Blocks Of Amphibolite Research Articles

Collisional orogeny between the Siberian and Sino-Korean paleoplates: Evidence from geochronology and metamorphism of the metamorphic ophiolite mélange in the Banlashan area near the Xar Moron River in the southeastern Central Asian orogenic belt

As the largest Phanerozoic accretionary orogenic belt, whether the Central Asian Orogenic Belt (CAOB) experienced a strong collisional orogenic process after Paleo-Asian Ocean (PAO) closure remains controversial. The age and P-T conditions recorded by metamorphic rocks could be used to study the thermal and tectonic regimes during orogenic processes, providing constraints for orogenic evolution. In this paper, the geochronology and metamorphism of a newly discovered metamorphic ophiolite mélange in the Banlashan area near the Xar Moron River in the southeastern CAOB were studied. The association and age of the metamorphic ophiolite mélange protoliths are similar to those of the Xar Moron River ophiolite belt to the north. The metamorphic ages of different ophiolite mélange lithologies are all late Permian (250.2–256.5 Ma), indicating that the Siberian and Sino-Korean paleoplates collided during the late Permian. The metamorphic pressure and temperature recorded by the garnet amphibolite block in the ophiolite mélange could reach 8.5–8.8 kbar and 700–725 ℃. The core-to-rim zoning of garnet indicates that the garnet amphibolite underwent a clockwise P–T path, which is the typical metamorphic feature of collisional orogens. The ophiolite mélange metamorphic features, combined with regional magmatic, metamorphic, sedimentary and tectonic deformation data, indicate that a strong collisional orogeny occurred rather than a soft orogeny between the Siberian and Sino-Korean paleoplates after the PAO closure.

Molybdenum isotope behavior during subduction zone metamorphism

The molybdenum (Mo) isotope composition (defined as δ98Mo measured per mil relative to NIST-3134) of many modern arc systems and the upper continental crust is heavier than the mantle and most subducting slab lithologies. This observation has led to a model whereby fluids leaving the slab transfer isotopically heavy Mo preferentially to the mantle wedge, leaving the residual slab isotopically lighter. We explore this model via an Mo isotope study of the metasedimentary and mélange lithologies of the Catalina Schist in California. These rocks record subduction zone metamorphism over a wide range of high-pressure/low-to-medium temperature conditions.Mo isotope compositions of the metasedimentary rocks decrease with increasing metamorphic grade, from a δ98Momean of −0.04 ± 0.13 ‰ (1σ, n = 3) for the lawsonite albite facies to −0.38 ± 0.07 ‰ (1σ, n = 5) for the epidote–amphibolite facies. The highest grade [amphibolite facies] samples show a slight uptick in δ98Mo values, with a mean of −0.19± 0.14 ‰ (1σ, n = 4). When coupled with major and trace element variations, the changes in δ98Mo appear to reflect metamorphic effects rather than sedimentary source rock heterogeneity. The positive relationship between δ98Mo and [Mo] and negative relationship with Ce/Mo argue for progressive loss of Mo with isotope fractionation during increasing P-T conditions; the reversal of this trend, seen in the increase in δ98Mo between the epidote amphibolite and amphibolite facies may reflect a subsequent partial re-fertilization by fluids carrying isotopically heavy Mo. δ98Mo values in the mélange across all grades are highly variable, ranging from −1.75 to −0.19, consistent with large scale mobilization of Mo with isotope fractionation. A metasomatized amphibolite block and reaction rind show similar light isotope compositions and signs of Mo depletion and transport. Together these three whole-rock data sets demonstrate pervasive open system behavior and mobility of Mo in the Catalina Schist. LA-ICP-MS Mo measurement of minerals in the differing metasedimentary metamorphic grades indicates that Fe oxides and/or hydroxides, titanite, rutile, and epidote are important reservoirs of Mo but when comparing mineral Mo content and modal abundance to whole-rock Mo concentrations, several samples were found to have “missing Mo”, something that has been observed in other Mo inventory studies. We attribute this to sample heterogeneity between the thin section and whole-rock powder scales.

Geology of the Mutis Complex, Miomaffo, West Timor

Mesozoic forearc assemblages are widespread across Indonesia, and these are also exposed in the overthrust terranes on Timor. The oldest rocks in the allochthonous Mutis Complex, West Timor are Mesozoic basaltic volcaniclastic rocks and melange containing blocks of normal mid-ocean ridge basalt (N-MORB) basalt (194 ± 5 Ma), amphibolite (metamorphism at 184 ± 6 Ma), garnet and actinolite-bearing schists, arkosic sandstone and volcanogenic sedimentary rocks. Based on the large component of Jurassic material, these rocks were probably derived from the Woyla terrane. The eastern part of the Miomaffo massif is complexly deformed with a dominant foliation overprinted by several fold and fault events including late normal cataclastic zones. This sequence is intruded by calc-alkaline andesitic dykes (possibly in the Eocene). The Central Sector of the massif is a block of high-strain greenschist facies rocks where the primary texture can no longer be recognised, but the bulk composition is the same as the volcaniclastic rocks in the east. Further west is an amphibolite facies metamorphic province. Amphibolite from the Western Sector have the same N-MORB composition as the basalt in the east, but the gneissic rocks have a higher proportion of arc-sourced protoliths. The peak metamorphic conditions were 650 °C and 0.9 GPa. This metamorphic event occurred at 37 Ma, reflecting subduction on the southeast margin of Sundaland, whereas the low-grade Mesozoic metamorphism of the Eastern Sector of the massif occurred in the accretionary prism along the Woyla Arc. The Mutis Complex at Miomaffo demonstrates the complex geological history of Mesozoic rocks in eastern Indonesia.

The slab–mantle wedge interface of an incipient subduction zone: Insights from the P–T–D evolution and petrological characteristics of the Dalrymple Amphibolite, Palawan Ophiolite, Philippines

AbstractIn this contribution, we present the petrological characteristics and pressure–temperature–deformation (P–T–D) history of the Dalrymple Amphibolite in Palawan Island, Philippines. This unit occurs below the residual mantle peridotites of the Palawan Ophiolite composed of interlayered dunite and harzburgite. The Dalrymple Amphibolite is predominantly composed of high‐grade metamafic blocks with minor metasedimentary units surrounded by a highly sheared hybridized matrix (kyanite + Ca‐amphibole + biotite + ilmenite ± garnet). Zr‐in‐rutile geothermometry and quartz‐in‐garnet geobarometry of the garnet amphibolite block reveal the P–T conditions of the prograde metamorphism from ~625°C, 11.5 kbar to ~700°C, 13 kbar. Zr‐in‐rutile geothermometer and the TZARS geobarometer (clinozoisite + rutile + quartz = anorthite + titanite + H2O) further indicate comparable peak metamorphic conditions for the amphibolite and epidote amphibolite (~605–710°C, 10–13 kbar) blocks. Geothermobarometry of the matrix surrounding the blocks reveals similar peak metamorphic conditions of ~700°C and 13 kbar to the garnet amphibolite blocks. The paleogeothermal gradient preserved in the Dalrymple Amphibolite (~16°C/km) and its block‐in‐matrix structure are atypical of metamorphic soles and are more akin to high‐T mélange complexes interpreted to have formed during incipient subduction. These P–T estimates and its block‐in‐matrix structure suggest that the Dalrymple Amphibolite likely represents the slab–mantle wedge interface of a young arc complex already transitioning from the much warmer paleogeothermal gradients of the slab–mantle wedge interface when subduction was first initiated. The similar peak metamorphic conditions constrained in the matrix and the metamafic blocks in the Dalrymple Amphibolite indicate that the matrix‐forming deformation started before the sequence reached peak metamorphic conditions at depths of around 45 km. Furthermore, the petrological and geochemical characteristics of this block‐in‐matrix sequence and the multiple generation of cross‐cutting veins reveal the complex processes that occur in the slab–mantle wedge interface of an arc in its infancy. Our results highlight the prevalence of Ca‐amphibole and kyanite over phyllosilicates (e.g., chlorite and serpentine) in the mélange matrix, mixing of crustal components along the slab–mantle wedge interface promoted by multiple deformation events and the ubiquity of fluids throughout its history. Subduction zones with similar geothermal gradients (e.g., warm subduction zones) possibly share these petrological characteristics.

Rb–Sr geochronology of metamorphic rocks from the Central Indonesian Accretionary Collision Complex: Additional age constraints for the Meratus and Luk Ulo complexes (South Kalimantan and Central Java)

Metamorphic rocks of the Central Indonesian Accretionary Collision Complex are exposed in widely scattered occurrences on the islands of Java, Sulawesi, and Kalimantan. Their relationship to one or more subduction zones is controversial which can partly be explained by insufficient geochronological investigation. Using Rb–Sr geochronology we have dated various schists from the Meratus Complex, South Kalimantan, and the Luk Ulo Complex, Central Java. The Meratus schists first experienced high-pressure/low-temperature (HP/LT) metamorphism and were later overprinted at medium pressure conditions. Due to the rarity and poor preservation of such rocks, isotopic ages for well-preserved HP/LT rocks could not be determined yet. Previously obtained K–Ar white mica dates (180–160 Ma) were attributed to the HP/LT event, however, this was speculative as the dated rocks only record greenschist- to amphibolite-facies P–T conditions. A newly dated partially overprinted but still glaucophane-rich sample yielded a well-defined Rb–Sr isochron age (phengite, epidote) of 112.9 ± 0.7 Ma (all uncertainties on Rb–Sr dates indicate 95% confidence level), providing a minimum age for the HP/LT stage. Internal mineral isochrons of more severely overprinted samples show poor linear fits between mica and associated phases indicating incomplete resetting of the Rb–Sr isotope system during the later lower pressure metamorphic event. Isochrons that are derived only from different grain-size fractions of white mica indicate dates between 115 and 110 Ma (± 3 to 6 Ma). In contrast to previous interpretations that linked high-grade rocks of the Meratus Complex to collision-induced crustal thickening it is here considered more likely that these rocks either represent exotic fragments from NW Australia or the metamorphic sole of the Meratus ophiolite. Differences in temperature estimates for HP/LT rocks from the Luk Ulo Complex probably indicate erroneous textural interpretation of equilibrium mineral assemblages and/or compositions. Neither petrological analysis nor geochronological dating has yet unambiguously confirmed different P–T–t paths for eclogites with and without tourmaline (Trm) from the Luk Ulo area. The newly dated Trm-bearing epidote blueschist from the serpentinitic Muncar River mélange yielded a well-defined Rb–Sr isochron of 119.0 ± 0.7 Ma (phengite, epidote, tourmaline) that perfectly matches with existing K–Ar and Rb–Sr data for Trm-free HP/LT rocks. Two samples of the more widespread garnet-phengite schists of the Luk Ulo area yielded Rb–Sr ages (phengite, feldspar) of 118.0 ± 0.8 Ma and 114.7 ± 1.3 Ma. A meter-sized epidote amphibolite block included in these clastic metasedimentary rocks provided a Rb–Sr isochron age of 116.8 ± 0.7 Ma (phengite, epidote, whole rock). These ages match existing K–Ar data for severely overprinted rocks from the Luk Ulo area and further corroborate the geological significance of Early Cretaceous metamorphism in Central Java. Petrological and geochronological data of the Meratus and Luk Ulo complexes are in accordance with a model suggesting that both occurrences belong to the same Cretaceous subduction system. The existence of an additional Cretaceous suture zone cutting through Sulawesi is considered likely.

Protolith and tectonic implications of the No. Ⅳ pluton in Xiarihamu area, East Kunlun, Qinghai Province

东昆仑造山带西段夏日哈木地区IV号岩体是镁铁-超镁铁质侵入体还是蛇绿岩单元仍存争议。本文对IV号岩体开展了野外地质特征、岩相学、地球化学及锆石U-Pb年代学的研究。IV号岩体主体为斜长角闪岩，呈透镜状产在片麻岩中。斜长角闪岩主要由角闪石、斜长石组成，含少量石英、黑云母、榍石和铁钛氧化物。具有基性岩的化学成分，属于拉斑玄武岩系列岩石，稀土配分模式中Eu显示正异常（δEu=1.3~1.6），ε_Nd（t）值为+3.5~+5.3，推测其原岩可能为堆晶辉长岩，明显不同于含铜镍矿的Ⅰ号镁铁-超镁铁岩体，与该地区出露的榴辉岩（退变榴辉岩）可以对比。斜长角闪岩中的锆石具有弱环带，采用LA-MC-ICP-MS方法得出的²⁰⁶Pb/²³⁸U加权平均年龄为421±4Ma （MSWD=2.8），Th/U比值为0.01~3.26，指示锆石为变质-深熔成因，代表了斜长角闪岩的形成时代。夏日哈木地区的斜长角闪岩和该地区出露的榴辉岩及榴闪岩的原岩可能都是早期形成于陆壳环境的基性岩（大陆边缘的基性侵入岩或喷出岩），在430Ma左右发生陆壳深俯冲作用时形成榴辉岩，420~410Ma榴辉岩发生不同程度退变形成榴闪岩和斜长角闪岩。由此推测，IV号岩体的构造属性与该地区的榴辉岩属性相同，形成于陆壳环境，其原岩不属于蛇绿岩单元。斜长角闪岩的出现指示了该地区原特提斯洋壳已俯冲消亡，深俯冲的陆壳开始折返，这对认识东昆仑早古生代的构造演化和铜镍矿形成的地质背景具有重要意义。;Whether the protolith of the No. IV pluton in Xiarihamu area in the western part of the East Kunlun Orogenic Belt belongs to a mafic-ultramafic intrusive body or a component (gabbro) of the ophiolitic unit remains debated. Here, we reported the occurrence and the petrographic, geochemical and zircon U-Pb geochronological analysis results of this pluton to clarify the above disagreement. Petrographically, this pluton is an amphibolite block occurring as lens within a set of gneisses. The amphibolite rocks mainly consist of hornblende and plagioclase, with minor quartz, biotite, titanite and Fe-Ti oxide. The samples collected from amphibolite pluton have SiO₂ contents varying from 43% to 50% with Mg^# values from 42 to 73, and chondrite-normalized REE patterns show they have a positive Eu anomaly (δEu=1.3~1.6), implying their protoliths were cumulative mafic rocks and belong to tholeiitic series. Furthermore, the amphibolite rocks have positive whole-rock ε_Nd(t) values of +3.5~+5.3, similar to those of the eclogite or retrograded-eclogite and different from those of the No. I Ni- and-Cu-bearing mafic-ultramafic rocks in Xiarihamu area. Zircon grains from the amphibolite pluton show weakly zonation on their CL images with Th/U ratios varying from 0.01 to 3.26, suggesting they have a metamorphic-anatexis origin rather than a magmatic one. The zircon grains yield a ²⁰⁶Pb/²³⁸U weight mean age of 421±4Ma (MSWD=2.8), which represent the formation time of the pluton. In other words, the formation of this pluton can be divided into two stages: the previous cumulate gabbro underwent a high-pressure metamorphism and formed eclogite in the subduction stage, then followed by a retrograde stage which led to the transformation from eclogite to amphibolite. To be more clearly, the protoliths of the amphibolite, eclogite and retrograded-eclogite in the Xiarihamu area are mafic rocks (mafic pluton or eruption) formed in a continental margin, they turned into eclogites during a deep continental subduction approximately at 430Ma, and finally became retro-eclogite and amphibolite with various retrograde metamorphism at 420~410Ma. Therefore, the protolith of the No. IV pluton is not a gabbro component of the ophiolitic unit. Our studying results indicate that the evolution of this region experienced a closing of Paleozoic Proto-Tethys oceanic basin and then an exhumation of subducted continental crust, which is important for understanding the Paleozoic tectonic evolution of East Kunlun Orogen and the geological setting of the Cu-Ni mineralization.

Paleoproterozoic oceanic subduction in the North China Craton: Insights from the metamorphic P–T–t paths of the Chicheng Mélange in the Hongqiyingzi Complex

How and when modern-style plate tectonics started in the Earth’s history is one of the most important scientific issues. The Chicheng mélange in the north margin of North China Craton (NCC) presents a new insight into the Paleoproterozoic cold oceanic subduction and the beginning of modern-style plate tectonic regime. The mélange comprises blocks of retrograde eclogites, amphibolites and peridotites in a matrix of metasediments. The retrograde eclogites mainly contain garnet, amphibole and symplectites consisting of fine-grained plagioclase, clinopyroxene and amphibole. Three metamorphic stages were derived, involving the prograde, peak, and post-peak decompression stages. The P–T conditions for the prograde and peak stages range from ~17 kbar/660 °C to >22/~750 °C kbar based on mineral composition isopleths in P–T pseudosections, involving core–to–mantle increasing pyrope and decreasing grossular in garnet and Ti content in amphibole. The post-peak decompression is proposed to be isothermal under melt (fluid)-absent conditions, recorded a pressure of 8–11 kbar on the basis of symplectite assemblages. For the amphibolites pre-peak heating, peak temperature, and post-peak cooling metamorphic stages were derived. They experienced anataxis during this overprinting metamorphism when heated to above the solidus. The peak temperature conditions are constrained to be 5–7 kbar/780–830 °C on the basis of the contours of the maximum Ti in amphibole and anorthite of plagioclase in P–T pseudosections. Zircon U–Pb dating for the selected samples and a summary of published age data reveal that the eclogite facies metamorphism may have occurred at 1.85–1.80 Ga, and the eclogitic rocks exhumed to a lower crust level during or after subduction to be overprinted by multi-phase thermal events at ~1.66 Ga and 460–420 Ma with undefined conditions, and at 360–270 Ma under amphibolite facies. The eclogite facies metamorphism from the Chicheng mélange shows a geothermal gradient of ~9 °C/km, pointing to a cold oceanic subduction at late Paleoproterozoic that occurred along the northern margin of the NCC. The overprinting metamorphism with anataxis recognized from the amphibolites represents a low-pressure type with a geothermal gradient of ~35 °C/km, which was interpreted to occur at 360–270 Ma governed by extension.

Subduction re-initiation at dying ridge of Neo-Tethys: Insights from mafic and metamafic rocks in Lhaze ophiolitic mélange, Yarlung-Tsangbo Suture Zone

Ophiolitic mélanges provide critical information on the history of subduction and metamorphism and, thus, on the evolution of ancient oceans. Blocks of mafic rocks, garnet-clinopyroxenites and amphibolites have recently been discovered in the Lhaze ophiolitic mélange of the Yarlung-Tsangbo Suture Zone, Southern Tibet. These blocks represent different origins from the Neo-Tethys. The unmetamorphosed MORB-type mafic rocks are considered as the oceanic crusts formed during the spreading of the dying mid-ocean-ridges. The garnet-clinopyroxenites and amphibolites are interpreted to be parts of dismembered metamorphic soles beneath the lowest part of the ophiolites. In spite of different mineral assemblages and metamorphic facies, both clinopyroxenites and amphibolites show strong affinities to the oceanic crust rocks, as evidenced by their geochemical and isotopic data. In addition, the inherited igneous zircon U–Pb results indicate that the protolith ages of these metamorphic rocks are 124–125 Ma, which are coeval with the formation ages of oceanic crusts and the metamorphic ages of amphibolites within the YTSZ. Geothermobarometry calculations indicate that these rocks went through metamorphism to peak conditions of granulite-facies in excess of 900°C at minimum pressure of 9 kbar for garnet-clinopyroxenites and of high-amphibolite facies of 650°C at 7.5 kbar for amphibolites, corresponding to a quite high geothermal gradient (25–30°C/km). Such a high geotherm implies that the lithospheric mantle and the subducted oceanic crusts were nascent and still hot when the mantle obducted onto the ocean crust of the Neo-Tethys. A scenario of subduction re-initiation at the dying mid-ocean-ridge close to the Eurasian continental margin is proposed to reveal the evolution of the Neo-Tethys and to provide new insights into the mechanism of formation of ophiolites and metamorphic soles worldwide.

Petrogenesis and 40Ar/39Ar dating of proto-forearc crust in the Early Cretaceous Caribbean arc: The La Tinta mélange (eastern Cuba) and its easterly correlation in Hispaniola

ABSTRACTThe La Tinta mélange is a small but singular ultramafic mélange sheet that crops out in eastern Cuba. It is composed of dolerite-derived amphibolite blocks embedded in a serpentinite matrix. The amphibolite blocks have mid-ocean ridge basalt (MORB)-like composition showing little if any imprint of subduction zone component, similar to most forearc and MOR basalts worldwide. Relict Cr-spinel and olivine mineral chemistry of the serpentinized ultramafic matrix suggest a forearc position for these rocks. These characteristics, together with a hornblende 40Ar/39Ar age of 123.2 ± 2.2 Ma from one of the amphibolite blocks, suggest that the protoliths of the amphibolite blocks correspond to forearc basalt (FAB)-related rocks that formed during the earlier stage of subduction initiation of the Early Cretaceous Caribbean arc. We propose that the La Tinta amphibolites correspond to fragments of sills and dikes of hypoabyssal rocks formed in the earlier stages of a subduction initiation scenario in the Pacific realm (ca. 136 Ma). The forearc dolerite-derived amphibolites formed by partial melting of upwelling fertile asthenosphere at the beginning of subduction of the Proto-Caribbean (Atlantic) slab, with no interaction with slab-derived fluids/melts. This magmatic episode probably correlates with Early Cretaceous basic rocks described in Hispaniola (Gaspar Hernandez serpentinized peridotite-tectonite). The dikes and sills cooled and metamorphosed due to hydration at low pressure (ca. 3.8 kbar) and medium to high temperature (up to 720ºC) and reached ca. 500ºC at ca. 123 Ma. At this cooling stage, serpentinite formed after hydration of the ultramafic upper mantle. This process might have been favoured by faulting during extension of the forearc, indicating an early stage of dike and sill fragmentation and serpentinite mélanges formation; however, full development of the mélange likely took place during tectonic emplacement (obduction) onto the thrust belt of eastern Cuba during the latest Cretaceous.

P–T conditions of metabasites within regional metapelites in far-eastern Nepal Himalaya and its tectonic meaning

Metabasites exposed in far-eastern Nepal provide an important insight into the metamorphic evolution of the Himalayan orogen independent from data obtained on metapelites. The P–T conditions and formation process of mafic granulite intercalated within Early Oligocene migmatites and two amphibolites surrounded by Early Miocene metapelites were inferred from pseudosection modeling and conventional geothermobarometry combined with the occurrences of field and microstructures. A mafic granulite in the Higher Himalaya Crystalline Sequence (HHCS) yields P–T conditions of 6.5–8 kbar, 730–750 °C. The similar peak P–T condition and retrograde path with low P/T gradient of mafic granulite and surrounding migmatite indicate that both rocks were simultaneously metamorphosed and exhumed together along the tectonic discontinuities in the HHCS. In contrast, the P–T conditions (2–5 kbar, 500–600 °C) of highly-deformed amphibolite block above the Main Central Thrust (MCT) records significantly lower pressure than garnet-mica gneisses in the country rock, suggesting that the amphibolite block derived from upper unit of the MCT zone and became tectonically mixed with the gneisses of hanging wall near the surface. An amphibolite lense below the MCT preserves the prograde P–T conditions (6–7.5 kbar, 550–590 °C) of Early Miocene syn-tectonic metamorphism that occurred in the MCT zone. This study indicates the top-to-the south movement of the MCT zone results in the tectonic assembly of rocks with different P–T–t conditions near the MCT.

Metamorphic evolution of garnet amphibolite in the western Dabieshan eclogite belt, Central China: Evidence from petrography and phase equilibria modeling

The western Dabieshan terrane in central China involves numerous blocks of garnet amphibolite and eclogite enclosed in felsic gneiss and schist. Two representative garnet amphibolite samples SHS22 and BSW05 were selected for detailed study. They are composed of garnet, hornblendic amphibole, sodic plagioclase, epidote, quartz, biotite and/or muscovite, and small amounts of rutile, sphene and ilmenite. A few phengite flakes are found as inclusions in garnet in sample SHS22. Garnet varies in grain size and rim composition. The coarse-grained garnet porphyroblasts are mostly anhedral and exhibit compositional zoning with increasing pyrope and decreasing or constant grossular content from core to rim. Pseudosections calculated in the system NCKMnFMASHO suggest that this garnet zoning reflects prograde P–T paths of early-stage eclogite facies from 1.9GPa at 490°C to 2.3GPa at 555°C for sample SHS26 and from 1.8GPa at 450°C to 2.2GPa at 540°C for sample BSW05. The peak P–T conditions are constrained by the garnet rim compositions with the maximum pyrope (Xprp) and minimum grossular (Xgrs) contents. The Si contents in phengite from sample SHS22 were modeled to yield pressure conditions similar to that constrained by garnet. The other garnet rim compositions with decreasing Xprp and increasing Xgrs contents were modeled in P–M (H2O) pseudosections to reflect different degrees of the modification of eclogite facies to amphibolite facies in fluid-unsaturated conditions, indicating decreasing pressures from 0.7 to 0.5GPa and increasing water contents from 3.6 to 5.2mol.% at T=575°C for sample SHS22, and decreasing pressures from 0.8 to 0.55GPa and increasing water contents from 3.5 to 4.7mol.% at T=530°C for sample BSW05, respectively. The garnet rims with minimum Xprp and maximum Xgrs were predicted to be close to an equilibrium state with the matrix minerals involving hornblendic amphibole, plagioclase, epidote, biotite and quartz. The constituting assemblage was closely saturated with water. As it is hard to evaluate if the relevant minerals are in equilibrium or not in the garnet amphibolite with multi-generation assemblages, the conventional thermobarometers are not very useful for estimating the P–T conditions for this type of rock.

Serpentinites and serpentinites within a fossil subduction channel: La Corea mélange, eastern Cuba

A variety of metaultramafic (serpentinite) rocks in La Corea melange, Sierra de Cristal, eastern Cuba, show differences in chemical, textural and mineralogical characteristics demonstrating a variety of protoliths. The melange originated during the Cretaceous as part of the subduction channel associated with the Caribbean island arc. This melange contains high pressure blocks in a serpentinite matrix and occurs at the base of the large tabular Mayari-Cristal ophiolite. Two principal groups of serpentinites have been identified in the melange: a) antigorite serpentinite, mainly composed of antigorite and b) antigorite-lizardite serpentinite, composed of mixtures of antigorite and lizardite and bearing distinctive porphyroblasts of diopsidic clinopyroxene. Antigorite serpentinites are closely related to tectonic blocks of amphibolite (representing subducted MORB) and constitute deep fragments of the serpentinitic subduction channel formed during hydration of the mantle wedge. The composition of the antigorite-lizardite serpentinites and the presence of clinopyroxene porphyroblasts in this type of rock suggest that abyssal lherzolite protoliths transformed into serpentinite before and during incorporation (as tectonic blocks) in the shallow part of the subduction channel. Although the studied rocks have different origin, mineralogical compositions and textures, they display similar PGE compositions, suggesting that these elements experienced no significant redistribution during metamorphism. Both types of serpentinites were exposed together in the La Corea melange during the Late Cretaceous, during obduction of the overriding Mayari-Baracoa ophiolitic belt that led to exhumation of the subduction channel (melange)

Mantle–crust interactions in a paleosubduction zone: Evidence from highly siderophile element systematics of eclogite and related rocks

Substantial differences in 187Os/188Os and absolute and relative abundances of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) in mantle peridotites compared to oceanic crust indicate that this suite of elements may prove useful in assessing mechanisms and pathways of mantle and slab mass transfer within the subduction channel. Currently, however, information regarding the mobility of the HSE in subduction zone environments is limited. To better understand the systematics of highly siderophile elements in subduction settings, we measured Os isotopic compositions and HSE abundances of cores and rinds of meter-scale blocks of eclogite, blueschist and garnet amphibolite from subduction-related mélanges within 1) the Franciscan Complex, CA, and a related terrane in Oregon; 2) the Catalina Schist, CA; and 3) the Samana Metamorphic Complex, Samana Peninsula, Dominican Republic. Rinds composed of amphibole, chlorite, ± phengite, talc, titanite and rutile partly enclose blocks at each location. Mineralogic, petrologic and geochemical data suggest that these rinds are metasomatic features that formed by fluid-mediated interaction between mafic blocks and an ultramafic matrix.The cores of high-grade blocks are characterized by high (Pt+Pd)/(Os+Ir+Ru), and variably radiogenic Os; initial 187Os/188Os ranges from 0.197 to 4.30. These characteristics are consistent with the HSE compositions of subducted basalts. In contrast, (Pt+Pd)/(Os+Ir+Ru) of rinds are generally much lower than cores, approaching values typical of mantle peridotites. The initial 187Os/188Os of most rinds are also typical of upper mantle peridotites; values for most rinds fall between 0.125 and 0.14. The similarities of the 187Os/188Os and HSE abundances between rinds and mantle peridotites indicate that the HSE contained within rinds reflect a contribution by mantle peridotite, whether by fluid-mediated transfer or through mechanical processes. If the HSE signatures of the rinds were generated by fluid-mediated metasomatic processes, these data require remarkably efficient and uniform transference of mantle HSE from peridotitic mélange matrix to the exteriors of mélange blocks during rind formation. More likely, the rinds are coatings of materials that contain HSE derived from peridotite precursors. These results suggest that centimeter-scale mass transfer by physical mechanisms may be important for some elements within the subduction channel.

Occurrence and petrology of amphibolite block in the ealiest Archean granitoid, the Acasta Gneiss Complex

Occurrence and petrology of amphibolite block in the ealiest Archean granitoid, the Acasta Gneiss Complex

Discovery of a dismembered metamorphic sole in the Saga ophiolitic mélange, South Tibet: Assessing an Early Cretaceous disruption of the Neo-Tethyan supra-subduction zone and consequences on basin closing

Blocks of strongly foliated garnet- and clinopyroxene-bearing amphibolites have recently been discovered in the Saga ophiolitic mélange, South Tibet. The Saga ophiolitic mélange is a sheared serpentinite matrix mélange that crops out along the Yarlung Zangbo Suture Zone (YZSZ), South Tibet. The YZSZ is the youngest and the southernmost of all sutures stretching across the Tibetan Plateau and contains the remnants of the Neo-Tethys ocean that once separated India from the Lhasa block. The garnet- and clinopyroxene-bearing amphibolite blocks are interpreted as parts of a dismembered sub-ophiolitic metamorphic sole. They are made mainly of hornblende, diopside and garnet and were strongly metasomatized during retrogression. Thermobarometry and 40Ar/39Ar dating on hornblende indicate that they were metamorphosed to peak conditions in excess of 12kbar and 850°C between 132 and 127Ma. Major and trace element geochemistry suggest an N- to E-MORB-type protolith, although a back- or inter-arc basin setting cannot be ruled out. This data supports a model in which the back-arc YZSZ ophiolites (central segment) were trapped in a fore-arc setting by the inception of a subduction at the back-/inter-arc ridge. The cause of this important Early Cretaceous tectonic event might be the presence of an oceanic plateau or hot-spot tracks in the Neo-Tethys, the collision of the Lhasa and Qiangtang blocks in the North and/or the onset of northward movement of the Indian continent in the South. The proposed conceptual model provides an explanation for the absence (or rarity) of Late Cretaceous ophiolites along the YZSZ which are mostly of Late Jurassic to Early Cretaceous age.

Timing of subduction and exhumation in a subduction channel: Evidence from slab melts from La Corea Mélange (eastern Cuba)

High pressure igneous rocks (tonalites), generated by partial melting of subducted basaltic rocks accreted to the mantle wedge, are present in the La Corea serpentinite-matrix mélange (eastern Cuba) as centimeter- to meter-sized blocks and as concordant to crosscutting veins within high-pressure parent amphibolite blocks. The slab melts have adakitic signatures, in agreement with formation after partial melting of metabasite. Thermobarometric calculations indicate 620–680°C and 13–15kbar during crystallization of tonalites and down to 250–300°C, 6kbar during retrogression, indicating counter-clockwise P–T paths (hot subduction-cool exhumation). Free water required for melting of amphibolite at moderate temperature (700–750°C) and moderate pressure (13–16kbar) close to the wet basaltic solidus is inferred to have been provided after dehydration of sediments, altered basaltic crust and serpentinite of the subducting Proto-Caribbean lithosphere. Single zircon (SHRIMP) and phengite 40Ar/39Ar age data constrain the P–T–t evolution of the mélange from the timing of crystallization of melts at ~110–105Ma to cooling at ~87–84Ma, ca. 350°C, ca. 9kbar. These figures are consistent with subduction of an oblique ridge, shortly before 115Ma. Furthermore, our data indicate very slow exhumation (ca. 1mm/yr) in the subduction channel during the oceanic convergence stage (120–70Ma) until final fast exhumation to the surface occurred at 70–65Ma during a regional arc-platform collision event.

The imprint of subduction fluids on subducted MORB-derived melts (Sierra del Convento Mélange, Cuba)

Major and trace element signatures and Sr–Nd–Pb isotope data for muscovite (Ms)-bearing amphibolite blocks and associated muscovite-bearing trondhjemite and quartz-muscovite rocks from the Sierra del Convento mélange (eastern Cuba) indicate that Proto-Caribbean oceanic crust underwent wet partial melting processes during Mesozoic subduction and after accretion to the upper plate. Trace element normalized patterns of Ms-bearing amphibolites are enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE) and evidence variable trace element transfer from the Proto-Caribbean subducting slab to the mantle wedge. Ms-bearing trondhjemites show LREE enrichment and HREE depletion and have geochemical features similar to adakites, including SiO 2 > 56 wt.%, high Na 2O contents (5.5–9.0 wt.%) and high Sr/Y (16–644). We consider that the trondhjemites represent primary and natural melts formed by deep partial melting of the subducting slab which did not significantly react with the mantle wedge before intrusion in the subduction channel. The Ms-bearing trondhjemites show different geochemical and petrological signatures compared with the Ms-free tonalites–trondhjemites from Sierra del Convento, that are interpreted as primary slab melts. These differences support the idea that partial melting processes in the Sierra del Convento subduction channel were triggered by the infiltration of fluids derived from three distinct subducted sources: sediments, altered mid-ocean ridge basalts (MORB), and serpentinites. In this scenario, the pegmatitic quartz-muscovite rocks, which are highly enriched in LILE, probably represent the crystallization products of fluids derived by differentiation of the trondhjemitic melts.

Petrography, mineralogy, and geochemistry of deep gravelly sands in the Eyreville B core, Chesapeake Bay impact structure

– The ICDP–USGS Eyreville drill cores in the Chesapeake Bay impact structure reached a total depth of 1766 m and comprise (from the bottom upwards) basement-derived schists and granites/pegmatites, impact breccias, mostly poorly lithified gravelly sand and crystalline blocks, a granitic slab, sedimentary breccias, and postimpact sediments. The gravelly sand and crystalline block section forms an approximately 26 m thick interval that includes an amphibolite block and boulders of cataclastic gneiss and suevite. Three gravelly sands (basal, middle, and upper) are distinguished within this interval. The gravelly sands are poorly sorted, clast supported, and generally massive, but crude size-sorting and subtle, discontinuous layers occur locally. Quartz and K-feldspar are the main sand-size minerals and smectite and kaolinite are the principal clay minerals. Other mineral grains occur only in accessory amounts and lithic clasts are sparse (only a few vol%). The gravelly sands are silica rich (∼80 wt% SiO2). Trends with depth include a slight decrease in SiO2 and slight increase in Fe2O3. The basal gravelly sand (below the cataclasite boulder) has a lower SiO2 content, less K-feldspar, and more mica than the higher sands, and it contains more lithic clasts and melt particles that are probably reworked from the underlying suevite. The middle gravelly sand (below the amphibolite block) is finer-grained, contains more abundant clay minerals, and displays more variable chemical compositions than upper gravelly sand (above the block). Our mineralogical and geochemical results suggest that the gravelly sands are avalanche deposits derived probably from the nonmarine Potomac Formation in the lower part of the target sediment layer, in contrast to polymict diamictons higher in the core that have been interpreted as ocean-resurge debris flows, which is in agreement with previous interpretations. The mineralogy and geochemistry of the gravelly sands are typical for a passive continental margin source. There is no discernible mixing with marine sediments (no glauconite or Paleogene marine microfossils noted) during the impact remobilization and redeposition. The unshocked amphibolite block and cataclasite boulder might have originated from the outer parts of the transient crater.

Lithium isotopes as a tracer of fluids in a subduction zone mélange: Franciscan Complex, CA

Abstract High-grade blocks of eclogite, blueschist, and garnet amphibolite found in melange terrains in the Franciscan Complex, CA display petrologic and geochemical evidence for a complicated history of interactions with aqueous fluids. These blocks have features (actinolitic rinds, a blueschist layer) that have petrologic and geochemical indicators of interaction with retrograde aqueous fluids. Lithium concentrations and isotopic compositions of different portions of three high-grade blocks are different from those of MORB (their presumed protolith) and show systematic variations associated with fluid-related features. Block cores and an eclogite layer have high whole-rock Li concentrations relative to MORB (17 to 39 ppm) and low to moderate δ7Li (0.4 to 2.1‰). This range of values is inconsistent with dehydration of MORB and is interpreted as the product of infiltration of aqueous fluids with an elevated Li concentration and low δ7Li. Rinds and a blueschist layer exhibit lower Li concentrations (7 to 24 ppm) and higher δ7Li (2.7 to 4.4‰). The differences between cores and rinds, and the eclogite and blueschist layers are interpreted as the product of retrograde infiltration of an aqueous fluid with low Li concentrations and high δ7Li. Results of a small-scale traverse across the boundary between the blueschist and eclogite layers show that the boundary is gradational over ∼ 0.5 cm, suggesting small-scale cross-layer diffusion/dispersion associated with the fluid infiltration. The high Li concentrations and low δ7Li of block cores suggests that the source of early fluids was likely subducted sedimentary rocks. The lower Li concentrations and higher δ7Li values of the rinds and blueschist layers do not fully constrain the source of retrograde fluids, but indicate that the fluid source likely had a lower Li concentration and higher δ7Li.

Jadeitite jade occurrence from the Sierra del Convento mélange (eastern Cuba)

A new jadeitite jade locality has been discovered in the serpentinite-matrix subduction melange of the Sierra del Convento (eastern Cuba) in a context associated with tectonic blocks of garnet-epidote amphibolite, tonalitic-trondhjemitic epidote gneiss and blueschist. The jade outcrops occur as blocks surrounded by serpentinite, as late Miocene-early Pliocene detritic sedimentary deposits formed during erosion of the melange, and as Recent river and beach deposits related to the Macambo and Guardarraya rivers and their mouths. Jade from these outcrops show gem-quality varieties with colours ranging from greenish white to dark green. Potentially, the deposits may be easily exploited. Former exploitation (probably of the beach deposits) by the pre-Columbian Taino inhabitants of Cuba is documented by a large number of jade artifacts (mostly, petaloid axes) found in eastern Cuba.