Large areal extent of ultrahigh-pressure metamorphism in the Sulu ultrahigh-pressure terrane of East China: new implications from coesite and omphacite inclusions in zircon of granitic gneiss

Coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite-facies granitic orthogneiss in the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China by Raman spectroscopy and microprobe analyses. The occurrences of these minerals in the voluminous granitic gneiss of Sulu support a regional and pervasive UHP metamorphic event that predated regional amphibolitic retrogression. Taking into account the widespread discoveries of coesite in other lithologies, we thus conclude that a substantial crustal component in the Sulu UHP metamorphic terrane appears to have shared a common history of Triassic subduction to mantle depths and later exhumation.     

Petrological investigation of the Ganyu peridotite in the Sulu ultrahigh-pressure terrane, eastern China

The ultramafic body sampled in the Chinese Continental Scientific Drilling (CCSD) Hole PP3 is located in the eastern part of the Dabie-Sulu UHP metamorphic belt near Donghai County. It is about 480 m thick, and consists chiefly of garnet peridotite, dunite and serpentinite. The principal minerals include olivine, chromium spinel, diopside, enstatite, garnet with minor secondary augite, phlogopite and amphibole. Both the olivine and orthopyroxene are highly magnesian, and the garnet is pyropic with 5.4-6.4% CaO and 0.3-3.3% Cr₂O₃. Two generations of clinopyroxene are present; an early diopside followed by augite. Chromium spinels are highly variable with Cr#s (100Cr / (Cr + Al)) between 51 and 89, and their compositions reflect different processes of formation, namely partial melting and eclogite, amphibole and greenschist facies metamorphism. The Mg#s (100 Mg / (Mg + Fe²⁺))of the spinels correlate positively with the Cr#s but negatively with oxygen fugacity. Based on the spinel compositions the ultramafic rocks originated in the shallow mantle, then subducted to depths of more than 100 km and finally exhumed to the surface. They underwent partial melting at shallow depths, mostly in the spinel facies, and were later transformed into garnet peridotites during deep subduction. All of the rocks were weakly metasomatized during exhumation and were subjected to retrograde metamorphism.     

An experimental study of spinel clinopyroxenite xenoliths from the Duncansby Ness vent,Caithness, Scotland

Xenoliths of coarse-grained spinel-clinopyroxenite up to 15 cm in size occur in tuff in an isolated Permian vent on the Caithness coast at Duncansby Ness. Highly altered fragments of chrome-spinel lherzolite and wehrlite are also found in the tuff and in a body of monchiquite within the vent. The spinel-clinopyroxenites consist of aluminous augite and aluminous pleonaste spinel (FeO/MgO = 0.9) and their texture suggests the spinel to have exsolved from the augite. Experiments on representative natural xenolith compositions at 18 kb (dry) indicate that all the spinel in the estimated average bulk composition (Sp_4.9Px_95.1) could have exsolved from an original homogeneous pyroxene. Initial fractionation of such a pyroxene from an alkali basaltic magma at P≥18 kb, 1450-1350 °C, would be followed by spinel exsolution at T< 1290 ° C. A similar origin by fractionation of a highly aluminous augite (± aluminous spinel) at high pressure, with subsequent spinel exsolution is proposed for spinel-clinopyroxenites from alkali basalts elsewhere in the world. The similarity of these xenoliths suggests that such a process may form an important stage in the evolution of some undersaturated basaltic rocks.     

SHRIMP U–Pb dating of coesite-bearing zircon from the ultrahigh-pressure metamorphic rocks, Sulu terrane, east China

This paper provides further evidence for the ongoing discussion as to whether the Dabie UHPM belt formed in Triassic or Palaeozoic time, and whether the Sulu UHPM belt formed in Triassic or Neoproterozoic time. Combined use of laser Raman spectrometer (LR), cathodoluminescence imaging (CL), and ion probe U–Pb in‐situ dating (SHRIMP) provided accurate ages of UHPM from rocks collected from Weihai, NE Sulu UHPM belt. LR was used to identify coesite and other UHP minerals as inclusions in zircon separates from an amphibolized peridotite and an eclogite. CL was used to examine the zoning structure of these zircon, and SHRIMP dating was performed on specific spots on zircon to obtain ages of different geological events. An age of 221 ± 12 Ma was obtained for coesite‐bearing zircon from the amphibolized peridotite; an age of 228 ± 29 Ma for eclogite was obtained from the lower intercept of a concordia plot. These ages are interpreted as the time of UHPM in the Weihai region. Ultramafic rocks to the east of Weihai yield a magmatic age at 581 ± 44 Ma. The zircon in the ultramafic rocks possibly also records a thermal event at c. 400 Ma, but no independent geological evidence for this event has been found. The eclogite protolith formed in the Middle Proterozoic (1821 ± 19 Ma), which is similar to the age of country rock gneisses of 1847–1744 Ma. The new geochronological data confirm that UHPM occurred in the Triassic in the Sulu area when subduction took the ultramafic body and the eclogite protolith, together with the adjacent supracrustal rocks, to mantle depths.     

Ultrahigh-pressure metamorphism in the forbidden zone: the Xugou garnet peridotite, Sulu terrane, eastern China

The Xugou garnet peridotite body of the southern Sulu ultrahigh‐pressure (UHP) terrane is enclosed in felsic gneiss, bounded by faults, and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite. The peridotite is composed of variable amounts of olivine (Fo₉₁), enstatite (En_92?93), garnet (Alm_20?23Prp_53?58Knr_6?9Grs_12?18), diopside and rare chromite. The ultramafic protolith has a depleted residual mantle composition, indicated by a high‐Mg number, very low CaO, Al₂O₃ and total REE contents compared to primary mantle and other Sulu peridotites. Most garnet (Prp_44?58) clinopyroxenites are foliated. Except for rare kyanite‐bearing eclogitic bands, most eclogites contain a simple assemblage of garnet (Alm_29?34Prp_32?50Grs_15?39) + omphacite (Jd_24?36) + minor rutile. Clinopyroxenite and eclogite exhibit LREE‐depleted and LREE‐enriched patterns, respectively, but both have flat HREE patterns. Normalized La, Sm and Yb contents indicate that both eclogite and garnet clinopyroxenite formed by high‐pressure crystal accumulation (+ variable trapped melt) from melts resulting from two‐stage partial melting of a mantle source. Recrystallized textures and P–T estimates of 780–870 °C, 5–7 GPa and a metamorphic age of 231 ± 11 Ma indicate that both mafic and ultramafic protoliths experienced Triassic UHP metamorphism in the P–T forbidden zone with an extremely low thermal gradient (< 5 °C km^?1), and multistage retrograde recrystallization during exhumation. Develop of prehnite veins in clinopyroxenite, eclogite, felsic blocks and country rock gneiss, and replacements of eclogitic minerals by prehnite, albite, white mica, and K‐feldspar indicate low‐temperature metasomatism.     

东海地区超高压石榴橄榄岩超微构造特征——快速折返的新证据

利用透射电子显微镜对中国大陆科学钻探(CCSD)主孔超高压石榴橄榄岩中石榴石、橄榄石及透辉石的超微构造特征进行了详细研究。石榴石、橄榄石及透辉石中的大部分区域都没有位错,在局部区域发育高密度位错及位错壁。石榴石局部区域的位错密度达到1．34×109／cm2以上,滑移系以1／2{110}为主。在石榴石中发现一部分熔融条带,条带由熔体和磁铁矿球粒构成,在附近的石榴石基体中有高密度的小位错圈。橄榄石局部区域的位错密度达到2．64×108／cm2以上,并发育杆状钛铁矿和含铬钛磁铁矿出溶体。透辉石局部区域发育有位错壁,在部分位错壁部位形成了微裂隙。透辉石内部存在类“多边化”区域,不同区域之间存在小角度差。分析认为该石榴橄榄岩加入俯冲板片之后,在超高压变质峰期及早期快速折返阶段发生了较强的塑性变形,在石榴石、橄榄石及透辉石中形成了高密度的位错、位错壁及亚颗粒。在折返过程中,该石榴橄榄岩曾经发生了减压部分熔融和静态重结晶,导致石榴石、橄榄石及透辉石中大部分区域的位错消失。石榴石中部分熔融条带得以保存,表明在发生减压部分熔融和静态重结晶之后所经历的折返过程非常迅速,具有很高的抬升速率及降温速率。     

An experimental study of the Fe-Mn exchange between garnet and ilmenite

The exchange equilibrium

Geobarometers involving clinopyroxene,garnet, plagioclase,ilmenite, rutile,sphene and quartz: estimation of pressure in quartz-absent assemblages

Enthalpy of formation and third law entropy for andradite (Ca₃Fe₂Si₃O₁₂) and hedenbergite (CaFeSi₂O₆) at standard state have been retrieved from experimental P-T-f(O₂) data on the stability relations of these phases. The data for hedenbergite were combined with the thermodynamic data for related phases (Berman 1988) to formulate the geobarometers based on the reactions CaFeSi₂O₆+2CaAl₂Si₂O₈+2FeTiO₃= Ca₃Al₂Si₃O₁₂+Fe₃Al₂Si₃O₁₂+2TiO₂ (CPG-IR), 3CaFeSi₂O₆+3CaAl₂Si₂O₈+3FeTiO₃=Ca₃Al₂Si₃O₁₂ +2Fe₃Al₂Si₃O₁₂+3CaTiSiO₅ (CPG-IS) and 3CaFeSi₂O₆+3CaAl₂Si₂O₈=2Ca₃Al₂Si₃O₁₂+ Fe₃Al₂Si₃O₁₂+3SiO₂ (CPG-O). The geobarometers involving rutile and sphene are proposed for the first time. In several granulite terranes, pressures have been estimated from the three geobarometers for basic granulites and charnockitic suite of rocks containing the above assemblages. In clinopyroxene-garnet-plagioclase-quartz-ilmenite-rutile assemblages, pressures obtained from CPG-IR are within ±500 bars of pressures calculated from the CPG-Q barometer. However, if rutile is absent, the pressures computed from the CPG-IR barometer are 300 to 1,200 bars higher than those calculated from the CPG-Q barometer. For maximum differences in pressures obtained from the two equilibria TiO₂ activity [a(TiO₂)] in the rocks is calculated to be 0.8. The sphene-bearing geobarometer (CPG-IS) also registers pressures comparable (<±500 bars) to those obtained from the CPG-Q barometer. The close agreement in pressure values obtained from the CPG-IR and the CPG-IS equilibria with those registered by the CPG-Q geobarometer in assemblages containing quartz implies that the rutile- and sphene-bearing geobarometers which do not involve quartz, can also be applied to estimate pressures from quartz-absent assemblages-assemblages which are not amenable to quantitative geobarometry using existing formulations.     

Oriented inclusions of pyroxene,amphibole and rutile in garnet from the Lüliangshan garnet peridotite massif,North Qaidam UHPM belt,NW China: an electron backscatter diffraction study

Oriented inclusions of clinopyroxene, orthopyroxene, sodic amphibole and rutile have been identified in garnet from the Lüliangshan garnet peridotite massif in the North Qaidam ultrahigh‐pressure metamorphic (UHPM) belt, northern Tibetan Plateau, NW China. Electron backscatter diffraction (EBSD) analyses demonstrate that nearly half of the measured intracrystalline clinopyroxene (8 out of 17) have topotactic crystallographic relationships with host garnet, that is, (100)_Cpx//{112}_Grt, (010)_Cpx//{110}_Grt and [001]_Cpx//<111>_Grt. One‐fifth of the oriented sodic amphibole (23 out of 110) inclusions of have topotactic crystallographic relationships with host garnet, that is, (010)_Amp//{112}_Grt, (100)_Amp//{110}_Grt and [001]_Amp//<111>_Grt. Over a third of rutile (36 out of 99) inclusions also show a close crystallographic orientation relationship with host garnet in that one <103>_Rt and one <110>_Rt parallel to two <111>_Grt while the axes of [001]_Rt exhibit small girdles centred the axes of <111>_Grt. But, no ‘well‐fit’ crystallographic relationship was observed between orthopyroxene inclusions and host garnet. Considering a very long and complex history for the Lüliangshan garnet peridotite, we suggest that the low fit rates for these oriented minerals may result from several possible assumptions including different generations or multi‐stage formation mechanisms, heterogeneous nucleation and growth under non‐equilibrium conditions, and partial changes of initial crystallographic orientations of some inclusions. However, the residual quantitative ‘well‐fit’ crystallographic information is sufficient to indicate that the nucleation and growth of many pyroxene, amphibole and rutile are controlled by the lattice of the host garnet. The revealed close topotactic relationships accompanied by clear shape orientations provide quantitative microstructural evidence demonstrating a most likely exsolution/precipitate origin for at least some of the oriented phases of pyroxene, sodic amphibole and rutile from former majoritic garnet and support an ultra‐deep (>180 km depth) origin of the Lüliangshan garnet massif.     

In situ characterization of forearc serpentinized peridotite from the Sulu ultrahigh-pressure terrane:Behavior of fluid-mobile elements in continental subduction zone

Serpentinized peridotites in the Yangkou(YK),Suoluoshu(SLS) and Hujialin(HJL) areas in the Sulu ultrahighpressure terrane represent the relic of ancient subcontinental lithospheric mantle below the North China Craton.Their protoliths,harzburgite and dunite,were variably hydrated by aqueous fluids released from subducting Yangtze continent.The rocks are enriched in fluid-mobile elements(FME) including Sb(42–333 times the depleted mantle value) and Pb(30–476 times).The degrees of the FME enrichment are comparable to that of the Himalayan forearc serpentinites,and greater than forearc mantle serpentinites from Marianas,suggesting that the degrees of FME enrichment in the forearc serpentinites are greater in continental subduction zones than those in the oceanic subduction zones.Lizardite after olivine in the SLS serpentinite shows higher degrees of enrichment in Sb and As than those for antigorite after both olivine and orthopyroxene in the YK area.The antigorite has highly enriched in Pb,U,Cs,and LREE,but not for the lizardite.The abundance of FME in two different species of serpentine reflects the different temperature of hydration.At temperature lower than 300 ℃,formed lizardite at shallow depths of the mantle wedge incorporates elements that are fluid mobile at low temperatures,such as Sb and As.When the temperature greater than 300 ℃,formed antigorite at a relatively deep mantle wedge incorporate more FME from the subducting continental slab(or fragments),including Pb,U,Cs,LREE as well as Sb and As.The eventual breakdown of antigorite(600–700 ℃) in prograde metamorphism would discharge water as well as FME into the subducting channel and/or the overlying mantle.     

Protolith and metamorphic ages of the Haiyangsuo Complex, eastern China: a non-UHP exotic tectonic slab in the Sulu ultrahigh-pressure terrane

Summary The Haiyangsuo Complex in the NE Sulu ultrahigh-pressure (UHP) terrane has discontinuous, coastal exposures of Late Archean gneiss with amphibolitized granulite, amphibolite, Paleoproterozoic metagabbroic intrusives, and Cretaceous granitic dikes over an area of about 15 km². The U–Pb SHRIMP dating of zircons indicates that theprotolith age of a garnet-biotite gneiss is >2500 Ma, whereas the granulite-facie metamorphism occurred at around 1800 Ma. A gabbroic intrusion was dated at ∼1730 Ma, and the formation of amphibolite-facies assemblages in both metagabbro and granulite occurred at ∼340–460 Ma. Petrologic and geochronological data indicate that these various rocks show no evidence of Triassic eclogite-facies metamorphism and Neoproterozoic protolith ages that are characteristics of Sulu-Dabie HP-UHP rocks, except Neoproterozoic inherited ages from post-collisional Jurassic granitic dikes. Haiyangsuo retrograde granulites with amphibolite-facies assemblages within the gneiss preserve relict garnet formed during granulite-facies metamorphism at ∼1.85 Ga. The Paleoproterozoic metamorphic events are almost coeval with gabbroic intrusions. The granulite-bearing gneiss unit and gabbro-dominated unit of the Haiyangsuo Complex were intruded by thin granitic dikes at about 160 Ma, which is coeval with post-collisional granitic intrusions in the Sulu terrane. We suggest that the Haiyangsuo Complex may represent a fragment of the Jiao-Liao-Ji Paleoproterozoic terrane developed at the eastern margin of the Sino-Korean basement, which was juxtaposed with the Sulu terrane prior to Jurassic granitic activity and regional deformation.     

Raman spectra of polycrystalline microdiamond inclusions in zircons,and ultrahigh-pressure metamorphism of a quartzofeldspathic rock from the Erzgebirge terrane,Germany

ABSTRACT

Polycrystalline microdiamonds are rare in ultrahigh-pressure (UHP) rocks worldwide. Among samples collected at Erzgebirge, Germany, we found abundant polycrystalline microdiamonds as inclusions in zircons from a quartzofeldspathic rock. To illuminate their origin and forming age, we investigated morphologies and Raman spectra of 52 microdiamond inclusions, and dated the zircon host. The zircons have low Th/U values (0.03–0.07) and a concordia U/Pb age of 335.8 ± 1.9 Ma. Polycrystalline diamond (10–40 µm) consists of many fine-grained crystals (1.5–3 µm) with different orientations; discrete single diamonds (2–20 µm) are rare. All measured Raman spectra show an intense diamond band at 1332–1328 cm^?1 and have a negative correlation with full width at half maximum (FWHM) of 5.8–11.3 cm^?1. These data combined with previously reported diamond band data (1331–1337 cm^?1) are compatible with those of diamond inclusions in various host minerals from other UHP terranes, but are different from those of ureilite diamonds. The Erzgebirge microdiamonds in zircon do not display visible disordered sp³-carbon, but show downshifting of the Raman band from the ideal value (1332 cm^?1), and have a broader diamond band (FWHM >3 cm^?1) than those of well-ordered diamonds. These features may reflect imperfect ordering due to rapid nucleation/crystallization during UHP metamorphism and rapid exhumation of the UHP terrane. Graphite inclusions in zircon show a typical G-band at 1587 cm^?1. Our study together with previously reported C-isotopic compositions (δ¹³C, ?17 to ?27‰) of diamond and occurrences of fluid/melt inclusions in diamond and garnet indicates that Erzgebirge microdiamonds are metamorphic, have an organic carbon source, and crystallized from aqueous fluids. Limited long-range ordering suggested by the Raman spectra is a function of the P–T time of crystallization and subsequent thermal annealing on decompression. Combined with regional geology, our work further constrains the tectonic evolution of the Erzgebirge terrane.     

Fluid inclusions in granulites, granulitized eclogites and garnet clinopyroxenites from the Dabie–Sulu terranes, eastern China

Minor granulites (believed to be pre-Triassic), surrounded by abundant amphibolite-facies orthogneiss, occur in the same region as the well-documented Triassic high- and ultrahigh-pressure (HP and UHP) eclogites in the Dabie–Sulu terranes, eastern China. Moreover, some eclogites and garnet clinopyroxenites have been metamorphosed at granulite- to amphibolite-facies conditions during exhumation. Granulitized HP eclogites/garnet clinopyroxenites at Huangweihe and Baizhangyan record estimated eclogite-facies metamorphic conditions of 775–805 °C and ≥15 kbar, followed by granulite- to amphibolite-facies overprint of ca. 750–800 °C and 6–11 kbar. The presence of (Na, Ca, Ba, Sr)-feldspars in garnet and omphacite corresponds to amphibolite-facies conditions. Metamorphic mineral assemblages and P–T estimates for felsic granulite at Huangtuling and mafic granulite at Huilanshan indicate peak conditions of 850 °C and 12 kbar for the granulite-facies metamorphism and 700 °C and 6 kbar for amphibolite-facies retrograde metamorphism. Cordierite–orthopyroxene and ferropargasite–plagioclase coronas and symplectites around garnet record a strong, rapid decompression, possibly contemporaneous with the uplift of neighbouring HP/UHP eclogites.

Carbonic fluid (CO₂-rich) inclusions are predominant in both HP granulites and granulitized HP/UHP eclogites/garnet clinopyroxenites. They have low densities, having been reset during decompression. Minor amounts of CH₄ and/or N₂ as well as carbonate are present. In the granulitized HP/UHP eclogites/garnet clinopyroxenites, early fluids are high-salinity brines with minor N₂, whereas low-salinity fluids formed during retrogression. Syn-granulite-facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene (granulitized HP garnet clinopyxeronite) or in quartz blebs in garnet and quartz matrices (UHP eclogite). For HP granulites, a limited number of primary CO₂ and mixed H₂O–CO₂(liquid) inclusions have also been observed in undeformed quartz inclusions within garnet, orthopyroxene, and plagioclase which contain abundant, low-density CO₂±carbonate inclusions. It is suggested that the primary fluid in the HP granulites was high-density CO₂, mixed with a significant quantity of water. The water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metasomatic reactions (“giant myrmekites”) occurring at quartz–feldspar boundaries. Compared with the UHP eclogites in this region, the granulites were exhumed in the presence of massive, externally derived carbonic fluids and subsequently limited low-salinity aqueous fluids, probably derived from the surrounding gneisses.     

Petrogenesis of the garnet peridotite and garnet-free peridotite of the Zhimafang ultramafic body in the Sulu ultrahigh-pressure metamorphic belt, eastern China

The CCSD‐PP1 drillhole penetrated a 110‐m‐thick sequence of the Zhimafang ultramafic body in the Sulu ultrahigh‐pressure (UHP) metamorphic belt, east China. The sequence consists of interlayered garnet‐bearing (Grt) and garnet‐free (GF) peridotite. Eleven layers of Grt‐peridotite, ranging from 1.2 to 9.5 m in thickness, have an aggregate thickness of 54.49 m, whereas eight layers of GF‐peridotite, ranging from 2.2 to 14.2 m in thickness, have a total thickness of 57.53 m. The boundaries between the two rock types are gradational. The Grt‐peridotites have slightly higher contents of Al₂O₃, CaO and SiO₂, and lower Mg^#s (0.90–0.92) than the GF‐peridotites (Mg^#s 0.91–0.93). Both contain low TiO₂ (<0.05 wt%) and have higher modal abundances of enstatite (average 10 vol.%) than diopside (1–5 vol.%), typical of depleted‐type upper mantle. The diopside in these rocks has high and relatively uniform Mg^# members (0.93–0.95), but highly variable Al₂O₃ (0.2–2.3 wt%), Na₂O (0.5–2.5 wt%) and Cr₂O₃ (0.38–2.09 wt%). Enstatite (En_92?93) contains very low Al₂O₃ (0–0.3 wt%). Both porphyroblastic and equigranular garnet are present. The equigranular varieties are zoned, from core to rim in Cr₂O₃ (3.4–4.2 wt%), MgO (18.4–17.5 wt%) and Al₂O₃ (21.1–20.1 wt%). Titania is very low in all the garnet, mostly <0.05 wt%. Chromite or chromium (Cr)‐spinel occur both in the Grt‐ and GF‐peridotite, and are characterized by high contents of Cr₂O₃ (49–58 wt%) and FeO (24–43 wt%), similar to that in iron‐rich Alpine‐type peridotites. Based on the bulk‐rock MgO–FeO compositions, the Zhimafang Grt‐peridotite probably underwent 20–30% partial melting, whereas the GF‐peridotite may have undergone as much as 35–40% partial melting, suggesting that the two rock types owe their differences to different degrees of partial melting rather than to pressure differences during metamorphism.     

Ar–Ar age determinations of eclogitic clinopyroxene and garnet inclusions in diamonds from the Venetia and Orapa kimberlites

Ar–Ar age measurements are reported for selected eclogitic clinopyroxene and garnet inclusions in Orapa diamonds and clinopyroxene inclusions in Venetia diamonds. Laser drilling of encapsulated clinopyroxene inclusions within Venetia diamonds released a maximum of 3% of the total ⁴⁰Ar, indicating little diffusive transfer and storage of radiogenic ⁴⁰Ar at the diamond–inclusion boundary. Apparent ages obtained during stepped heating of three diamonds are consistent with diamond crystallisation occurring just prior to the kimberlite eruption 520 Ma ago. Stepped heating of three clinopyroxene-bearing Orapa diamonds gave ages of 906–1032 Ma, significantly above the eruption age, but consistent with previously determined isotopic ages. A few higher apparent ages hint at the presence an older generation of Orapa diamonds that formed >2500 Ma ago. Orapa garnets also contain measurable K contents, and record a range of ages between 1000 and 2500 Ma. The old apparent ages and lack of significant interface ⁴⁰Ar released by the laser probe, suggests that pre-eruption radiogenic ⁴⁰Ar and mantle-derived ⁴⁰Ar components are trapped in microinclusions within the pyroxene and garnet inclusions.     

鲁东南岚山头超高压变质岩流体包裹体特征与板片折返史

鲁东南岚山头含柯石英榴辉岩主要产于花岗质片麻岩内,是苏鲁超高压变质带主要榴辉岩体密集分布区之一。流体包裹体研究表明,榴辉岩矿物及细脉石英中捕获有四种类型包裹体:在超高压-高压榴辉岩相条件下捕获的 N_2±CO_2包裹体;在高压榴辉宕重结晶阶段被捕获的 CO_2-H_2O 包裹体和含子矿物高盐度 H_2O 溶液包裹体:在超高压岩石折返过程中的最晚(角闪岩相退变质甚至更晚)阶段捕获的低盐度 H_2O 溶液包裹体。利用榴辉岩矿物及脉体石英中捕获的流体包裹体相互期次关系,可以对本区超高压变质作用板片折返过程中的流体演化史进行重建。     

南苏鲁超高压带和高压带边界的准确限定——来自变质岩锆石中矿物包体的证据

南苏鲁地区由 4个岩片组成 ,自西北至东南依次为正片麻岩岩片 (Ⅰ )、表壳岩岩片 (Ⅱ )、含蓝晶石石英岩大理岩岩片 (Ⅲ )和副片麻岩变火山岩片岩岩片 (Ⅳ )。上述构造岩片均经历了强烈的角闪岩相绿片岩相退变质作用的改造。采用激光拉曼、阴极发光和电子探针分析技术 ,对南苏鲁方圆约 380 0km2 范围内 93件锆石样品中的矿物包体进行了系统鉴定。结果表明 ,在第Ⅰ和Ⅱ岩片样品的锆石中均发现以柯石英为代表的超高压 (UHP)矿物包体 ,而在第Ⅲ和Ⅳ岩片锆石中则保存以文石和多硅白云母为代表的高压 (HP)矿物包体。由此推断 ,第Ⅰ和Ⅱ岩片应归属超高压变质带 ,而第Ⅲ和Ⅳ岩片应归属高压变质带。结合变质作用温压条件的研究结果 ,确定超高压岩石峰期变质温压条件为t=72 3～ 85 2℃ ,p≥ 2 .8× 10 3 MPa ;而高压变质岩石峰期变质温压条件为t =5 0 0～ 6 0 0℃ ,p =1.2× 10 3 ～ 2 .5× 10 3 MPa。最新区域填图结果表明 ,南苏鲁东海地区超高压和高压带之间的接触界线为一典型的韧性剪切带。     

A TEM study of the oriented orthopyroxene and forsterite inclusions in garnet from Otrøy garnet peridotite,WGR, Norway: new insights on crystallographic characteristics and growth energetics of exsolved pyroxene in relict majoritic garnet

Regularly oriented orthopyroxene (opx) and forsterite (fo) inclusions occur as opx + rutile (rt) or fo + rt inclusion domains in garnet (grt) from Otrøy peridotite. Electron diffraction characterization shows that forsterite inclusions do not have any specific crystallographic orientation relationships (COR) with the garnet host. In contrast, orthopyroxene inclusions have two sets of COR, that is, COR‐I: <111>_grt//<001>_opx and {110}_grt～//～{100}_opx (～13° off) and COR‐II: <111>_grt//<011>_opx and {110}_grt～//～{100}_opx (～14° off), in four garnet grains analysed. Both variants of orthopyroxene have a blade‐like habit with one pair of broad crystal faces parallel/sub‐parallel to {110}_grt plane and the long axis of the crystal, <001>_opx for COR‐I and <011>_opx for COR‐II, along <111>_grt direction. Whereas the lack of specific COR between forsterite and garnet, along with the presence of abundant infiltrating trails/veinlets decorated by fo + rt at garnet edges, provide compelling evidence for the formation of forsterite inclusions in garnet through the sequential cleaving–infiltrating–precipitating–healing process at low temperatures, the origin of the epitaxial orthopyroxene inclusions in garnet is not so obvious. In this connection, the reported COR, the crystal habit and the crystal growth energetics of the exsolved orthopyroxene in relict majoritic garnet were reviewed/clarified. The exsolved orthopyroxene in a relict majoritic garnet follows COR‐III: {112}_grt//{100}_opx and <111>_grt//<001>_opx. Based on the detailed trace analysis on published SEM images, these exsolved orthopyroxene inclusions are shown to have the crystal habit with one pair of broad crystal faces parallel to {112}_grt//{100}_opx and the long crystal axis along <111>_grt//<001>_opx. Such a crystal habit can be rationalized by the differences in oxygen sub‐lattices of both structures and represents the energetically favoured crystal shape of orthopyroxene inclusions in garnet formed by solid‐state exsolution mechanism. Considering the very different COR, crystal habit, as well as crystal growth direction, the orthopyroxene inclusions in garnet of the present sample most likely had been formed by mechanism(s) other than solid‐state exsolution, regardless of their regularly oriented appearance in garnet and the COR specification between orthopyroxene and garnet. In fact, the crystallographic characteristics of orthopyroxene and the similar chemical compositions of garnet at opx + rt inclusion domains, fo + rt inclusion domains/trails and garnet rim suggest that the orthopyroxene inclusions in the garnet are most likely formed by similar cleaving‐infiltration process as forsterite inclusions, though probably at an earlier stage of metamorphism. This work demonstrates that the oriented inclusions in host minerals, with or without specific COR, can arise from mechanism(s) other than solid‐state exsolution. Caution is thus needed in the interpretation of such COR, so that an erroneous identification of exhumation from UHP depths would not be made.     

南苏鲁芝麻房石榴石橄榄岩中橄榄石的“C”类组构及其形成条件探讨

上地幔地震的各向异性主要归因于橄榄石的优选方位,不同的橄榄石优选方位模式可以作为上地幔不同动力学作用的指示剂。不同应力和含水量条件下的高温变形实验已经确定出五类橄榄石组构模式(“A”型、“B”型、“C”型、“D”型和“E”型)。本文运用电子背散射(EBSD)技术对来自苏鲁超高压变质带南部的芝麻房石榴石橄榄岩的橄榄石进行了优选方位测定,不同变形程度的橄榄石均显示了[100]轴近垂直于面理和[001]轴近平行于线理的特征,为“C”类组构模式,可见组构类型与变形程度没有关系,并且橄榄石组构所显示的NW向SE的剪切指向,与围岩-正、副片麻岩中形成于折返过程的石英优选方位所显示的SE向NW的剪切指向完全不同,说明芝麻房石榴石橄榄岩中橄榄石的“C”类组构是折返前形成的。结合橄榄石结构水的测量和已有的芝麻房石榴石橄榄岩形成的温压条件,推测该组构形成于含水俯冲带中,认为芝麻房石榴石橄榄岩的原岩来自于高含水的上部地幔楔碎块,与俯冲的陆壳物质一起经历了超高压变质作用并最终折返至地表。