Abstract Despite the importance of Tethys Himalayan or North Himalayan gneiss domes for discussing extrusive flow of the underlying Greater Himalayan Sequence, these metamorphic domes in general remain poorly documented. The main exception is the Kangmar dome. The Malashan metamorphic complex, a newly documented North Himalayan gneiss dome, is shown to have strong similarities with the Kangmar dome, suggesting that the North Himalayan gneiss domes have the following features in common: (i) Barrovian-type metamorphism with grade increasing towards a centrally located two-mica granite; (ii) the presence of two dominant ductile deformation stages, D 1 and D 2 , with D 2 showing an increasing strength towards the granite contacts; and (iii) the development of a strong D 2 foliation (gneissosity) in the outermost part of the granite cores. In addition, field and bulk-chemical studies show: (i) D 2 is associated with a dominant top-to-the-north sense of shear (in disagreement with the most recent kinematic studies in Kangmar dome); (ii) the deposition age of associated metasediments is upper Jurassic suggesting that the Malashan dome is located not at the base, but within the middle section of the Tethys Himalaya; and (iii) in contrast to the Kangmar granitic gneiss that is interpreted as Indian basement, three granitic bodies in Malashan all formed as young intrusive bodies during the Himalayan orogeny. These results suggest that the formation mechanism of the North Himalayan gneiss domes needs to be re-evaluated to test the rigidity of the hanging wall assumed in channel flow models.
Abstract The degree of graphitization of carbonaceous material (CM) has been widely used as an indicator of metamorphic grade. Previous work has demonstrated that peak metamorphic temperature ( T ) of regional metamorphic rocks can be estimated by an area ratio (R2) of peaks recognized in Raman spectra of CM. The applicability of this method to low‐pressure (<3 kbar) contact metamorphism was tested using Raman spectroscopic analyses of samples from two contact‐metamorphic aureoles in Japan (Daimonji and Kasuga areas). A suitable measurement procedure allows the dependence of the geothermometer on sample type (thin section, chip) and incident angle of laser beam relative to the c ‐axes of CM to be tested. Two important general results are: (i) in addition to standard thin sections, chips are also suitable for spectral analysis; and (ii) the incident angle of the laser beam does not significantly affect the temperature estimation, i.e. spectral measurements for the geothermometer can be carried out irrespective of the crystallographic orientation. A laser wavelength of 532 nm was used in this study compared with 514.5 nm in an independent previous study. A comparison shows that the use of a 532‐nm laser results in a slightly, but systematically larger R2 ratio than that of a 514.5‐nm laser. Taking this effect into account, our results show that there is a slight but distinct difference between the R2– T correlations shown by contact and regional metamorphic rocks: the former are slightly better‐crystallized (have slightly lower R2 values) than the latter at the same temperature. This difference is interpreted as due to the degree of associated deformation. Despite the slight difference, the results of this study coincide within the estimated errors of ±50 °C with those of the previously proposed Raman CM geothermometer, thus demonstrating the applicability of this method to contact metamorphism. To facilitate more precise temperature estimates in regions of contact metamorphism, a new calibration for analyses using a 532‐nm laser is derived. Another important observation is that the R2 ratio of metamorphosed CM in pelitic and psammitic rocks is highly heterogeneous with respect to a single sample. To obtain a reliable temperature estimate, the average R2 value must be determined by using a substantial number of measurements (usually N > 50) that adequately reflects the range of sample heterogeneity. Using this procedure (with 532‐nm laser) and adapting our new calibration, the errors of the Raman CM geothermometer for contact metamorphic rocks decrease to ∼±30 °C.
In general, derivation of subduction-stage P-T (pressure-temperature) paths for high-P metamorphic rocks is difficult, because in most cases they are affected by subsequent metamorphism, during which the pre-peak P mineral assemblages are significantly modified. However, petrological studies during the last 10 years in the Sambagawa belt have recognized unusually well-constrained sections of the subduction-stage P-T path. In particular, derivation of the subduction-stage P-T path of the Kotsu glaucophane eclogite was backed-up by insights obtained from a combination of petrological and structural analyses. Application of a new thermal model shows that the derived subduction-stage P-T paths are most likely to have been generated in a setting where an active spreading ridge is close to being subducted. This tectonic interpretation suggests that exhumation of high-P/T metamorphic rocks in oceanic subduction zones is triggered by regional heating events such as ridge subduction. A review of recent studies on the Sambagawa belt indicates the significance of promoting a 'comprehensive petrology', which encompasses the full set of constraints provided by disciplines such as petrology, structural geology, geochronology and thermal modeling.
Research Article| September 01, 2003 Cenozoic and Mesozoic metamorphism in the Longmenshan orogen: Implications for geodynamic models of eastern Tibet Simon Wallis; Simon Wallis 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Search for other works by this author on: GSW Google Scholar Tatsuki Tsujimori; Tatsuki Tsujimori 2Research Institute of Natural Sciences, Okayama University of Science, Okayama 700-0005, Japan Search for other works by this author on: GSW Google Scholar Mutsuki Aoya; Mutsuki Aoya 3Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Search for other works by this author on: GSW Google Scholar Tetsuo Kawakami; Tetsuo Kawakami 4Department of Earth Sciences, Faculty of Education, Okayama University, Okayama 700-8530, Japan Search for other works by this author on: GSW Google Scholar Kentaro Terada; Kentaro Terada 5Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Hiroshima 739-8526, Japan Search for other works by this author on: GSW Google Scholar Kazuhiro Suzuki; Kazuhiro Suzuki 6Nagoya University Center for Chronological Research, Nagoya University, Nagoya 464-8602, Japan Search for other works by this author on: GSW Google Scholar Hironobu Hyodo Hironobu Hyodo 7Research Institute of Natural Sciences, Okayama University of Science, Okayama 700-0005, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information Simon Wallis 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Tatsuki Tsujimori 2Research Institute of Natural Sciences, Okayama University of Science, Okayama 700-0005, Japan Mutsuki Aoya 3Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Tetsuo Kawakami 4Department of Earth Sciences, Faculty of Education, Okayama University, Okayama 700-8530, Japan Kentaro Terada 5Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Hiroshima 739-8526, Japan Kazuhiro Suzuki 6Nagoya University Center for Chronological Research, Nagoya University, Nagoya 464-8602, Japan Hironobu Hyodo 7Research Institute of Natural Sciences, Okayama University of Science, Okayama 700-0005, Japan Publisher: Geological Society of America Received: 01 Feb 2003 Revision Received: 27 May 2003 Accepted: 29 May 2003 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2003) 31 (9): 745–748. https://doi.org/10.1130/G19562.1 Article history Received: 01 Feb 2003 Revision Received: 27 May 2003 Accepted: 29 May 2003 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Simon Wallis, Tatsuki Tsujimori, Mutsuki Aoya, Tetsuo Kawakami, Kentaro Terada, Kazuhiro Suzuki, Hironobu Hyodo; Cenozoic and Mesozoic metamorphism in the Longmenshan orogen: Implications for geodynamic models of eastern Tibet. Geology 2003;; 31 (9): 745–748. doi: https://doi.org/10.1130/G19562.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract New zircon U-Pb and mica 40Ar/39Ar dating combined with structural studies in the Longmenshan orogen confirm that most of the upper crustal deformation in the eastern margin of Tibet is Mesozoic. However, at lower structural levels, apatite U-Pb and monazite electron microprobe dating reveals a previously unknown domain of Cenozoic (ca. 65 Ma) Barrovian-type metamorphism and deformation. This discovery shows that the crust in the eastern margin of Tibet was already a substantial thickness around the time of the India-Asia collision. Associated deformation has a N-S-oriented stretching lineation, implying that deformation was not driven by topographic gradients in the Tibetan Plateau. The observed moderate amounts of distributed postmetamorphic E-W shortening can probably explain the present thickness of the continental crust in the area. These results do not support models of crustal thickening caused by solid-state lateral flow of midcrustal metamorphic rocks. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
