Sulu Terrane Research Articles

Petrofabrics and seismic properties of garnet peridotite from the UHP Sulu terrane (China): Implications for olivine deformation mechanism in a cold and dry subducting continental slab

Lattice-preferred orientations (LPO) of olivine, diopside, enstatite and garnet from the Zhimafang garnet peridotite body in the Sulu ultrahigh-pressure (UHP) metamorphic terrane (China) were measured using the electron backscatter diffraction (EBSD) technique. The peridotite was captured from a mantle wedge immediately adjacent the subducted Yangtze slab and then experienced the UHP metamorphism at 750–950 °C and 4–7 GPa. The olivine LPO is characterized by the [001] axis close to the stretching lineation and the (100) plane subparallel to the foliation, indicating the prevailing of (100) [001] slip. Enstatite LPO displays the dominance of (100) [001] slip. Diopside developed complex LPO patterns that are difficult to explain using a single slip system of (100) [001]. Garnet is almost randomly oriented due to its low volume fractions, cubic symmetry and the presence of numerous slip systems. Calculated seismic properties of the peridotite yield a maximum P-wave velocity normal to the foliation and a minimum along the foliation, with anisotropy up to 8% in strongly sheared samples. The S-wave velocity pattern is complex but the fast polarization plane generally normal to the foliation. The inferred shear sense from the olivine LPO is top-to-SE, in contrary to exhumation-induced top-to-NW thrusting recorded in the quartz LPO, implying that the olivine LPO formed at early UHP metamorphic conditions. The olivine crystals have relatively low water contents (141–475 H/10 6 Si), indicating a fluid-deficient environment for the LPO formation. The present study suggests that a combination of low temperature and UHP plays a much more important role than the water content to promote the activation of (100) [001] slip in olivine.

Ca-rich Garnet–Clinopyroxene Rocks at Hujialin in the Su–Lu Terrane (Eastern China): Deeply Subducted Arc Cumulates?

Layers of Ca-rich garnet–clinopyroxene rocks enclosed in a serpentinite body at Hujialin, in the Su–Lu terrane of eastern China, preserve igneous textures, relict spinel in garnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite, Fe-rich spinel, and also amphibole in clinopyroxene. In terms of their major and trace element compositions, the studied samples form a trend from arc cumulates towards Fe–Ti gabbros. Reconstructed augite compositions plot on the trend for clinopyroxene in arc cumulates. These data suggest that the rocks crystallized from mantle-derived magmas differentiated to various extents beneath an arc. The Ca-rich garnet þ diopside assemblage is inferred to have formed by compressing Ca-rich augite, whereas the relatively Mg-rich cores of garnet porphyroblasts may have formed at the expense of spinel. The protolith cumulates were subducted from near the crust–mantle boundary (c. 1 GPa) deep into the upper mantle (4 8 0 6 GPa and 750 50 C). Negatively sloped P–T paths for the garnet– clinopyroxene rocks and the corollary of corner flow induced subduction of mantle wedge peridotite are not supported by the available data. Cooling with, or without, decompression of the cumulates after the igneous stage probably occurred prior to deep subduction.

Petrogenesis of UHP Metamorphic Crustal and Mantle Rocks from the Chinese Continental Scientific Drilling Pre-pilot Hole 1, Sulu Belt, Eastern China

The Pre-pilot Hole No. 1 of the Chinese Continental Scientific Drilling Project (CCSD-PPH1) in the southern Sulu terrane recovered a continuous core of eclogite, garnet peridotite, orthogneiss, paragneiss, and minor schist and quartzite. Geochemical characteristics indicate that the garnet peridotite was derived from depleted mantle; all other rock types are metamorphosed supracrustal rocks with continental affinities. The eclogite consists of garnet, omphacite, phengite, quartz (coesite), amphibole, rutile, and zircon; P-T estimates of peak metamorphism are 785-820°C and >2.7 to 3.7 GPa. The gneisses show common amphibolite-facies mineral assemblages consisting of plagioclase, K-feldspar, muscovite, and quartz, with minor garnet, epidote (or zoisite), and biotite; coesite inclusions in zircon indicate that the gneisses together with eclogites were subjected to an early UHP metamorphism prior to amphibolite-facies retrogression. The Garnet peridotites show porphyroblastic textures, and consist mainly of garnet, clinopyroxene, orthopyroxene, and olivine with minor phlogopite. Garnet and clinopyroxene porphyroblasts show significant compositional zoning. Applying relevant geothermobarometers, core compositions of the minerals indicate P-T conditions of 6.0-7.0 GPa and 1100-1200°C. For the rim, similar or slightly higher P-T conditions compared to the eclogites were obtained. We suggest that the former represent crystallization conditions of garnet peridotite in the upper mantle, whereas the latter reflect reequilibrium conditions during incorporation of mantle rocks into the subducted slab. We conclude that the garnet peridotite may have been derived from the mantle wedge above the subduction zone. If so, these mantle-derived rocks were sandwiched between continental-derived country rocks.

Multiphase solid inclusions in UHP rocks (Su-Lu, China): Remnants of supercritical silicate-rich aqueous fluids released during continental subduction

Primary multiphase solid (MS) inclusions without preserved fluid are found within peak minerals in kyanite quartzite ± topaz and kyanite–phengite–epidote eclogite from Donghai area (Su-Lu terrane). Typical mineral association in inclusions is: paragonite + muscovite + anhydrite ± corundum ± “alunite-type” sulphate ± zircon ± calcite ± chlorite ± SiO 2 ± barite ± pyrite ± apatite in quartzites, and paragonite + rutile + apatite ± amphibole ± Zn-staurolite ± magnetite ± plagioclase ± zircon ± pyrite ± “alunite-type” sulphate ± Zn–Mg–Fe–Al–Ti spinel in eclogites. On the basis of the fluid inclusion textures and of the daughter-phase assemblage, calculated fluid composition is as follows: in quartzites 24 wt.% SiO 2, 30 wt.% Al 2O 3, 9 wt.% CaO, 5 wt.% K 2O, 3 wt.% Na 2O, 11 wt.% SO 3, 18 wt.% H 2O, with traces of TiO 2, Fe 2O 3, FeO, MgO, BaO, P 2O 5, Cl −, F, and (CO 3) 2−, and in eclogites 26 wt.% SiO 2, 21 wt.% TiO 2, 20 wt.% Al 2O 3, 2 wt.% MgO, 4 wt.% FeO, 6 wt.% Fe 2O 3, 7 wt.% CaO, 3 wt.% Na 2O, 3 wt.% P 2O 5, 7 wt.% H 2O, 1 wt.% Cl, and traces of ZnO, MnO, K 2O, SO 3, and F. Originally trapped chloride-poor aqueous fluids contained very high amounts – in the order of tens of wt.% – of Si 4+, Al 3+, and Ti 4+. Solute species reflect the chemical composition of the host rocks: Mg 2+, Fe 2+, Ti 4+, P 5+, and Na +, are abundant in the fluids present in eclogites, while fluids are enriched in Al 3+, K +, Na +, (SO 4) 2−, (CO 3) 2− in quartzites. We propose that multiphase solid inclusions represent remnants of high-density supercritical silicate-rich aqueous fluids that were in equilibrium with peak minerals at UHP conditions. These fluids show characters which are transitional between aqueous fluids and silicate melts, and were probably produced by dehydration reactions of the host rocks during the latest stages of subduction.

40Ar-39Ar Thermochronological Constraints on the Exhumation of Ultrahigh-Pressure Metamorphic Rocks in the Sulu Terrane of Eastern China

In order to better understand the uplift history of the Sulu ultrahigh-pressure metamorphic terrane, 40Ar-39Ar thermochronological analysis was conducted on minerals extracted from both metamorphic (gneiss and retrograded eclogite) and igneous (pegmatite, granite, and mafic intrusion) rocks from this terrane. Samples of the major lithotectonic units were collected from key localities, including Rongcheng, Laoshan, Yangkou, Taohang, and Rizhao. The analyses of metamorphic rocks yielded ages for hornblende ranging from 222 to 186 Ma, biotite from 130 to 119 Ma, and K-feldspar from 112 to 108 Ma. Hornblende, biotite, and K-feldspar from igneous rocks yielded exclusively late Mesozoic ages from 137 to 97 Ma. A two-stage cooling path for the metamorphic rocks were revealed. A slow cooling (1.9 to 3.8°C/m.y.) during the period of ∼220 to ∼130 Ma was followed by fast cooling (6.8 to 16.7°C/m.y.) during ∼130 to ∼100 Ma. The time span and cooling rate of the second stage coincide with those of the analyzed post-collisional igneous rocks as well as with the formation of rift basins in the region. The study strongly indicates that extensional tectonics played an important role in the final stage of rapid exhumation of the ultrahigh-pressure rocks in the Sulu terrane.

SHRIMP U-Pb Dating of Zircon from the Xugou UHP Eclogite, Sulu Terrane, Eastern China

Eclogites, together with garnet clinopyroxenites, occur as lenses within the Xugou garnet peridotite body in the southern Sulu ultrahigh-pressure (UHP) terrane. Combined cathodoluminescence (CL) imaging and SHRIMP U-Pb dating of zircon from two Xugou mafic eclogites provide added constraints on the timing of UHP metamorphism in this area. Zircons from both samples show subrounded to rounded shapes and patchy CL patterns without inherited igneous cores, indicating that they are metamorphic zircons. SHRIMP U-Pb analyses of these zircons yielded apparent U-Pb ages of 214-280 Ma, with a weighted mean age of 237 ± 8 Ma, which is consistent with previous reported UHP metamorphic ages from eclogite pods and country-rock gneisses. The Xugou mafic lenses may have formed by partial melting of the enclosing peridotites in the mantle before subduction (Zhang et al., 2003); then these eclogites, together with the host peridotites, were tectonically emplaced into the subduction zone and subjected to UHP metamorphism at 237 ± 8 Ma.

U-Pb SHRIMP ages recorded in the coesite-bearing zircon domains of paragneisses in the southwestern Sulu terrane, eastern China: New interpretation

Laser Raman spectroscopy and cathodoluminescence (CL) images reveal that abundant and complicated mineral inclusions occur in zoned zircon separates from paragneisses in drill holes CCSDPP2, ZK-2304, and CCSD-MH, at Donghai, southwestern Sulu terrane, China. Many low-P mineral inclusions are preserved in inherited zircon cores. Coesite + quartz within a single inclusion, and some low-P minerals (e.g., quartz and albite) are identified in zircon mantles. Textures showing the quartz to coesite transformation in zircon indicate that both core and mantle of the zircon are detrital. Inclusions of coesite, together with garnet, jadeite, and phengite, are common in zircon rims formed during ultrahigh-pressure (UHP) metamorphism. SHRIMP U-Pb analyses of these zoned zircon grains identify three discrete and meaningful age groups. Proterozoic ages of 840 to 658 Ma for zircon cores indicate that the detrital zircons have a variety of sources. An early Paleozoic group of 505 to 455 Ma ages for zircon mantles constrain neither the timing of a discrete UHP metamorphic event, nor the transformation age from quartz to coesite. The 227 ± 9 Ma (the weighted mean age) obtained from coesite-bearing zircon rims represents the UHP metamorphic age. Thus, one Late Triassic UHP metamorphic event, rather than two UHP metamorphic events, is well established in Sulu UHP terrane by the method of U-Pb SHRIMP dating for coesite-bearing zircon rims in garnet-biotite-amphibole-albite gneisses.

High-pressure/ultrahigh-pressure eclogites from the Hong’an Block, East-Central China: geochemical characterization, isotope disequilibrium and geochronological controversy

The Hong’an Block (western Dabieshan) exposes a series of HP/UHP metamorphic rocks, with a S-to-N distribution from blueschist–greenschist, kyanite-free, to kyanite- and coesite-bearing eclogites. The available age data are inconclusive that hinder our understanding of the tectonic evolution of the Block. The metamorphic temperatures in the Hong’an Block (Tmeta ≈ 700 to 500°C) are lower by 50–150°C than that of the Dabie and Sulu terranes. In this work, we undertook new trace element and Sr–Nd–O isotopic analyses on minerals in order to gain more insight into the geochronological problems. The results are as follows: (1) Trace element distribution patterns suggest that garnet and omphacite in many cases are out of chemical equilibrium; and the presence of high-temperature LREE-rich mineral inclusions (e.g., epidote) in garnet and omphacite has contributed to isotope disequilibrium. (2) Sm–Nd isotope analyses yielded no isochron ages for the Hong’an eclogites. (3) Rb–Sr isotope analyses gave mixed results; in some cases, coexisting minerals are completely out of isotope equilibrium, and in others, isochron relationship is established, yielding ages from 210 Ma to 225 Ma. The pattern of Rb–Sr isotope disequilibrium appears to be independent of the petrological and O-isotope temperatures. (4) In contrast to the unequilibrated Sm–Nd isotopic systems, oxygen isotopes of the eclogite minerals seem to have attained isotope equilibrium or near-equilibrium. Oxygen isotope temperatures are comparable with petrological temperatures. However, this is an apparent feature due to mass balance constraints. (5) Whole-rock δ18O values show a large variation from +10‰ to −8‰, suggesting that their protoliths have undergone very different processes of water–rock interaction. In view of the overall geochronological information, we conclude that the HP/UHP metamorphism in the Hong’an Block took place in the Triassic at about 220–230 Ma, as observed in the Dabie and Sulu terranes. The significance of published Paleozoic dates (450–300 Ma) for the Xiongdian eclogite is not clear. However, any hypotheses advocating two periods of UHP metamorphic events for the same tectonic unit or in the same locality are not constrained by the geochronological data.

A combined study of SHRIMP U-Pb dating, trace element and mineral inclusions on high-pressure metamorphic overgrowth zircon in eclogite from Qinglongshan in the Sulu terrane

Methods recently advanced for discrimination on the genesis of metamorphic zircon, such as analysis of mineral inclusions and trace elements, provide us powerful means to distinguish zircon overgrowth during high-pressure metamorphism. Zircons in ultrahigh-pressure eclogite from Qinglongshan in the Sulu terrane were studied by the SHRIMP U-Pb method in combining with trace element and mineral inclusion analyses. No inherited core was identified in the analyzed zircons by means of cathodoluminescence images. The occurrence of high-pressure metamorphic mineral inclusions in zircon, such as garnet, omphacite, rutile, and the flat HREE pattern in zircon indicate that the zircon formed at high-pressure metamorphic conditions. Therefore, a weighted average U-Pb age of 227.4 ± 3.5 Ma obtained from such a kind of zircon is interpreted to represent the timing of peak metamorphism for the Qinglongshan eclogite.

Mineral inclusions in zircon domains and geological significance of SHRIMP U-Pb dating for coesite-bearing zircons of paragneiss in Sulu terrane, eastern China

Laser Raman spectroscopy and cathodoluminescence (CL) image reveal that zircons separated from paragneisses in the southwestern Sulu terrane (eastern China) preserve multi-stage mineral assemblages in different zircon domains. In the same paragneiss zircon sample, some zircon grains retain inherited (detrital) cores with abundant low-pressure mineral inclusions of Qtz+Phe+Ap+impurities and Qtz+Phe+impurities. The ultrahigh-pressure (UHP) metamorphic overgrowths mantles of these zircons preserve Coe, Coe+Phe and other UHP mineral inclusions, indicating that these inherited (detrital) zircons from protoliths experienced metamorphic recrystallization during the Sulu UHP metamorphic event. However, other zircon grains preserve UHP mineral inclusions of Coe, Coe+Ap and Coe+Phe in the cores and mantles, whereas the outmost rims contain quartz (Qtz) and other low-pressure mineral inclusions. These phenomena prove that the second group zircons were crystallized at UHP meta-morphic stage and overprinted by the late retrogressive metamorphism related to the Sulu ter-rane exhumation. SHRIMP U-Pb dating reveals that zircons separated from the paragneisses recorded complicated geochronological traces. Some zircons retain inherited cores with the 206Pb/238U ages of 284–754 Ma, indicating the complexity of the detrital zircons in protoliths. Coesite-bearing zircon domains recorded 238–266 Ma (with the average age of 245±14 Ma) for the UHP metamorphic event. The retrogressive rims of zircons recorded 213–223 Ma (with the average age of 217±15 Ma) for the late-stage amphibolite facies metamorphic event, indicating that the rapid exhumation time for the Sulu UHP terrane is about 220 Ma.

Fluid inclusions evidence for differential exhumation of ultrahigh pressure metamorphic rocks in the Sulu terrane

Differential exhumation was petrologically recognized in ultrahigh pressure metamorphic rocks from the southern and northern parts of the Sulu terrane. While a normal exhumation occurred for eclogites and gneisses in south Sulu, granulite-facies overprinting of ultrahigh pressure metamorphic rocks took place with high retrograde temperatures in north Sulu. A study of fluid inclusions reveals trapping of five type fluid inclusions in high and ultrahigh pressure eclogite minerals and vein quartz in the Sulu terrane. These are A-type N2±CO2 inclusion trapped at high and ultra-high pressure eclogite-facies metamorphic condition, B-type pure-CO2 liquid phase inclusion with higher density trapped during granulite-facies overprinting metamorphism of eclogites, C-type CO2−H2O inclusion and D-type hypersaline inclusion trapped in high pressure eclogite-facies re-crystallization stage, and E-type low salinity H2O inclusion trapped in the latest stage of ultrahigh pressure exhumation (amphibolite-facies retrogression). Identification of crowded-distributing pure-CO2 liquid inclusions with higher density trapped in garnet of eclogites provides an evidence for granulite-facies overprinting metamorphism in the north Sulu terrane.

A combined study of SHRIMP U-Pb dating, trace element and mineral inclusions on high-pressure metamorphic overgrowth zircon in eclogite from Qinglongshan in the Sulu terrane

A combined study of SHRIMP U-Pb dating, trace element and mineral inclusions on high-pressure metamorphic overgrowth zircon in eclogite from Qinglongshan in the Sulu terrane

Zircon SHRIMP U-Pb age of garnet olivine pyroxenite at Hujialin in the Sulu terrane and its geological significance

Garnet olivine pyroxenite at Hujialin is situated in the Sulu ultrahigh pressure (UHP) metamorphic belt, Shandong Province. Most of the zircon separated from the rock is well crystallized, prismatic and granular with a length to width ratio of 1:1.3-1:2.5. CL and BSE images show the magmatic oscillatory zoning in the zircon. Th/U ratio ranges from 0.99 to 2.81. These suggest a magmatic origin for the zircon studied. SHRIMP dating yields206pb/238U ages of 207–223 Ma, with a weighted average of 216±Ma. This age corresponds to zircon growth during exhumation of UHP slab and thus the timing of amphibolite-facies retrogression. The garnet olivine pyroxenite was wrapped and brought to the crust by the UHP slab during exhumation, and then suffered from metasomatism by fluid from the UHP slab itself. The zircon U-Pb age records the timing of the crystallization of metasomatic melt. Therefore, fluid that was released during exhumation of deeply subducted continental slab may be the important source for zircon growth.

Tracing the protolith, UHP metamorphism, and exhumation ages of orthogneiss from the SW Sulu terrane (eastern China): SHRIMP U–Pb dating of mineral inclusion-bearing zircons

Orthogneisses are the major country rocks hosting eclogites in the Sulu UHP terrane, eastern China. All of the analyzed orthogneiss cores from the main drilling hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) have similar major and trace element compositions and a granite protolith. These rocks have relatively high LREE/HREE ratios, strong negative Eu anomalies (Eu/Eu*=0.20–0.39), and negative Ba anomalies (Ba/Ba*=0.25–0.64). Coesite and coesite-bearing UHP mineral assemblages are common inclusions in zircons separated from orthogneiss, paragneiss, amphibolite, and (retrograded) eclogite of the CCSD-MH. This suggests that the eclogite, together with its country rocks, experienced in situ ultrahigh-pressure (UHP) metamorphism. Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons from the orthogneisses are zoned and that they have distinct mineral inclusions in the different zones. Most zircons retain early magmatic cores with abundant low-pressure mineral inclusions, which are mantled with metamorphic zircon-containing inclusions of coesite and other UHP minerals. The outermost rims on these grains contain low-pressure mineral inclusions, such as quartz and albite. SHRIMP U–Pb dating of the zoned zircons gives three discrete and meaningful groups of ages: Proterozoic ages for the protolith, 227±2 Ma for the coesite-bearing mantles, and 209±3 Ma for the amphibolite facies retrograde rims. The widespread occurrence of UHP mineral inclusions in zircons from the Sulu metamorphic belt dated at about 227 Ma suggests that voluminous continental crust experienced late Triassic subduction to depths of at least 120 km and perhaps more than 200 km. Eighteen million years later, the terrane was rapidly exhumed to midcrustal levels, and the UHP rocks were overprinted by amphibolite facies metamorphism. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P– T data is estimated to be 5.6–11.0 km/Ma. Such rapid exhumation of the Sulu UHP terrane may be due to the buoyancy forces produced by subduction of low-density continental material into the deep mantle.

Retrograded textures and associated mass transfer: evidence for aqueous fluid action during exhumation of the Qinglongshan eclogite, Southern Sulu ultrahigh pressure metamorphic terrane, eastern China

AbstractThe Qinglongshan eclogites in the Southern Sulu ultrahigh pressure metamorphic (UHPM) terrane show very different retrograded textures from their counterparts in the Northern Sulu terrane, implying a different thermal history. Scanning electron and optical microscope observations indicate that the peak assemblage of the Qinglongshan eclogite is anhydrous, composed of Grt + OmpI + Rt + (Ky + coesite). These primary minerals were replaced by second and third stage minerals, resulting in symplectite pseudomorphs or coronas. The following relationships are inferred: OmpI → OmpII + Ab + Fe‐oxide symplectite (type I) and Rt → Rt + Ilm intergrowth; and, Ky → Pg, OmpII (+Pl) → Amp (+Pl) symplectite (type II), and Grt → Prg (+Fe‐oxide). Mineral chemistry and mass‐balance demonstrate that the pseudomorphed textures were developed by metasomatism involving dissolution and precipitation intensified by fluids along grain boundaries. The formation of symplectite type I produced Fe, Mg and Na but consumed Ca and Si. The Mg and Fe diffused to garnet where exchange of (Mg, Fe) with Ca of the garnet resulted in compositional zonation with decreased Ca towards the edge of garnet grains where Ca was consumed during symplectite formation. The replacement of kyanite by paragonite consumed the extra Na. In the later stage, fluid infiltration partially transformed symplectite type I to type II, and narrow rims of pargasite resorbed garnet from their boundaries. Mass balance suggests that the transformation and resorption would have been coupled during fluid infiltration. In the latest stage, epidote and quartz were precipitated at very late stage as a result of fluid activity along microfractures. Tentative P–T conditions based on mineral reactions and thermocalc software suggest that the retrograded eclogite did not record the granulite facies retrograde evolution characteristic of eclogites from the Northern Sulu terrane. The difference in retrograde evolution between the Southern and Northern Sulu eclogites suggests a different exhumation history.

Concepts for diamond exploration in “on/off craton” areas—British Columbia, Canada

The tectonic setting of British Columbia (BC) differs from classic diamond-bearing intracratonic regions such as the Northwest Territories and South Africa. Nevertheless, several diamond occurrences have been reported in BC. It is also known that parts of the province are underlain by Proterozoic and possibly Archean basement. Because the continents of today are composites of fragments of ancient continents, it is possible that some of the regions underlain by old crystalline basement in eastern British Columbia were associated with a deep crustal keel. The keel may have predated the break-up of the early Neoproterozoic supercontinent called Rodinia and was preserved possibly until the Triassic. Some of these old continental fragments may have been displaced relative to their position of origin and dissociated from their keel, or the keel may have since been destroyed. Such fragments represent favourable exploration grounds in terms of the “Diamondiferous Mantle Root” model (DMR model) if they were intersected by kimberlites or lamproites prior to displacement or destruction of their underlying deep keel. Therefore, extrapolation of fragments of the diamond-bearing Precambrian basement from the Northwest Territories or Alberta to BC provides a sufficient reason for initiating reconnaissance indicator mineral surveys. The “Eclogite Subduction Zone” model (ES model) predicts formation of diamonds at lower pressure (i.e., depth) than required by the DMR model in convergent tectonic settings. Although not proven, this model is supported by thermal modeling of cold subduction zones and recent discoveries of diamonds in areas characterized by convergent tectonic settings. If the ES model is correct, then the parts of BC with a geological history similar to today's “cold” subduction zones, such as Honshu (Japan), or to continental collision zones, such as Kokchetav massif (Kazakhstan) and the Dabie–Sulu Terrane (east central China), may be diamondiferous. The terranes where geological evidences suggest an ultrahigh pressure (UHP) metamorphic event followed by rapid tectonic exhumation (which could have prevented complete resorption of diamonds on their journey to the surface) are worth investigating. If UHP rocks were intercepted at depth by syn- or post-subduction diamond elevators, such as kimberlites, lamproites, lamprophyres, nephelinites or other alkali volcanic rocks of deep-seated origin, the diamond potential of the area would be even higher.

Tracing the Boundary between UHP and HP Metamorphic Belts in the Southwestern Sulu Terrane, Eastern China: Evidence from Mineral Inclusions in Zircons from Metamorphic Rocks

The southwestern Sulu terrane, eastern China is divided into four fault-bounded lithologic units. From northwest to southeast, they are: (1) orthogneiss unit (Unit I); (2) supracrustal rock unit (Unit II); (3) kyanite-bearing quartzite—marble unit (Unit III); and (4) paragneiss—schist unit (Unit IV). Inclusions of index minerals in zircon separates from more than 90 samples from these units were identified using laser Raman spectroscopy and electron microprobe analysis. Coesite and coesitebearing ultrahigh-pressure (UHP) mineral assemblages occur in zircon separates from various amphibolite-facies metamorphic rocks in Units I and II. In contrast, aragonite- and phengite-bearing high-pressure (HP) mineral assemblages are preserved in zircons from many amphibolite-to greenschist-facies metamorphic rocks in Units III and IV. These together with previous studies yield P-T estimates of peak metamorphic recrystallization at 723°-852°C, and P > 2.8 GPa for UHP rocks, and 500°-600°C, and P = 1.2-2.5 GPa for HP rocks. The spatial distribution of these P-T estimates and recent surface mapping results constrain an exact boundary between UHP and HP belts in the southern Sulu terrane along a ductile shear zone in the Donghai region.

SHRIMP U–Pb ages of ultrahigh‐pressure and retrograde metamorphism of gneisses, south‐western Sulu terrane, eastern China

AbstractLaser Raman spectroscopy and cathodoluminescence (CL) images reveal that most zircon separated from paragneiss and orthogneiss in drillhole CCSD‐PP2 at Donghai, south‐western Sulu terrane, retain low‐P mineral‐bearing inherited cores, ultrahigh‐pressure (UHP) mineral‐bearing mantles and low‐P mineral‐bearing (e.g. quartz) rims. SHRIMP U–Pb analyses of these zoned zircon identify three discrete and meaningful age groups: Proterozoic protolith ages (> 680 Ma) are recorded in the inherited cores, the UHP metamorphic event in the coesite‐bearing mantles occurred at 231 ± 4 Ma, and the late amphibolite facies retrogressive overprint in the quartz‐bearing rims was at 211 ± 4 Ma. Thus, Neoproterozoic supracrustal protoliths of the Sulu UHP rocks were subducted to mantle depths in the Middle Triassic, and exhumed to mid‐crustal levels in the Late Triassic. The exhumation rate deduced from the SHRIMP data and metamorphic P–T conditions is 5.0 km Ma−1. Exhumation of the Sulu UHP terrane may have resulted from buoyancy forces after slab break‐off at mantle depths.

Fluid inclusions hidden in coesite-bearing zircons in ultrahigh-pressure metamorphic rocks from southwestern Sulu terrane in eastern China

Primary fluid inclusions, together with coesite mineral inclusions, are identified in the same zircon domains by laser Raman spectroscopy, cathodoluminescence (CL) image and micro-texture analysis in paragneiss and eclogite from the main drilling hole of Chinese Continental Scientific Drilling Project in southwestern Sulu terrane. Most fluid inclusions are characterized by CO2 (gas)-H2O (liquid) two-phase, a few by H2O one-phase liquid inclusions. These features indicate that the eclogite and its country-rocks may be located in the “wet system” rather than in the “dry system” during UHP metamorphism. SHRIMP U-Pb dating indicates that the timing of trapping the fluid and coesite inclusions in metamorphic zircon domains is about 233.7 ±4.3 Ma, which may represent the age of zircon growth in the stage of pressure decrease but temperature increase during the retrograde period of UHP metamorphism thus indicating the fluid activity still under the UHP conditions. The zircons further overgrew at about 213.2 ±5.2 Ma in response to amphibolite-facies retrogression. Therefore, fluid activity in the Sulu UHP metamorphic rocks principally occurred during the exhumation of UHP slab in the Middle to Late Triassic. The present results not only provide insight into the fluid property and fluid-rock interaction mechanism in the Sulu-Dabie UHP terrane, but also present a new means to exactly identify the primary fluid inclusions preserved in zircons from the UHP metamorphic rocks.

Zircon SHRIMP U-Pb age of garnet olivine pyroxenite at Hujialin in the Sulu terrane and its geological significance

Zircon SHRIMP U-Pb age of garnet olivine pyroxenite at Hujialin in the Sulu terrane and its geological significance