Protolith and ultranhigh-pressure (UHP) metamorphic ages of ultramafic rocks in Weihai area,North Sulu UHP terrane
8
Citation
0
Reference
20
Related Paper
Citation Trend
Abstract:
Ultramafic rocks as irregular lenses and blocks are commonly identified within gneissic rocks in the Sulu ultrahigh-pressure (UHP) metamorphic terrane,eastern China.A combined study of mineral inclusions,cathodoluminescence (CL) images,and in situ U-Pb LA-ICP-MS dates of zircons provided insight into the protolith and UHP metamorphic ages of these rocks.Zircon grains separated from olivine-bearing pyroxenite (samples W1 and W2) in Weihai area,North Sulu can be subdivided into three types:(1) inherited (magmatic) zircons with clear magmatic CL images and Ol+Cpx+Ap inclusions,which record Paleoproterozoic protolith ages of 1835~1845Ma;(2) some inherited zircons with blurry CL images and Ol+Cpx+Ap inclusions give a younger age group of 250~784Ma related to partial recrystallization and Pb loss during late magmatic and metamorphic events;and (3) metamorphic zircons with homogeneous CL images and Grt+Cpx inclusions that record UHP metamorphism at 230~234Ma,which are consistent with the UHP ages (235~225Ma) recorded by the coesite-bearing zircons from eclogite and its country-rocks in Sulu UHP terrane.Existence of UHP metamorphic zircons in ultramafic rocks indicates that the fluid may play an important role in the UHP metamorphic process of the Sulu terrane.The results from this study not only exactly constrain the protolith and UHP metamorphic ages of ultramafic rocks,but also show important significance for an in-depth insight into of the fluid property and evolution feature,fluid-rock interaction mechanism in the Sulu-Dabie UHP terrane.Keywords:
Protolith
Ultramafic rock
Coesite
Cite
Using the Laser Raman spectroscopy,cathodoluminescence method and electron microprobe analysis,the authors recognized coesite inclusions together with garnet, omphacite, phengite, magnesite, rutile and apatite inclusions in zircons from all amphibolite samples in pre pilot drillhole CCSD PP1 and CCSD PP2 as well as in outcrops of Sulu terrane. The mineral inclusion assemblage preserved in zircons is similar to that in the matrix of eclogites formed during the UHP metamorphism. These data indicate that amphibolites might have been formed by the retrogression of UHP eclogites in the uplifting of Sulu terrane. The research is of great significance in probing into the genetic mechanism of amphibolites and the fluid rock interaction during the tectonic uplift of Sulu terrane.
Omphacite
Coesite
Phengite
Cite
Citations (0)
Coesite
Protolith
Omphacite
Cite
Citations (139)
Metamorphic grade and ages of UHP eclogites and its country rocks are the key of understanding the formation mechanism of a UHP terrane.Coesite inclusions in zircons from paragneiss and some micro-exsolution textures in omphacite in eclogites imply that the eclogite and the paragneiss in the Yematan area,the North Qaidam all together experienced UHP metamorphism.Previous studies obtained metamorphic age of 458±7Ma for the eclogite,but the age of the paragneiss are different,which largely affect the discussion of formation mechanism of the Yematan UHP terrane.This paper focuses on the metamorphic age and geochemistry of paragneiss interbeds in the Yematan UHP eclogite.CL images of zircons from the paragneiss obviously show a core-rim internal texture and are of typical metamorphic detrital zircons.LA-ICP-MS U-Pb dating yield an older age of greater than 1000Ma for the formation of the protolith and a younger age of 458±6Ma for the peak metamorphism of the paragneiss.The metamorphic age of 458±6Ma is the same as that of the host UHP eclogite,which indicates that the eclogite and the paragneiss formed in the same tectonic event. The geochemical study suggests that the protolith of the paragneiss was chert and likely formed in a setting of the marginal basin. The consistent ages for both eclogites and country rock gneisses,together with their geological background,especially field occurrence, suggesting the in situ HP-UHP metamorphism,and likely resulted from deep continental subduction.
Protolith
Coesite
Omphacite
Cite
Citations (12)
Coesite
Omphacite
Cite
Citations (260)
Abstract Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircon from Sulu‐Dabie dolomitic marbles is characterized by distinctive domains of inherited (detrital), prograde, ultrahigh‐pressure (UHP) and retrograde metamorphic growths. The inherited zircon domains are dark‐luminescent in CL images and contain mineral inclusions of Qtz + Cal + Ap. The prograde metamorphic domains are white‐luminescent in CL images and preserve a quartz eclogite facies assemblage of Qtz + Dol + Grt + Omp + Phe + Ap, formed at 542–693 °C and 1.8–2.1 GPa. In contrast, the UHP metamorphic domains are grey‐luminescent in CL images, retain the UHP assemblage of Coe + Grt + Omp + Arg + Mgs + Ap, and record UHP conditions of 739–866 °C and >5.5 GPa. The outermost retrograde rims have dark‐luminescent CL images, and contain low‐ P minerals such as calcite, related to the regional amphibolite facies retrogression. Laser ablation ICP‐MS trace‐element data show striking difference between the inherited cores of mostly magmatic origin and zircon domains grown in response to prograde, UHP and retrograde metamorphism. SHRIMP U‐Pb dating on these zoned zircon identified four discrete 206 Pb/ 238 U age groups: 1823–503 Ma is recorded in the inherited (detrital) zircon derived from various Proterozoic protoliths, the prograde domains record the quartz eclogite facies metamorphism at 254–239 Ma, the UHP growth domains occurred at 238–230 Ma, and the late amphibolite facies retrogressive overprint in the outermost rims was restricted to 218–206 Ma. Thus, Proterozoic continental materials of the Yangtze craton were subducted to 55–60 km depth during the Early Triassic and recrystallized at quartz eclogite facies conditions. Then these metamorphic rocks were further subducted to depths of 165–175 km in the Middle Triassic and experienced UHP metamorphism, and finally these UHP metamorphic rocks were exhumed to mid‐crustal levels (about 30 km) in the Late Triassic and overprinted by regional amphibolite facies metamorphism. The subduction and exhumation rates deduced from the SHRIMP data and metamorphic P–T conditions are 9–10 km Myr −1 and 6.4 km Myr −1 , respectively, and these rapid subduction–exhumation rates may explain the obtained P–T–t path. Such a fast exhumation suggests that Sulu‐Dabie UHP rocks that returned towards crustal depths were driven by buoyant forces, caused as a consequence of slab breakoff at mantle depth.
Protolith
Cite
Citations (214)
Peridotite
Metasomatism
Enstatite
Geochronology
Cite
Citations (119)
The pre-pilot drillhole CCSD-PP1 of the Chinese Continental Scientific Drilling Project, with a depth of 432. 08 m, is located in the Donghai area in the southeastern Sulu terrane. The core samples are comprised mainly of paragneiss, granitic gneiss and ultramafic rock with minor intercalated layers of phengite-bearing kyanite quartzite and eclogite. All the analyzed paragneiss and granitic gneiss samples experienced amphibolite-facies retrogressive metamorphism. According to the whole-rock analyses, mineral inclusions in zircons and cathodoluminescence images, the granitic gneisses may be divided into two types. In the zircons in the first type of granitic gneiss samples form pre-pilot drillhole CCSD-PP1, many non-UHP mineral inclusions, comprised of Qtz + Phe + Ap, were identified from the core (C) through the mantle (M) to the rim (R) of zircon. All analyzed zircons show magmatogenetic features and are characterized by euhedral crystal shapes. The cathodoluminescence images show typical crystallization growth zoning from the core to the rim. These features indicate that the first type of granitic gneisses in pre-pilot drillhole CCSD-PP1 has not experienced UHP metamorphism. However, in the second type of granitic gneiss, a low-pressure mineral inclusion assemblage, mainly consisting of Qtz + Phe + Ab + Ksp + Ap, was identified in the core (C) of a zircon, while coesite or coesite-bearing UHP mineral inclusions were identified in the mantle (M) and rim (R) of the same zircon. These features suggest that the second type of granitic gneiss may have formed prior to UHP metamorphism and then together with paragneisses, phengite-bearing kyanite quartzite and eclogite experienced UHP metamorphism. This research is of great significance for an in-depth study of the subduction-exhumation mechanism and magmatism of Sulu UHP metamorphic belt and the selection of the drilling site for the Chinese Continental Scientific Drilling Project.
Coesite
Cite
Citations (0)
The Dabie-Sulu Triassic collisional orogen in eastern Asia was created by northward subduction of the Yangtze continental-crust capped plate beneath the Sino-Korean craton. Eclogites, garnet peridotites, and surrounding country rock gneisses and marbles were all subjected to in situ UHP metamorphism, as indicated by the presence of rare but widespread coesite inclusions in eclogitic minerals and in zircon crystals in the country rocks, as well as by virtually identical metamorphic ages of various UHP rock types. Metamorphic P-T estimates, combined with investigations of mineral exsolution textures and high-P polymorphs, indicate that recovered depths of continental subduction may have exceeded 200 km. Parageneses of mineral inclusions in zoned zircon domains combined with U-Pb ages delineate a well-constrained P-T-time path, suggesting exhumation rates of 5-10 km/Myr. A similar P-T-time trajectory has been established for the microdiamond-bearing Kokchetav Massif. Thus far, however, diamond inclusions have not been confirmed from coesite-bearing zircon domains of Dabie-Sulu UHP rocks despite numerous detailed studies. Oxygen isotopes of minerals from many outcrop samples and the Chinese Continental Scientific Drilling (CCSD) project main hole cores indicate that δ18O depletion took place in a volume of Proterozoic protoliths exceeding 100 000 km3 along the northern edge of the Yangtze craton. Evidently, passive-margin sediments and bimodal igneous rocks that had formed during rifting and breakup of the supercontinent Rodinia were subjected to extensive meteoric waterrock interactions attending terminal Neoproterozoic Snowball Earth conditions. Such hydrothermal alteration volatilized and depleted C from the relatively oxidized protoliths, accounting for the rare occurrences of graphite and apparent lack of microdiamond in Dabie-Sulu UHP rocks.
Coesite
Protolith
Rodinia
Geochronology
Cite
Citations (32)
Protolith
Massif
Geochronology
Geothermobarometry
Metamorphic core complex
Cite
Citations (184)
Protolith
Coesite
Omphacite
Phengite
Cite
Citations (18)