| Subduction zones are the tectonic belts that enable the interaction at the interface of convergent plates.Subduction of oceanic slab provides a direct driving force for plate motion,which is closely related to the earthquake,volcanic eruption and mineralization in the subduction zone.The oceanic subduction zone is an important site for the material exchange between crust and mantle,where the oceanic crust materials were recycled back to mantle,contributing to the mantle heterogeneity.As a result,a combined study of petrology,geochronology and geochemistry was carried out for the high-pressure(HP)metamorphic rocks in oceanic subduction zone.The result can not only trace the metamorphic evolution but also probe the geochemical behavior of elements and isotopes as well as the fluid properties and activities during the subduction zone processes.The present study deals with the serpentinite melange of the Rió San Juan Complex(RSJC),Dominican Republic,which is a part of the exhumed forearc and accretionary prism of the Early Cretaceous to Paleogene Greater Antilles intra-oceanic subduction zone,representing the remains of oceanic subduction channel,and providing a natural ideal laboratory for studying metamorphic activity.The rocks in the northern RSJC are generally enriched in jadeite,and thus they were named as the jadeite-rich rocks.Because most jadeitites are thought to crystallize directly from high-pressure aqueous fluids,or require considerable chemical modification of any reasonable protolith via such fluids,they can provide important information on fluids and mass flux in the oceanic subduction zone.In this study,the detailed petrographic,chronological and geochemical study have been performed on jadeite-rich rocks from the northern RSJC.The results show that the zircons are euhedral with large grain size with obvious oscillatory zoning.They preserve rare mineral inclusions,mostly muscovite,sphene,and albite.SIMS dating of zircons on epoxy mount and LA-ICPMS dating of zircons on thin sections give relatively uniform U-Pb ages of 121-113 Ma.They have high Th/U ratios(>0.1),steep REE patterns with positive Ce anomalies(Ce/Ce*=44.7-167)and negative Eu anomalies(Eu/Eu*=0.71-0.99)except some domains.SIMS oxygen isotopic analysis of zircons on mount gives relatively uniform ?18O values of 5.0 to 6.8‰,which are mainly concentrated in the range of 5.0-5.8‰.Such values are similar to the O isotope composition of the mantle zircon values.All these observations indicate that these zircons are magmatic origin,which might be inherited from the igneous protolith of the jadeite-rich rocks,and the U-Pb ages of 121-113 Ma represent the formation ages of the protolith.Due to the absence of well-shaped jadeites with obvious oscillatory zoning and zircons that co-crystallized with jadeite,it is inferred that the jadeite-rich rocks in this area are metasomatic genesis(R-type).The petrological study on the eclogites and blueschists in the northern RSJC shows that the garnets have pronounced chemical compositional zoning of both the major and trace elements.Garnets from the eclogites have almandine-enriched cores(Alm57-58Prp14-16Grs20-22Sps4-5),grossular-enriched rims(Alm51-54Prp17-21Grs24-26Sps2-3),and the increases of grossularite content and Mg#from the core to rim,indicating that they were formed during the prograde stage with increasing temperature and pressure.The cores of garnets from the garnet-blueschists are enriched in spessartite(Alm56-58Prp4-5Grs28-30Sps4-6),whereas the rims are more enriched in grossular(Alm53-57Prp5-7Grs34-38Sps0.4-0.7).From the core to the rim,the contents of grossular and Mg#increase obviously,the spessartite contents decrease,representing that these garnets were formed during the prograde stage with increasing temperature and pressure.According to the eclogite pressuremeter provided by and The garnet-clinopyroxene exchange geothermobarometry by Nikitina&Simakov(1994)and Nakamura(2009)yields the metamorphic peak condition for eclogite of 753? and 27 kbar.The study of zircons from eclogites and albite-rich rocks in this region provides geochemical and geochronological data for the protolith origin of subducted HP metamorphic rocks.The petrographical observations show that the zircons have similar feature,which are euhedral,small grain size of about 100 ?m mostly,and most of them show oscillatory zoning or no obvious zoning.SIMS zircon dating gives a wide range of U-Pb ages of 2613-149 Ma.SIMS O isotope analysis indicates that ?18O values of zircon also have a wide range of 4-12‰ with mostly concentrated in 6-10‰,which are greater than ?18O values of mantle zircon.LA-ICPMS trace element analysis indicates that the trace element compositions of zircon have high Th/U ratios(>0.1)and steep REE distribution patterns,exhibiting obvious positive Ce anomalies(Ce/Ce*=3.50-244)and negative Eu anomalies(Eu/Eu*=0.11-0.79).Combine with previous studies of the northern RSJC,we proposed that these zircons are detrital zircons that derived from the surrounding continental crust,which is primarily in the Mexico and Columbia regions and deposited on subducted oceanic slabs prior to subduction. |
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