Orogenic Rocks Research Articles

Metamorphism of Dolomitic and Magnesitic Rocks in Collisional Orogens and Implications for Orogenic CO2 Degassing

Abstract Carbonate-bearing sediments, containing calcite, dolomite or magnesite as major carbonate components, are important constituents of sedimentary sequences deposited on passive margins through Earth’s history. When involved in collisional orogenic processes, these sediments are metamorphosed at variable temperatures and pressures, and undergo decarbonation reactions. While the orogenic metamorphism of some of these lithologies (i.e. impure limestones and dolostones, marls sensu stricto and calcareous pelites) is relatively well understood, very little is known about the metamorphic evolution and decarbonation history of mixed carbonate–silicate rocks in which either dolomite or magnesite is the dominant carbonate component. Here we present the results of a petrologic study of representative samples of metasediments from Central Nepal, derived from Proterozoic dolomitic and magnesitic protoliths metamorphosed during the Himalayan orogeny. The main metamorphic assemblages developed in sediments originally containing different amounts of dolomite or magnesite are characterised in detail. Forward thermodynamic modelling applied to seven samples allows constraints to be placed on (i) the main decarbonation reactions, (ii) the P–T conditions under which these reactions took place, (iii) the composition of the fluids, and (iv) the amounts of CO2 released. We conclude that the CO2 productivity of dolomitic and magnesitic pelites and marls originally containing 15–40% carbonate is significant (>5.5 ± 1.0 CO2 wt% and up to 10.5 ± 1.5 CO2 wt%), whereas for carbonate contents above 60–70%, CO2 productivity is negligible unless aqueous fluids infiltrate from the outside and trigger decarbonation reactions. Since the dolomitic and magnesitic protoliths are significantly abundant in the sedimentary sequences involved in the still active Himalayan orogen, the decarbonation processes described here could contribute to the diffuse CO2 degassing currently observed at the surface. Furthermore, we propose for the first time that the peculiar magnesium-rich assemblages investigated in this study may derive from evaporitic protoliths, and that the whole Upper Lesser Himalayan Sequence may therefore represent the metamorphic product of a Proterozoic sequence consisting of alternating layers of carbonatic, evaporitic and pelitic sediments.

Does zircon geochemistry record global sediment subduction?

Abstract Global compilations of zircon geochemistry have been used as evidence for changes in plate tectonic styles and surface environments. In particular, zircon δ18O has been used as a proxy for global sediment subduction and incorporation into igneous melts. However, research employing such compilations commonly ignores geologic and geographic context. We analyze a newly georeferenced zircon δ18O database from 1000 Ma to present. The compilation shows positive and negative isotopic excursions, both of which have been interpreted in the context of global phenomena. Sensitivity testing demonstrates that these excursions are the product of regional tectonic environments. Specifically, low Tonian–Cryogenian δ18O, previously interpreted as a climate signal of snowball Earth glacial meltwater, is isolated to ultra-high-pressure rocks of the Dabie Sulu orogen in central Asia, ophiolites in Egypt, and juvenile arc development in Madagascar, predating Cryogenian glaciation. Positive anomalies in the Ediacaran–Cambrian and Devonian, previously interpreted to record snowball Earth erosion and the rise of land plants, are the result of upper-plate, supracrustal sources of Gondwana. A Neogene anomaly is the result of volcanic rocks associated with Yellowstone and Iceland. Sampling bias in zircon geochemical compilations is analogous to that of Lagerstätten (sites of exquisite fossil preservation) and the “monograph effect” (large contributions from individual researchers) in paleontology, which both result in artifacts of over-representation. Long-term zircon δ18O trends broadly track the generation of collisional granites and continental arcs through time, recording the melting of sediments and sedimentary rocks in individual orogens, and not variations in sediment subduction globally.

Carboniferous sills and related metasomatic activity in the area of Asaritsa Peak, West Stara Planina Mountain

Two diorite to monzodiorite sills are recently found among the Silurian sedimentary succession near to Asaritsa peak, West Stara Planina Mountain. Several levels with diagenetic metalliferous nodules occur within shale interval in the sediments. The magmatic rocks are affected by strong propylitic to sericitic alteration (chlorite, sericite, albite and quartz) and only relicts of the rock-forming minerals (feldspar, biotite and clinopyroxene) are observed. The bodies are crosscut by thin quartz veins and disseminated pyrite is observed. The chondrite normalized spidergrams show patterns similar to orogenic rocks, with very high peaks of Th and U and depletion in Sr and Ti. The zircon U-Pb geochronology reveals that the magmatic crystallization is during the Carboniferous period – 326.4±3.2 Ма, with low content of xenocryst most probably from the sedimentary basement. The magmatic activity coincides with the Variscan orogeny formed during the late Paleozoic and most probabaly is related to the evolution of the alkaline Buhovo-Seslavtsi Pluton which is situated just to the south, at a direct proximity. The different composition of the pyrite from the sills and the diagenetic nodules from the sediments is most probably due to their different genesis. While the alterations of the nodules can be attributed to metasomatic processes related to the magmatic activity.

Boron isotope behavior during metamorphic dehydration and partial melting of continental crust: Constraints from tourmaline in metapelites and leucocratic dikes, Himalayan orogen

Crustal rocks in collisional orogens often underwent the metamorphic transition from dehydration to anatexis in response to an increase of geothermal gradients, but it remains obscure how the two types of metamorphism influenced each other. This issue can potentially be addressed by an integrated study of mineralogy and geochemistry for tourmaline in protoliths and their anatectic products. In this paper, we present new results from comprehensive analyses of major-trace elements and B isotopes in tourmaline together with whole-rock Sm–Nd isotopes in well-characterized metapelites and leucocratic dikes from the Himalayan orogen. The leucocratic dikes show whole-rock εNd(t) values of −15.7 to −15.4, falling within an εNd(t) range of −17.7 to −13.8 for the metapelites. In the metapelites, metamorphic tourmaline (Tur-M) is dravitic and has relatively low δ11B values of −12.0 to −9.0‰. As the metamorphic grade increases, the δ11B values of Tur-M decrease, which is linked to progressive metamorphic dehydration. In the leucocratic dikes, three types of tourmaline are identified by different petrological and geochemical criteria. The tourmaline cores (Tur-LC) are schorlitic and have relatively high δ11B values of −8.7 to −7.7‰, and this type is ascribed to an anatectic origin. The tourmaline rims (Tur-LR) corroding the Tur-LC are dravitic and have relatively high δ11B values of −9.6 to −7.6‰, and this type is attributed to a fluid origin. Another type of anatectic tourmaline (Tur-L) occurs as grains with homogeneous oscillatory zoning that are dravitic and have slightly higher δ11B values of −10.4 to −5.9‰. The anatectic Tur-LC in the leucocratic dikes has higher δ11B values than the metamorphic Tur-M in the metapelites. We attribute this difference to the addition of external fluids with high δ11B values to the metapelite source at the overlying crustal levels. The influxed fluids most likely originated from metamorphic dehydration of lithologically similar metapelites at deeper crustal levels, as evidenced by the Mg-rich characteristics of Tur-L and Tur-LR. The metamorphic fluids would ascend into shallow crustal levels through faults and shear zones, which could not only trigger fluid-fluxed (hydration) melting of the metapelites but also result in fluid activity in later stages. This is further verified by phase equilibrium modelling and model calculation of B geochemical behavior during both metamorphic dehydration and partial melting. On this basis, a spatiotemporally coupled dehydration-hydration melting mechanism is proposed to link the early dehydration metamorphism to the later anatectic metamorphism in the collisional orogen.

Provenance of the Lower Cambrian Khewra Sandstone: Implications for Pan-African Orogeny

This study describes the sedimentological aspects of the Lower Cambrian Khewra Sandstone, along with its lithostratigraphic correlation of globally distributed coeval red sandstones to decipher the imprints of Pan-African Orogeny. Despite the economic significance of Khewra Sandstone, the understanding of its environment of deposition, provenance and lower contact with the suite of igneous body namely Khewrite is poor. The Khewra Sandstone is an allostratigraphic unit bounded by unconformities. The upward coarsening sequence with abundant sedimentary structures and varying sedimentological properties of the topmost bed suggest that the Khewra Sandstone is tempestites deposited under fluvio-deltaic depositional environment. This sandstone is dominated by subarkoses, feldspathic litharenites and sublitharenites which were derived from the craton interior to recycled orogen provenance during the Pan-African orogeny under humid to semi-humid paleoclimatic conditions. The plutonic to metamorphic characteristics of framework grains suggest that the Bundlekhand Craton, Delhi Fold Belt and Aravalli Orogen rocks were the primary provenances. The lower and middle parts of the Khewra Sandstone are significantly affected by diagenesis while the diagenetic action in upper part is negligible and it acts as a potential hydrocarbon reservoir rock. The lithostratigraphic correlation suggests a worldwide series of rifting events before the deposition of Khewra Sandstone and the presence of Khewrite at lower contact marks a similar rifting event on the Indian plate.

Decoupling of Mg from Sr–Nd isotopic compositions in Variscan subduction-related plutonic rocks from the Bohemian Massif: implications for mantle enrichment processes and genesis of orogenic ultrapotassic magmatic rocks

Decoupling of Mg from Sr–Nd isotopic compositions in Variscan subduction-related plutonic rocks from the Bohemian Massif: implications for mantle enrichment processes and genesis of orogenic ultrapotassic magmatic rocks

Mineralogy of a rare Ediacaran epidote‐bearing trondhjemitic complex from the Nubian Shield: Insights into P‐T conditions of magma emplacement

Igneous epidote occurring in tonalite–trondhjemite–granodiorite (TTG) rocks provides valuable insight into the physiochemical parameters of their magmatic system. Epidote has been identified within a newly discovered, rare, Ediacaran, high‐Al TTG‐suite at the Fannani Igneous Complex (FIC), Nubian Shield. The objective is to assess P–T conditions of magma emplacement using petrographic‐, mineral chemical data, and thermobarometry methods. The TTG‐suite is made up of ubiquitous oligoclase, variable contents of quartz, K‐feldspar, amphibole, biotite, epidote, accessory titanite, zircon, apatite and magnetite. Feldspars vary from An14 to An26 in plagioclase, with K‐feldspar having near‐end‐member compositions (Or91–Or95). Amphiboles are Al‐rich (AlT = 1.65–2.04 atoms per formula unit [apfu]), calcic (Ca = 1.83–1.93, apfu), with average Mg/(Fe + Mg) of 0.44, and are ferroedenite–edenite–ferropargasite. Biotites are moderately Fe‐rich (Fe/(Fe + Mg) = 0.50–0.58) characteristic of type ‘C’–biotite of calc–alkaline orogenic rocks. Epidote exhibits pistacite components [Ps = Fe3+/(Fe3++Al)] of Ps24–Ps33, typical of magmatic epidote. The average crystallization pressure of 5.41 kbar (calculated using the Al‐in‐Hb barometer) is consistent with the presence of magmatic epidote and reflects the mesozonal crustal levels of emplacement. The Hb–Plag–geothermometer produced an average crystallization temperature of 721°C. These values fall within experimentally‐determined P–T ranges of stability of magmatic epidote with fO2 buffered from nickel‐nickel‐oxide (NNO) to hematite (HM). The FIC rocks were developed from a wet, subsolvus, orogenic, Al‐rich magmatic system. Findings reported here could have implications for conditions of magma emplacement and mineralogical characterization of TTGs occurring within various continental masses.

40Ar/39Ar AGES OF HYDROTHERMAL FLUIDS OF CABAÇAL Au-Zn-Cu DEPOSIT: IMPLICATIONS OF THE CACHOEIRINHA OROGENY ON MINERALIZATION EVENT IN SW AMAZONIAN CRATON

The Cabaçal deposit is located in SW Amazonian Craton, Mato Grosso State, Brazil, with important Au-Zn-Cu mineralization. The ore is hosted by the Alto Jauru orogenic rocks (1.79-1.74 Ga) comprised of felsic volcanic and volcanoclastic rocks and mineralizations occur as bands concordant with the mylonitic foliation, breccias, quartz-carbonate veins and rock-disseminated, and the polymetallic ore comprises chalcopyrite, pyrite, marcasite, pyrrhotite, sphalerite and galena. The mineralization is related to hydrothermal alteration and includes quartz, chlorite, carbonate, sericite and biotite. Six sericite grains from the hydrothermal zones yielded 40Ar/39Ar ages from 1521.3 ± 1.3 Ma to 1510.4 ± 1.2 Ga. These ages are evidence that hydrothermal solutions originated related to the Cachoeirinha orogen (1.58-1.52 Ga) cutting the Alto Jauru orogen (1.79-1.74 Ga) host rocks. Detailed geology and petrography indicate banded iron formation played an important role in the metal deposition as a chemical barrier for the mineralizing fluids.

Origin of potassic postcollisional volcanic rocks in young, shallow, blueschist-rich lithosphere.

Potassium-rich volcanism occurring throughout the Alpine-Himalayan belt from Spain to Tibet is characterized by unusually high Th/La ratios, for which several hypotheses have brought no convincing solution. Here, we combine geochemical datasets from potassic postcollisional volcanic rocks and lawsonite blueschists to explain the high Th/La. Source regions of the volcanic melts consist of imbricated packages of blueschist facies mélanges and depleted peridotites, constituting a new mantle lithosphere formed only 20 to 50 million years earlier during the accretionary convergence of small continental blocks and oceans. This takes place entirely at shallow depths (<80 km) without any deep subduction of continental materials. High Th/La in potassic rocks may indicate shallow sources in accretionary settings even where later obscured by continental collision as in Tibet. This mechanism is consistent with a temporal trend in Th/La in potassic postcollisional magmas: The high Th/La signature first becomes prominent in the Phanerozoic, when blueschists became widespread.

Combined use of Sm–Nd isotopes and lithogeochemistry in the sedimentary provenance of the southern Ediacaran-Cambrian Bambuí foreland basin system, Brazil

Sedimentary provenance analysis based on lithogeochemistry and isotope data has been extensively applied in the investigation of ancient sedimentary successions. This approach contributes to understand the tectonic setting in which these strata were deposited and to recognize major provenance patterns in time and space. The Ediacaran to Cambrian Bambuí Group is exposed in the São Francisco craton (SE Brazil) and represents the sedimentary record of a foreland basin system developed during the assembly of the West Gondwana. Different lines of evidence indicate that this basin system evolved into a confined setting, due to the overloads imposed on the western and eastern margins of the São Francisco paleoplate by the diachronic Brasília and Araçuaí orogenic systems, respectively. New whole-rock lithogeochemistry and Sm–Nd isotopic data were applied to evaluate the overall sedimentary provenance patterns of the Bambuí strata exposed in the southern portion of the São Francisco craton (i.e., within the Sete Lagoas basement high) and to identify its possible sources. Coupled (La/Yb)n and Th/Sc ratios with 147Sm/144Nd and εNd isotopes allowed the recognition of three contrasting source patterns associated with different 2nd order sequences. The lower sequence comprises carbonate ramp deposits and shows highly heterogeneous and more negative εNd ranging from −10.5 to −4.5, as well as Th/Sc and (La/Yb)n ratios between 0.6 and 0.7 and 0.54 and 0.76, respectively. The provenance patterns point toward intermediate-dominated and isotopically evolved crustal sources, with mean TDM model ages between 1.5 and 2.0 Ga. The overlying 2nd-order sequence is composed of fine-to medium-grained siliciclastics grading upward into platformal oolitic calcarenites and calcilutites. This succession exhibits homogeneous and less negative εNd(t) values between −6.8 and −5.2, with low Th/Sc (0.6–0.8) and (La/Yb)n (0.60–0.84) ratios. The uppermost Bambuí 2nd-order sequence, marked by greenish siltstones and arkoses grading upward into storm-bedded arkoses and lithic sandstones, shows homogeneous εNd(t) ranging from −8.5 to −6.1, slightly more negative than the underlying successions, and relatively higher Th/Sc and (La/Yb)n ratios ranging from 0.7 to 1.3 and 0.85 to 1.28, respectively. The Sm–Nd isotopic signature of the upper deposits indicates felsic-dominated and more isotopically juvenile crustal sources with TDM model ages of 1.5–1.7 Ga, characteristics compatible with Ediacaran arc-related rocks and post-tectonic granites exposed in the Araçuaí orogen. Our data indicate that the Proterozoic cratonic covers, their correlatives in the surrounding orogens, and magmatic rocks from the Araçuaí orogen acted as major sedimentary sources during the first evolutionary stages of the Bambuí basin-cycle in the Sete Lagoas basement high, which behaved as a forebulge depocenter. During the deposition of the overlying sequences, the Araçuaí orogen rocks became increasingly important in sourcing the Ediacaran- Cambrian foreland system in this depocenter. Besides revealing the major provenance patterns through the evolution of an ancient and partially confined foreland basin system, our data also demonstrates that even minor changes in the Sm–Nd system might be useful as reliable elements on the provenance analysis of sedimentary systems elsewhere.

Anatomy of an extensional shear zone leading to the exhumation of the middle crust within the Canigou dome (Eastern Pyrenees, Axial Zone)

To investigate the mechanism of exhumation of deep crustal rocks in hot orogens, we focus on the southwestern part of the Variscan Canigou gneiss dome (Pyrenees) where the transition between the middle and upper crust can be observed. On the basis of new structural data, geological mapping, cross sections, microstructural analyses, and petrologic observations, we propose that flat-lying foliation within the gneiss dome and steep cleavage developed above it formed coevally. We note the presence of regional-scale shear zones at the boundary between the middle crust (Núria-Canigou Unit) and upper crust (Puigmal unit). At this boundary, the E-W South Canigou Shear Zone is an extensional shear zone showing top-to-the southwest kinematics. Deformation, which is localized within the andalusite-biotite transition zone, occurred under retrogressive conditions, from high-grade ductile to brittle conditions during the main deformation stage (D2). The tectonic contact was folded during a later stage (D3). We propose that the shape of the Canigou dome resulted from the activation of extensional shear zones at the top of the orthogneiss, leading to partial exhumation of high-grade rocks. The Canigou gneiss dome can be considered an extensional dome that formed during late Variscan (ca. 310-290Ma) transpression.

Permian Hydrothermal Alteration Preserved in Polymetamorphic Basement and Constraints for Ore-genesis (Alpi Apuane, Italy)

The reconstruction of the polymetamorphic history of basement rocks in orogens is crucial for deciphering past geodynamic evolution. However, the current petrographic features are usually interpreted as the results of the metamorphic recrystallization of primary sedimentary and/or magmatic features. In contrast, metamorphic rocks derived by protoliths affected by pre-metamorphic hydrothermal alterations are rarely recognized. This work reports textural, mineralogical and geochemical data of metasedimentary and metaigneous rocks from the Paleozoic succession of the Sant’Anna tectonic window (Alpi Apuane, Tuscany, Italy). These rocks were recrystallized and reworked during the Alpine tectono-metamorphic event, but the bulk composition and some refractory minerals (e.g., tourmaline) are largely preserved. Our data show that the Paleozoic rocks from the Alpi Apuane were locally altered by hydrothermal fluids prior to Alpine metamorphism, and that the Permian magmatic cycle was likely responsible for this hydrothermal alteration. Finally, the Ishikawa Alteration Index, initially developed for magmatic rocks, was applied to metasedimentary rocks, providing a useful geochemical tool for unravelling the hydrothermal history of Paleozoic rocks, as well as a potential guide to the localization of hidden ore deposits in metamorphic terranes.

Subduction-collisional processes between the Eurasian and Philippine Sea plates: Constraints from thermal-age paths of the Taiwan Orogen

Records of the thermal-age path of orogenic rocks are crucial in understanding subduction and collisional processes between a continent and an arc. Tectonic events and thermochronological data of the Taiwan Orogen have been extensively investigated. We use the zircon UPb youngest ages as the (initial) subduction age, the Raman Spectroscopy of Carbonaceous Material, 40Ar/39Ar age of mica porphyroblasts, whole-rock LuHf isochron age and thermal results as prograde peak metamorphic condition during subduction and basal accretion, and the apatite fission-track data for cooling age of the Taiwan Orogen to illuminate the subduction and collisional processes between the Eurasian Plate and the Luzon Arc. The Hsuehshan Range Nappe in the west of the Taiwan Orogen, whose subduction was initiated at 12 Ma, reached peak metamorphism with a temperature of ~250 °C to 490 °C at a depth of 8 km during the interval 6–2.5 Ma, mainly by basal superimposed accretion. The isothermal dwelling during peak metamorphism condition of the Hsuehshan Range suggests that tectonic underplating through basal superimposed accretion contribute significantly to the formation of the Taiwan Orogen, and led to a sudden increase of subduction and décollement depths, and slow downward thickening of the orogenic wedge and mountain root growth. Located in the east of the Taiwan Orogen, the Yuli Beltsubduction was initiated at 15.4–16 Ma, and attained peak metamorphism at a depth of 40 km, with a temperature of 550–560 °C and a pressure of 10.5–13 kbar during the interval 12.2–4.4 Ma due to basal superimposed accretion. The exhumation of the Hsuehshan Range commenced at ~2.5 Ma, which is later than that of the Yuli Belt at ~3.4 Ma. Such distinct and different thermal-age paths of the Hsuehshan Range and the Tananao Complex indicate that different tectonic wedges in the Taiwan Orogen are formed through different subduction and accretional processes.

Formation of metasomatic tourmalinites in reduced schists during the Black Hills Orogeny, South Dakota

Abstract Tourmaline is a common mineral in granites and metamorphic rocks in collisional orogens. This paper describes graphite-bearing, metasomatic tourmalinites in sillimanite-zone schists of the Proterozoic Black Hills Orogen, South Dakota. The tourmalinites bound quartz veins and beyond about 1 m grade into schists with disseminated tourmaline, and ultimately tourmaline becomes only a trace, intrinsic phase in the schists. Next to the quartz veins, tourmaline has almost completely replaced schist minerals, including biotite, muscovite, and plagioclase. The tourmaline is generally anhedral and follows the original foliation direction of the schist. However, tourmaline is euhedral in quartz veinlets cutting through the tourmalinites. Tourmaline is compositionally zoned from having about 22 to 2% of apparent Al occupancy on the Y sites. There are very good negative correlations of Y(Fe2++Mg2+), XCa2+, and YTi4+ with YAl3+, and a very good positive correlation of X-site vacancies with YAl3+. Mg# [molar Mg2+/(Mg2++Fe2+)] is fairly invariant at approximately 0.5, which is somewhat higher than that in the precursor biotite. This is in contrast to tourmaline in the neighboring peraluminous Harney Peak leucogranite where the range of Y site occupancy of Al is small at about 20%, but the Mg# ranges from 0.12 to 0.5. The compositional trends in the metasomatic tourmaline are dominated by the exchange X☐ + 4 YAl3+ = XCa2+ + 3 Y(Fe2++Mg2+) + YTi4+. Mass-balance calculations suggest the metasomatizing fluid brought in H+ and B(OH)3 and removed K+, SiO2, and some Fe2+ during tourmalinization. Other elements in the tourmaline largely reflect the bulk composition of the replaced schist. The calculations show that silica in the quartz veins was locally derived, not brought in by the metasomatizing fluid. Interstitial graphite in the tourmalinites shows precipitation of carbon from the methane-bearing fluid. The study demonstrates an important effect of boron transfer by fluids during metamorphism and magmatism in the Earth's crust.

Remarkable petrographic features of Sri Lankan granulites; with special regard to unexpected occurrence of “felsite-nanogranite inclusions” in garnet

Fine spherulitic intergrowths of quartz and feldspar in felsite inclusions (FIs) in garnet in five representative granulite-facies rocks from northern, central and southeastern parts of the Highland Complex (HC), Sri Lanka, are presented together with an experimentally reproduced spherulitic intergrowth of similar dimensions. FIs show characteristics indicative of unexpectedly rapid cooling after far-from-equilibrium crystallization of trapped partial melts formed during high-grade metamorphism, because granulite-facies rocks in continental collision orogens generally have been presumed to be cooled and exhumed slowly from the depths for more than millions of years. Sri Lanka is the key place to unravel the mystery of rapid cooling of huge granulite masses and relevant geological processes because of the most widespread but apparently restricted occurrence of FIs along the marginal part of the HC in addition to accumulated geological, geochemical, and geochronological data.

Precambrian/Early Paleozoic orogenic rocks in the Himalaya - Remnants of the leading edge of the Indian plate

Precambrian/Early Paleozoic orogenic rocks in the Himalaya - Remnants of the leading edge of the Indian plate

Partial melting of subduction zones

俯冲带是地幔对流环的下沉翼，是地球内部的重要物理与化学系统。俯冲带具有比周围地幔更低的温度，因此，一般认为俯冲板片并不会发生部分熔融，而是脱水导致上覆地幔楔发生部分熔融。但是，也有研究认为，在水化的洋壳俯冲过程中可以发生部分熔融。特别是在下列情况下，俯冲洋壳的部分熔融是俯冲带岩浆作用的重要方式。年轻的大洋岩石圈发生低角度缓慢俯冲时，洋壳物质可以发生饱和水或脱水熔融，基性岩部分熔融形成埃达克岩。太古代的俯冲带很可能具有与年轻大洋岩石圈俯冲带类似的热结构，俯冲的洋壳板片部分熔融可以形成英云闪长岩-奥长花岗岩-花岗闪长岩。平俯冲大洋高原中的基性岩可以发生部分熔融产生埃达克岩。扩张洋中脊俯冲可以导致板片窗边缘的洋壳部分熔融形成埃达克岩。与俯冲洋壳相比，俯冲的大陆地壳具有很低的水含量，较难发生部分熔融，但在超高压变质陆壳岩石的折返过程中可以经历广泛的脱水熔融。超高压变质岩在地幔深部熔融形成的熔体与地幔相互作用是碰撞造山带富钾岩浆岩的可能成因机制。碰撞造山带的加厚下地壳可经历长期的高温与高压变质和脱水熔融，形成S型花岗岩和埃达克质岩石。;Subduction zones are descending limbs of mantle convection cells, and the major physical and chemical system of Earth's interior. Subduction zones have lower temperature than the adjacent mantle, and therefore it is proposed that subducted materials including oceanic crust and continental crust do not undergo partial melting, whereas dehydration of the subducted oceanic crust and underlying mantle causes partial melting of the overriding mantle wedge to generate arc magmatic rocks. However, some studies concluded that the hydrated oceanic crust could be partly molten during the subduction. In particular, partial melting of subducted oceanic slabs is probably main mechanism of arc magmatism in the following circumstances. The water-saturated and dehydration melting of subducted oceanic materials can generate the adakitic rocks during the shallow and slow subduction of the younger oceanic lithosphere. The Archean subduction zones have similar features to those of the younger oceanic lithosphere, and therefore the partial melting of the subducted Archean oceanic crust resulted in the formation of tonalite-trondhjemite-granodiorite (TTG). The partial melting of mafic rocks in the shallowly subducted oceanic plateau generated the adakitic rocks. The partial melting of oceanic crust near the margins of slab window during the subduction of spreading mid-oceanic ridge produced the adakitic rocks. Contrary to the subducted oceanic crust, partial melting of deeply subducted continental crusts should be rare due to very low water content. However, the ultrahigh-pressure rocks underwent intensive dehydration melting during the exhumation from the mantle to crust. Interaction between the melt derived from the ultrahigh-pressure metamorphic rocks and the mantle is a potential formation mechanism of the potassic or ultrapotassic magmatic rocks in the collisional orogens. The thickened lower crust of the collisional orogens experienced prolonged high-temperature and high-pressure metamorphism and associated partial melting, and in turn resulted in the formation of S-type granitoids and adakitic rocks.

Geology, geochronology and geochemistry of the Miocene Sulutas volcanic complex, Konya-Central Anatolia: genesis of orogenic and anorogenic rock associations in an extensional geodynamic setting

ABSTRACT Subduction-related environments are complex geological sites where various magmas occur in close spatial and temporal association. To better understand geological processes led to the generation of compositionally distinct magmas, we need to know the origin and evolution of magmas formed in these settings. Our purpose here is to determine the genesis of the Sulutas Volcanic Complex (SVC), located near Konya (Central Anatolia, Turkey) and to understand its relation to the subduction-related processes. Based on a detailed field study of the SVC, we present here Ar-Ar geochronology, mineral chemistry, whole-rock major, trace element and Sr-Nd-Pb isotope data. The SVC can be subdivided into three distinct chemical groups. The oldest group (~16 Ma) characterized by bimodal association is composed of Na-alkaline basaltic rocks and adakitic dacite. The Na-alkaline basaltic rocks were derived from an OIB-like enriched asthenospheric mantle source, but the adakitic dacites from a mafic lower crust. Geochemistry of potassic lamprophyres (~13.5 to 12.5 Ma), the second group, can be best explained by melting of a depleted lithospheric mantle source that was metasomatized by slab-derived sediment melts. This metasomatic component generated phlogopite-rich veins in their source. The youngest group (~13 to 11 Ma), high-K calc-alkaline rocks (HKCA) is composed of basalt, andesite and mafic enclave-bearing dacite. Parental magmas to the HKCA are consistent with the derivation from a lithospheric mantle source metasomatized by fluid-like subduction components. Geochemical variations in the HKCA can be explained by magmatic processes (e.g. assimilation-fractional crystallization for basalt and dacite, fractional crystallization for andesite) from a common parental magma similar to the mafic enclave. The SVC composed of orogenic and anorogenic rocks occurred in a graben-like extensional basin in the Central Anatolia, and this extensional tectonics is related to the retreating subduction zone along the Cyprus arc within the convergence system of the African-Eurasian plates during Miocene.

Structural evolution of the Paleoproterozoic Trans-North China Orogen: Evidence from the Xiaoqinling region, central China

High-grade metamorphic rocks of Archean to Paleoproterozoic age, collectively called the Taihua Complex, are well exposed in the Xiaoqinling region in the western part of the southern segment of the Trans-North China Orogen (TNCO), central China. However, the evolution of orogenic events in this part of the TNCO remains unclear. Here we present structural and zircon U–Pb data to constrain the structural evolution of the rocks in the Xiaoqinling region. The first phase of deformation (D1) produced penetrative, ductile fabrics and localized ductile shear belts, and it was accompanied by a regional high-grade metamorphism. The D1 structures were the result of WNW-directed thrusting movements that have been dated at 1863–1840 Ma, and the microstructures suggest temperatures of 600–650 °C, which correspond to the conditions of exhumation of these orogenic rocks. These data support the ca. 1.85 Ga collision model for the Western and Eastern blocks that involves the SE-directed subduction of the Western Block. The second phase of deformation (D2) produced localized E–W-striking normal shear belts, and the D2 structures provide reliable evidence for N–S extension during the late Paleoproterozoic. Intense magmatic activity accompanied this period of extension. Zircon U–Pb dating constrains the extension to the period between 1829 and 1703 Ma. Post-orogenic extension in the TNCO accounts for D2 and the associated magmatism, and orogen-parallel extension is indicated by the structures. Thus, the Paleoproterozoic structural evolution of the rocks in the Xiaoqinling region of the TNCO records orogenic processes from exhumation to post-orogenic extension.

Pb, Sr and Nd Isotope Ratios of Permian-Triassic Flood Basalts in the Basement of the West Siberian Plate

.