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Ishwar-Kumar C.,Indian Institute of Science | Sajeev K.,Indian Institute of Science | Windley B.F.,University of Leicester | Kusky T.M.,Wuhan University | And 8 more authors.

The occurrence of high-pressure mafic-ultramafic bodies within major shear zones is one of the indicators of paleo-subduction. In mafic granulites of the Andriamena complex (north-eastern Madagascar) we document unusual textures including garnet-clinopyroxene-quartz coronas that formed after the breakdown of orthopyroxene-plagioclase-ilmenite. Textural evidence and isochemical phase diagram calculations in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 system indicate a pressure-temperature (P-T) evolution from an isothermal (780°C) pressure up to c. 24kbar to decompression and cooling. Such a P-T trajectory is typically attained in a subduction zone setting where a gabbroic/ultramafic complex is subducted and later exhumed to the present crustal level during oceanic closure and final continental collision. The present results suggest that the presence of such deeply subducted rocks of the Andriamena complex is related to formation of the Betsimisaraka suture. LA-ICPMS U-Pb zircon dating of pelitic gneisses from the Betsimisaraka suture yields low Th/U ratios and protolith ages ranging from 2535 to 2625Ma. A granitic gneiss from the Alaotra complex yields a zircon crystallization age of ca. 818Ma and Th/U ratios vary from 1.08 to 2.09. K-Ar dating of muscovite and biotite from biotite-kyanite-sillimanite gneiss and garnet-biotite gneiss yields age of 486±9Ma and 459±9Ma respectively. We have estimated regional crustal thicknesses in NE Madagascar using a flexural inversion technique, which indicates the presence of an anomalously thick crust (c. 43km) beneath the Antananarivo block. This result is consistent with the present concept that subduction beneath the Antananarivo block resulted in a more competent and thicker crust. The textural data, thermodynamic model, and geophysical evidence together provide a new insight to the subduction history, crustal thickening and evolution of the high-pressure Andriamena complex and its link to the terminal formation of the Betsimisaraka suture in north-eastern Madagascar. © 2015 Elsevier B.V. Source

Sawada Y.,The University of Shimane | Sakai T.,The University of Shimane | Itaya T.,Okayama University of Science | Yagi K.,Hiruzen Institute for Geology and Chronology Co. | And 3 more authors.
Palaeobiodiversity and Palaeoenvironments

Pumice beds of the Maragheh Formation, which is distributed on the plains at the foot of the late Cenozoic Sahand volcano, NW Iran, consist of the Mordaq tuff bed, lower pumice beds (A and B), middle pumice bed and upper pumice bed, in ascending order. New hornblende and plagioclase K–Ar ages of the pumices from these are: Mordaq tuff bed: 8.14 ± 0.27 Ma, lower pumice beds (A): 7.54 ± 0.22 Ma, lower pumice beds (B): 6.95 ± 0.28 Ma, middle pumice bed: 7.87 ± 0.29 Ma, upper pumice bed: 6.96 ± 0.31 Ma. The middle pumice bed is a flood deposit, and the chemical compositions of glass, hornblende and biotite in pumices overlap those from the lower pumice beds (A). These features indicate that the pumices of the middle pumice bed were derived from a similar source to the lower pumice beds (A). Chemical compositions of pumices from the Maragheh Formation fall in the dacite field of the total alkali-silica (TAS) classification diagram and slightly differ from those of the Plio–Pleistocene rocks from the Sahand volcano. Chemical compositions of late Miocene Maragheh Formation pumices show adakitic features with high Sr contents (698–545 ppm), and low Y (12–8 ppm). It is inferred that the source materials of the adakitic dacite magma were amphibole eclogite or garnet amphibolite originating from the Neo-Thetys plate subducted beneath the Eurasia plate. © 2016 Senckenberg Gesellschaft für Naturforschung and Springer-Verlag Berlin Heidelberg Source

Sasaki Y.,Yamaguchi University | Imaoka T.,Yamaguchi University | Nagashima M.,Yamaguchi University | Nakashima K.,Yamagata University | And 3 more authors.
Resource Geology

The relationship between the magmatism of the Cretaceous Ofuku pluton and mineralization in and around the Akiyoshi Plateau, Yamaguchi Prefecture, Japan was investigated using a combination of field observation, petrographic and geochemical analyses, K-Ar geochronology, and fluid inclusion data. The Ofuku pluton has a surface area of 1.5×1.0km, and was intruded into the Paleozoic accretionary complexes of the Akiyoshi Limestone, Ota Group and Tsunemori Formation in the western part of the Akiyoshi Plateau. The pluton belongs to the ilmenite-series and is zoned, consisting mainly of early tonalite and granodiorite that share a gradational contact, and later granite and aplite that intruded the tonalite and granodiorite. Harker diagrams show that the Ofuku pluton has intermediate to silicic compositions ranging from 60.4 to 77.9wt.% SiO2, but a compositional gap exists between 70.5 to 73.4wt.% SiO2 (anhydrous basis). Modal and chemical variations indicate that the assumed parental magma is tonalitic. Quantitative models of fractional crystallization based on mass balance calculations and the Rayleigh fractionation model using major and trace element data for all crystalline phases indicate that magmatic fractionation was controlled mainly by crystal fractionation of plagioclase, hornblende, clinopyroxene and orthopyroxene at the early stage, and quartz, plagioclase, biotite, hornblende, apatite, ilmenite and zircon at the later stage. The residual melt extracted from the granodiorite mush was subsequently intruded into the northern and western parts of the Ofuku pluton as melt lens to form the granite and aplite. The age of the pluton was estimated at 99-97Ma and 101-98Ma based on K-Ar dating of hornblende and biotite, respectively. Both ages are consistent within analytical error, indicating that the Ofuku pluton and the associated Yamato mine belong to the Tungsten Province of the San-yo Belt, which is genetically related to the ilmenite-series granitoids of the Kanmon to Shunan stages. The aplite contains Cl-rich apatite and REE-rich monazite-(Ce), allanite-(Ce), xenotime and bastnäsite-(Ce), indicating that the residual melt was rich in halogens and REEs. The tonalite-granodiorite of the Ofuku pluton contains many three-phase fluid inclusions, along with daughter minerals such as NaCl and KCl, and vapor/liquid (V/L) volume ratios range from 0.2 to 0.9, suggesting that the fluid was boiling. In contrast, the granite and aplite contain low salinity two-phase inclusions with low V/L ratios. The granodiorite occupies a large part of the pluton, and the inclusions with various V/L ratios with chloride daughter minerals suggest the boiling fluids might be related to the mineralization. This fluid could have carried base metals such as Cu and Zn, forming Cu ore deposits in and around the Ofuku pluton. The occurrence and composition of fluid inclusions in the igneous rocks from the Akiyoshi Plateau are directly linked to Cu mineralization in the area, demonstrating that fluid inclusions are useful indicators of mineralization. © 2016 The Society of Resource Geology. Source

Gouzu C.,Hiruzen Institute for Geology and Chronology Co. | Yagi K.,Hiruzen Institute for Geology and Chronology Co. | Thanh N.X.,Hanoi University of Mining and Geology | Itaya T.,Okayama University of Science | Compagnoni R.,University of Turin

High-pressure and ultra-high pressure (HP-UHP) blueschist- and eclogite-facies metabasaltic and metasedimentary rocks occur in four different tectonic units near Lago di Cignana, western Alps. We have determined K-Ar ages for white micas (matrix phengite and paragonite) from the Lago di Cignana UHP unit (LCU; 39-41 Ma); the lower and upper units of the Zermatt-Saas meta-ophiolite (LU and UU; 37-38 Ma and 38-41 Ma respectively), and the Combin unit (CU; 36-40 Ma). These K-Ar ages overlap with single-grain Ar-Ar plateau ages (36-42 Ma) previously determined for phengites from LCU metasediments. Matrix white micas have been severely deformed during exhumation, and their chemistries differ from those of micas included in garnet. Although individual mica grains in the matrix could have experienced different degrees of deformation which have reset their K-Ar systems, "bulk" white mica separates provide the average age of all the individual grains in the separate. The similarity of ages determined for white micas from the LCU, LU, UU and CU units, regardless of rock type and mineral species, suggests that these four units were metamorphosed together as part of a single metamorphic sequence in the Piemonte-Liguria paleosubduction zone and were subsequently exhumed together. However, present-day structural relationship among those units and the limited occurrence of UHP minerals in LCU suggests that the exhumation of LCU was more rapid than that for LU, UU and CU. The age gaps between the youngest value of white mica K-Ar ages in each unit and the inferred timing of the metamorphic peak (U-Pb age: 44 Ma) is 5, 7, 6 and 8 Myr for LCU, LU, UU and CU, respectively. These intervals are considerably shorter than that determined for the Sanbagawa HP metamorphic belt of Southwest Japan (>. 31 Myr). The short interval observed for the Lago di Cignana units that we have studied is consistent with the model of rapid exhumation of the UHP-bearing metamorphic domain, suggesting the exhumation rate is higher than 18 mm/y in the early stage of exhumation from the deepest level (ca. 120 km) to the lower crust (ca. 30 km). © 2016 Elsevier B.V. Source

Ishwar-Kumar C.,Indian Institute of Science | Windley B.F.,University of Leicester | Horie K.,Japan National Institute of Polar Research | Kato T.,Nagoya University | And 5 more authors.
Precambrian Research

We report detailed evidence for a new paleo-suture zone (the Kumta suture) on the western margin of southern India. The c. 15-km-wide, westward dipping suture zone contains garnet-biotite, fuchsite-haematite, chlorite-quartz, quartz-phengite schists, biotite augen gneiss, marble and amphibolite. The isochemical phase diagram estimations and the high-Si phengite composition of quartz-phengite schist suggest a near-peak condition of c. 18. kbar at c. 550. °C, followed by near-isothermal decompression. The detrital SHRIMP U-Pb zircon ages from quartz-phengite schist give four age populations ranging from 3280 to 2993. Ma. Phengite from quartz-phengite schist and biotite from garnet-biotite schist have K-Ar metamorphic ages of ca. 1326 and ca. 1385. Ma respectively. Electron microprobe-CHIME ages of in situ zircons in quartz-phengite schist (ca. 3750. Ma and ca. 1697. Ma) are consistent with the above results. The Bondla ultramafic-gabbro complex in the west of the Kumta suture compositionally represents an arc with K-Ar biotite ages from gabbro in the range 1644-1536. Ma. On the eastern side of the suture are weakly deformed and unmetamorphosed shallow westward-dipping sedimentary rocks of the Sirsi shelf, which has the following upward stratigraphy: pebbly quartzite/sandstone, turbidite, magnetite iron formation, and limestone; farther east the lower lying quartzite has an unconformable contact with ca. 2571. Ma quartzo-feldspathic gneisses of the Dharwar block with a ca. 1733. Ma biotite cooling age. To the west of the suture is a c. 60-km-wide Karwar block mainly consisting of tonalite-trondhjemite-granodiorite (TTG) and amphibolite. The TTGs have U-Pb zircon magmatic ages of ca. 3200. Ma with a rare inherited core age of ca. 3601. Ma. The K-Ar biotite cooling age from the TTGs (1746. Ma and 1796. Ma) and amphibolite (ca. 1697. Ma) represents late-stage uplift. Integration of geological, structural and geochronological data from western India and eastern Madagascar suggest diachronous ocean closure during the amalgamation of Rodinia; in the north at around ca. 1380. Ma, and a progression toward the south until ca. 750. Ma. Satellite imagery based regional structural lineaments suggests that the Betsimisaraka suture continues into western India as the Kumta suture and possibly farther south toward a suture in the Coorg area, representing in total a c. 1000. km long Rodinian suture. © 2013 Elsevier B.V. Source

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