal-chemical characteristics of apatite, we suggest that the trace element variations reflect the complicated growth of apatite. The gradually decreasing from the core to the rim for Na, Sr, LREE, MREE, U, Th and Pb indicate apatite growth in the UHP metamorphism, while the secondary changes in local parts imply multi-stage dissolution and re-growth. The relatively homogeneous HREE compositions of the main part of the apatite suggest that apatite growth was mainly associated with the UHP metamorphism. HREE enrichments in the outmost rim of apatite could have resulted from garnet breakdown in amphibolite-facies retrograde metamorphism and/or HREE-releasing of garnet due to a short-lived heating during the exhumation. The high-87Sr/86Sr rim could indicate apatite growth coupled with phengite breakdown in the amphibolite-facies retrograde metamorphism.(3) In situ U-Pb dating and trace element analysis of zircons combined with a textural relationship investigation in thin section is a powerful tool to constrain the UHP stage of high-grade metamorphism. Two types of zircon grains have been identified in thin sections of a retrograde eclogite from the CCSD-MH in the Sulu UHP terrain. Type 1 zircon grains occur as inclusions in fresh garnet and omphacite, and Type 2 zircon grains were found in symplectite around omphacite. The fresh rims of Type 1 zircons and mantles of a few Type 2 zircons exhibit remarkably lower REE, Y, Nb and Ta contents than the inherited zircon cores, suggesting coeval growth with garnet, rutile and apatite during UHP metamorphism. These may have formed in the UHP metamorphism and survived retrograde metamorphism. The weighted average 206Pb/238U age of these zircon domains (230±4 Ma,2σ) agrees well with the published age of coesite-bearing zircon separates (230±1 Ma,2σ), suggesting that the peak UHP metamorphism in the Sulu terrain may have occurred at-230 Ma. Zircon domains surrounded or cut across by symplectite could have been altered by retrograde metamorphism. Together, they provide a younger weighted average 206Pb/238U age of 209±4 Ma (2σ). These retrograde zircon domains have similar REE compositions to the-230 Ma UHP zircon domains. These observations imply that the-209 Ma zircon domains could have formed by fluid activity-associated alternations in the amphibolite-facies metamorphism, which could have resulted in the complete loss of Pb but not REE in these domains.(4) Partial melting of the gneiss could have triggered the fluid flow in the continental subduction zone. In order to decipher the fluid activity in the Sulu UHP terrane in eastern China, quartz veins together with an adjacent eclogite lens and the host gneiss were studied. The deformed quartz vein is located at the boundary between the host gneiss and the eclogite lens. The amphibolite-facies metamorphic degree of the eclogite lens decreases from the rim to the core. The foliated gneiss contains felsic veins. Zircon rims from the gneiss are characterized by melt-related signatures in steep HREE patterns, high Hf contents and negative Eu anomalies and pool a weighted average 206Pb/238U age of 219±3 Ma (2σ), which is obviously younger than the peak metamorphic age of the Dabie-Sulu UHP terrane (-230 Ma). This suggests that the gneiss in the Sulu UHP terrane could have suffered from partial melting during the initial exhumation stage.The formation age of the quartz vein (219±2 Ma,2σ) defined by zircon rims agrees well with the partial melting time (219±3 Ma,2σ) of the host gneiss. The initial 176Hf/177Hf ratios of zircon rims from the quartz vein are obviously lower than zircons from eclogite lens, but overlap with the coeval zircon domains from the nearby granite dikes produced by partial melting of orthogneiss. These observations suggest that the quartz vein and corresponding fluid flow could be associated with partial melting of the host gneiss. On the other hand, amphibole-bearing and HREE-rich zircon rims from the amphibolite pool an amphibolite-facies metamorphic age of 217±5 Ma (2σ), overlaping with the formation age of the quartz vein. This implies that retrogression of the eclogite lens could have been caused by melting-induced fluid flow. Based on the above observations, we speculate that partial melting of the gneiss in the continental subduction-related UHP belt could have induced significant fluid flow during the exhumation stage, and thus contributed a lot to the extensive retrogression of eclogites in the Sulu UHP terrane.