Geosystems and Geoenvironment最新文献

Petrography and scanning electron microscopy (SEM) investigations aided by energy dispersive X-ray (EDX) analysis and quantitative measurement of reservoir properties were used to extensively examine the physical and diagenetic characteristics of the Joyan Member Sandstone of Jaisalmer Formation. The Joyan Member Sandstone is fine- to medium-grained, moderate to well sorted, sublitharenite to litharenite. Mechanical compaction, precipitation of calcareous, ferruginous and silica cements, clay minerals, dissolution and alteration of unstable clastic grains such as feldspar and rock fragments, and grain fracturing are the identified diagenetic features. Feldspar and rock fragments underwent significant changes to kaolinite and chlorite while silica cement primarily originated from the dissolution and alteration of these grains and pressure solution. Mechanical compaction and the authigenic cements like calcareous, ferruginous, and silica reduced primary porosity, while secondary porosity was created by dissolution of clastic grains and cements. Compaction reduced porosity from an anticipated original 40% to around 13.4%. Porosity was reduced by cementation to 20.8%. Cementation reduced the porosity of the Joyan Member Sandstone somewhat more than compaction. Calcareous cementation played a major role in the porosity evolution of Joyan Member Sandstone. During early burial, the early calcareous cement occupied most of the pore spaces, leading to a significant reduction in porosity. However, incomplete filling or scattered patches of calcareous cement helped to preserve some primary porosity. In addition to calcareous cement, clay minerals like kaolinite and chlorite also acted as pore-filling and pore-lining cements. Kaolinite had a booklet-like or lamellar pattern contributing to minor porosity loss through pore-occlusion, while pore lining chlorite helped to retain porosity by preventing syntaxial silica overgrowth. Extensive dissolution of calcareous cement significantly increased the secondary porosity. Diagenesis affects reservoir quality by reducing initial porosity through cementation and compaction, and then increasing it through dissolution of early calcareous cement and unstable grains. The diagenesis of the studied sandstone is closely linked to its potential as a reservoir.

Subduction zones play a critical role in the global carbon cycle by regulating carbon exchange between the Earth's surface and interior. Processes that are known to release carbon from the slab, including metamorphic decarbonation and carbonate dissolution, cannot explain the high CO₂ flux in magmatic arcs. Slab melting is the least considered mechanism for carbon mobilization at subarc depths based on the high solidus temperatures of carbonated lithologies, which were experimentally determined under dry or H₂O-absent conditions. Subducted sediments are major carbon carriers, however, their melting behaviour with excess H₂O remains largely unexplored. Here, we perform fluid-present melting, high-pressure experiments at 750–1100 °C and 2.5–4 GPa using starting compositions similar to global average subducted sediments to determine the solidus, melting relations and carbonate stability fields. The onset of melting is between 750 and 800 °C at 2.5 GPa and between 850 and 900 °C at 4 GPa. Dolomite melts out on or close to the solidus, whereas crystalline aragonite persists >150 °C above the solidus. Flux melting of carbonated sediment at moderately hot subduction zones is examined to be feasible in the framework of the previously constructed dehydration history of the underlying serpentinites, providing a pathway to transfer carbon from the slab to the subarc mantle. However, complete breakdown of refractory aragonite requires at least 50 °C higher than that predicted for the hottest slab P‒T paths. Thus, even in the presence of H₂O, partial subducted carbon may survive the melting event occurring at shallow regions and reach considerable mantle depths.

Subduction mélange, with its distinctive block-in-matrix structure, is documented in exhumed fossil subduction zones worldwide. Rocks from these terranes preserve features that record tectonic processes from the time that the rocks were at the subduction interface. Careful study of these features allows for connections to be made with tectonic processes occurring in active subduction zones. The Catalina Schist mélange has served as an exhumed analog in such studies as it records abundant evidence for tectonic processes occurring at the subduction interface. Focusing of fluids in mélange matrix at the subduction interface is documented, and this fluid-rich environment may have contributed to seismic activity. Deformation and tectonic mixing juxtaposed disparate materials (mafic, ultramafic, and sedimentary rocks) over length-scales of 10s of km along the interface, occurring in concert with metasomatism and mass transport by fluids to create mineralogically, chemically, and rheologically distinct compositions. These processes may have impacted seismic behavior and plate geodynamics along with influencing the chemistry of arc magmas that form above the subduction interface. Evidence suggests that the duration of tectonic formation of mélange may be variable from one locality to another, with relatively small differences in peak ages of blocks of <10 Myrs recorded in the amphibolite facies rocks of the Catalina Schist.

We present new U-Pb detrital zircon ages, depositional history and tectonic model for the Liuqu Conglomerate (LQC) in southern Tibet that represents a critical geochronometer for the collision history of the Tibetan-Himalayan Orogenic Belt. LQC is a ∼5 km–thick, late Mesozoic–Cenozoic molasse deposit occurring strictly within the Yarlung Zangbo Suture Zone (YZSZ) and is tectonically overlain to the north by the Cretaceous Xigaze ophiolite and to the south by the Mesozoic Tethyan Himalaya sequence. It consists of matrix- and clast-supported conglomerates with sandstone intercalations, and its matrix includes poorly to moderately sorted sandstone and mudstone. New U–Pb detrital zircon dating of LQC sandstones has revealed a youngest zircon age of 307 ± 13 Ma and an oldest zircon age of 3362 ± 51 Ma. The age spectrum of zircons displays a prominent peak of ∼935 Ma, two large peaks at ∼516 Ma and 1474 Ma, and two small clusters of ∼2429 Ma and ∼2772 Ma that point to East Gondwana as the likely provenance for the LQC depocenter. The LQC represents fluvial deposits of an axial river system, which developed in an orogen-parallel, transtensional accommodation space within the YZSZ, after the collision of the Late Jurassic-Early Cretaceous Trans–Tethyan arc–trench system with the northern edge of India in the latest Cretaceous. The Indian subcontinent with the accreted Tethyan ophiolites and the intra–suture LQC depocenter arrived at and collided with the active margin of Eurasia during the latest Oligocene (∼23 Ma). The LQC depocenter started receiving clastic material and zircons for the first time from the Gangdese Magmatic Belt and the Xigaze forearc basin to the north by ∼20 Ma. The ensuing continent–continent collision resulted in significant crustal uplift across the collision zone, and in the inversion and rapid exhumation of the LQC strata by the early–Middle Miocene. The depositional and exhumation history of the fluvial LQC formation within the YZSZ involved two discrete collision events during the evolution of the Tibetan-Himalayan Orogenic system.

Ophiolitic mélanges, units that contain components of an ophiolite suite, provide crucial information on earth history and orogenic evolution. In this paper, four ophiolitic mélanges are characterized, including the Baijiantan-Yeyagou, Hebukesair, Zhaheba and Hongguleleng mélanges in the Junggar region (NW China), southern Altaids. Detailed geological mapping, structural, geochronological, and geochemical analyses constrain ages, geochemical affinities and relationships of magmatic with clastic rocks. MORB-/OIB-type (meta)gabbros and plagiogranite are the oldest mélange components, slightly older or coeval with associated chert and MORB-/OIB-type basalt; these rocks collectively constitute an ophiolite suite. Ophiolitic rocks predate associated clastic sedimentary rocks (conglomerate and turbidite) by ∼90-25 My, and associated SSZ-type hornblende gabbro, basaltic andesite, diabase, diorite and rhyolite by ∼78-37 My, except in the Hongguleleng mélange, where ophiolitic rocks predate latter units by ∼9-25 My. The ophiolitics are repeated by imbricate thrusts and duplexes, and folded. Ophiolitic blocks in mélanges locally preserve similar structures. Such blocks commonly have MORB and/or OIB geochemical affinities. Significantly older ages of MORB/OIB igneous rocks compared to ages of associated clastic sedimentary/SSZ-type igneous rocks shows that the former rocks formed as part of the crust of a large ocean, far from a convergent margin. The far-traveled oceanic crustal slices were imbricated and disrupted into block-in-matrix structures during accretion and incorporation into a subduction complex. SSZ-type magmatic rocks locally intrude into and extrude onto clastic rocks, demonstrating that a mélange contains multi-stage magmatic rocks. Folds, tilted structures and shear band cleavages are locally cross-cut by dikes, and these rocks are themselves have been dismembered into blocks. An intruded conglomerate in the Hongguleleng mélange contains pebbles of gabbro and basaltic andesite, the latter of which overlies sandstone. Superimposed folds in clastic rocks and chert record the polydeformation of the mélanges. Determination of the complex relationships of multi-stage magmatism and deformation illuminates the tectonic history of ophiolitic mélanges in the Junggar region. This history includes formation and subduction-accretion of the crust of a large ocean and post-subduction intracontinental deformation.

The Northern Calcareous Alps in the Western Tethys realm were affected in Middle to Late Jurassic times by a mountain building process triggered by ophiolite obduction similar to that in the Inner Western Carpathians or Inner Dinarides. In contrast to these other mountain ranges, in the Northern Calcareous Alps the obducted ophiolites or ophiolite derived components in the Bathonian-Oxfordian mélanges are missing. Cr-spinels in Kimmeridgian basinal deposits are the oldest known relics of a Jurassic ophiolite obduction. This study reveals new data from a newly detected Hallstatt Mélange below the Late Jurassic Plassen Platform in the southeastern Northern Calcareous Alps (Raucherschober/Schafkogel area). Upper Triassic Hallstatt Limestone blocks from the former distal northwestern continental margin of the Neo-Tethys Ocean, as well as ophiolite and radiolarite blocks from the Neo-Tethys Ocean floor rest within an upper Middle to lower Upper Jurassic radiolaritic-argillaceous matrix. Ophiolitic blocks show calc-alkaline volcanic arc affinity, defining the rocks as the product of intra-oceanic subduction and the formation of an early arc during stacking of the oceanic crust. Resedimented ribbon radiolarite blocks deposited above the newly formed suprasubduction (SSZ) ophiolites in the Neo-Tethys Ocean east of the island arc have a Middle Jurassic age. Later, at a time of decreasing tectonic activity, the Hallstatt Mélange was sealed by the Kimmeridgian-Tithonian Plassen Carbonate Platform, showing a shallowing-upward trend.

The Remeshk-Mokhtarabad and Fannuj-Maskutan ophiolites represent two major ophiolitic units in the North Makran Domain (Makran Accretionary Prism). Volcanic rocks and dykes of these ophiolites mainly consist of basalts and rare basaltic andesites, andesites and dacites. No chemical distinction can be seen in basalts from these two ophiolitic units, or between volcanic rocks and dykes. Basaltic rocks show a broad MORB-type nature but variable chemical composition (e.g., SiO₂= 42.64–52.63 wt%; TiO₂= 0.98–2.43 wt%; Mg# = 71–50). They show both N-MORB (Type 1) and E-MORB (Type 2) compositions (MORB: mid-ocean ridge basalt; N: normal; E: enriched). Type 1 rocks are very rare in both ophiolitic units, whereas Type 2 rocks are predominant. Type 1 rocks show low Th (0.10–0.16 ppm), Nb (1.86–2.82 ppm), Ta (0.09–0.17 ppm) abundance and low (La/Yb)_N (0.50–0.75), (La/Sm)_N (0.48–0.72) ratios. Compared to N-MORBs, Type 2 basalts show slight enrichment in Th (0.42–1.60 ppm), Nb (6.09–14.6 ppm), and Ta (0.227–0.792 ppm), as well as (La/Yb)_N and (La/Sm)_N ratios >1 like those observed in typical E-MORB. Trace element petrogenetic models indicate that primitive basalts derived from partial melting of a heterogeneous sub-oceanic mantle variably metasomatized by plume-type (OIB-) components. Type 1 basalts derived from partial melting of mantle regions with no enrichment in OIB-type components, whereas Type 2 basalts derived from partial melting of DMM sources variably enriched by OIB-components. These rocks formed in an oceanic basin that was strongly affected by mantle plume activity and different extents of plume-ridge interaction.

The North Qaidam belt is an important polymetallic metallogenic belt in the northwestern region of China. However, the tectonic setting and age of related VMS deposits remain debated. Here we performed an integrated analysis of field relationship, geochemistry, and geochronology for hosting rocks of the Lüliangshan VMS-type Cu deposit and surrounding mafic-ultramafic rocks. These rocks, including serpentinite, pyroxenite, chromitite, mafic dykes with associated meta-plagiogranite, lava, chert, and limestone, constitute a relatively complete ophiolite complex, indicating that the Lüliangshan Cu deposit can also be introduced as an ophiolite-hosted VMS deposit. Geochemical data show that the meta-dolerite and ore-hosting lava exhibit geochemical features similar to tholeiitic forearc basalt and are probably generated by partial melting of a depleted mantle source metasomatized by hydrous fluids. Some lavas have boninitic compositions and are formed by partial melting of residual mantle after extraction of forearc basalt. Some ore-hosting lavas also have geochemical affinities to island arc tholeiites as a result of more SSZ components involved in their magma source. The chert samples have remarkably high Fe₂O₃^T contents and are classified as iron-rich one of hydrothermal origin, which is deposited in a ridge-proximal environment. These rocks, together with chromitites with subducted-related geochemical features, collectively indicate that the ophiolite-hosted VMS-type Cu deposit was formed in the forearc setting. Meta-gabbros intruding the ore-hosting lavas yield zircon U-Pb ages mainly ranging from 527 Ma to 518 Ma. The new ages of forearc ophiolite and the oldest age of island-arc rocks (514 Ma) suggest that the Lüliangshan Cu deposit formed in the early Cambrian during early-stage subduction of Proto-Tethys Ocean.

The Kızıldağ (Hatay) ophiolite in southern Turkey represents a complete, although fault-dissected, remnant of oceanic lithosphere that formed within the South Tethyan ocean during the late Cretaceous. The Kızıldağ forms part of the Upper Cretaceous belt which includes the Troodos (Cyprus), Baer-Bassit (Syria), Amanos (S Tukey), S Iran and Semail (Oman) ophiolites. Ion-probe (SIMS) dating of seven samples of crustal rocks (cumulate gabbro, isotropic gabbro and isolated dykes in mantle tectonite), and a plagiogranite intrusion provides important clues concerning the temporal development of the emplaced oceanic crust. Single grain ²⁰⁶Pb/²³⁸U dates that overlap within analytical uncertainty for four samples, including the plagiogranite (93.83 ± 0.46 Ma), the isotropic gabbro (92.9 ± 0.52 Ma) and the isolated dykes (92.54 ± 0.44 Ma to 93.6 ± 0.75 Ma), are interpreted as magmatic crystallisation ages, and suggest that the Kızıldağ ophiolite formed within 1-2 Ma. Three other samples with single grain ²⁰⁶Pb/²³⁸U dates that are outside the range of analytical uncertainty yielded nearly identical lower intercept ages of 94.2 ± 2.5 Ma to 94.4 ± 0.97 Ma for two cumulate gabbros and 90.0 ± 6.4 Ma for an isotropic gabbro. Comparison of the new and published radiometric ages of the Kızıldağ suggest that this ophiolite is ∼1.5 Ma older than previously believed, and is similar to the crystallisation ages of plagiogranites from the Troodos (Cyprus) and the Semail (Oman) ophiolites. The new age data emphasise the value of dating a range of ophiolitic rocks. Geochemically, the crustal rocks of the Kızıldağ ophiolite formed from boninitic magmas (cumulate gabbros and isolated dykes) and from island arc tholeiitic magmas (isotropic gabbro). The new whole-rock chemical data support a subduction-initiation (fore-arc) setting for the Kızıldağ ophiolite, in common with the Troodos, Semail, Baër-Bassit and other Upper Cretaceous ophiolites of the South-Tethyan region.

In the Neyriz area of southern Iran unusual skarns are found above serpentinised peridotites at the contact with crystalline limestones. They have been interpreted as the high-temperature product of intrusion by hot peridotite into limestones, as low-temperature rodingites (the product of calcium metasomatism associated with serpentinisation), or a fortuitous juxtaposition of unrelated rocks. Their age is not known. The skarns are wollastonite-pyroxene-calcite rocks in which dark green pyroxenes are fassaites with high Al, Fe and Ti with high Ca-Tschermak's components. The field relations, textures, mineral assemblages and compositions, and melt inclusions in wollastonite and fassaite indicate the skarns formed by melting at the contact between peridotites and limestones with retrograde reactions during cooling forming garnet and anorthite. There are uncertainties in temperature estimates since pressure, X_CO2 and other compositional variables are unknown, but melting temperatures were likely to have been close to 1100°C with garnet formation at approximately 900°C. Later alteration of some skarns and formation of rodingites close to the limestone-peridotite contacts occurred during low-temperature Ca metasomatism, probably after emplacement of the ophiolite during Zagros collision. A hot intrusive origin for the skarns appears incompatible with an arc-related supra-subduction origin of the ophiolite inferred from geochemical studies, but recent work in eastern Indonesia shows that during late Neogene subduction rollback, melts formed above hot mantle that intruded highly extended continental crust in a forearc setting. The scale, timing and temperatures of melting and metamorphism are very similar to those of the Late Cretaceous Neyriz ophiolite.