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We combine zircon sensitive high-resolution ion microprobe U-Pb spot dating and mica [sup.40][Ar-.sup.39]Ar plateau ages with field-geological and geochemical constraints from the Malashan area of Southern Tibet to show that the deformed granite core of the North Himalayan metamorphic domes in this area is not Indian basement, but was intruded and deformed during the Himalayan orogeny. Microstructural observations reveal that a transition from top-to-the-south thrust-related to top-to-the-north extension-related deformation occurred during granite intrusion and related metamorphism. This suggests that intrusion triggered the onset of extensional tectonics in the Tibetan middle to upper crust. Expected positive feedback mechanisms between decompression melting leading to more intrusion and more extensional deformation suggest that this mechanism may have been important on a regional scale. Keywords: north Himalayan gneiss domes, granite, age dating, extensional tectonics.

The approach of oceanic ridges to subduction zones finally resulting in ridge subduction is observed on the present Earth's surface. The thermal effects of the ridge approach event are studied using a two-dimensional numerical model around subduction zones with particular emphasis on the implications for the associated high-P/high-T metamorphism. The ridge approach occurs when the subduction rate (v) is larger than the half-spreading rate at the ridge. The difference between the two rates represents the ridge approach rate to the trench, u, which is newly incorporated in the present thermal model. The calculated thermal structure along the slab surface when a ridge reaches the trench generally depends only on u/v when shear heating or mantle flow is not taken into account. If u/v is small, or the ridge approach is slow compared to the subduction rate, the thermal effect of the ridge approach is large enough to explain the 'hot' general P-T conditions off high-P/ high-T metamorphism relative to the steady, state conditions predicted by numerical simulations. In addition, when the ridge is close to the trench, the subduction stage P-T paths experienced by high-P/ high-T metamorphic rocks show a strongly curved trajectory with the dP/dT progressively increasing with pressure. This suggests that the slope of petrologically determined subduction P-T paths may indicate if ridge approach contributed to the formation of the metamorphic rocks. doi: 10.1029/2004TC001715.

Petrologic studies of the Kotsu eclogite from the Sambagawa high-pressure (P), low-temperature (T) metamorphic belt show that the eclogite has recorded a very steep subduction-stage P-T path reaching 20 kbar and 600 [degrees]C; the dP/dT is >7.1 kbar/100 [degrees]C. Compilation of previously derived subduction-stage P-T paths from the Sambagawa belt and the result of this study show that these P-T paths are not on a straight line from the origin, but define a curve with dP/dT increasing with pressure. A new model incorporating the progressive approach of an oceanic ridge to a subduction zone shows that the Sambagawa P-T paths are best explained by a setting in which a ridge is close to being subducted at a slow rate relative to the plate-subduction rate. Exhumation of high-P, low-T metamorphic rocks in oceanic subduction zones may be associated with the slow approach and subsequent subduction of oceanic ridges. Keywords: pressure-temperature path, subduction, oceanic ridge, modeling, eclogite.

New zircon U-Pb and mica [40.sup]Ar/[sup.39]Ar 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. Keywords: Tibetan Plateau, radiometric dating, metamorphism, Cenozoic tectonics.

[1] Clockwise pressure-temperature (P-T) paths with a temperature increase after peak P have been reported from a number of subduction-related metamorphic belts around the world. However, an adequate tectonic interpretation of these paths has been lacking. One of the principal difficulties is the lack of a sufficient heat source to account for the post peak P temperature rise. However, two-dimensional thermal calculations incorporating accretion of a thick metamorphic unit in a subduction zone reveal that, in general, an accreted sheet with a width of several kilometers experiences a post accretion heating without having to appeal to any special internal heat source. The heating effect is due to the spatial shift of the subduction boundary, or subduction boundary jump. This heating can account for the generation of subduction-related clockwise P-T paths within thick metamorphic units. For such units the styles of clockwise P-T paths vary considerably and can be used as a general indicator of the exhumation rate by considering the balance between conduction and advection during exhumation. In contrast, a thin unit with a width of [congruent to] 1 km tends to record a nonclockwise P-T path regardless of its exhumation rate. This is because the thinness decreases the potential both for post accretion heating and for maintaining the higher achieved temperature during exhumation. In addition, a subduction boundary jump can cause a post accretion transition of metamorphic P-T conditions from high to low P-T ratios, and it may therefore be incorrect to use peak T metamorphic conditions as an indicator of the geotherm along the coeval subduction boundary. INDEX TERMS: 3210 Mathematical Geophysics: Modeling, 3660 Mineralogy and Petrology: Metamorphic petrology, 5418 Planetology: Solid Surface Planets: Heat flow, 5475 Planetology: Solid Surface Planets: Tectonics (8149); KEYWORDS: Thermal modeling, clockwise P-T path, subduction, nappe accretion

The Raman spectra of carbonaceous material ( CM) from 19 metasediment samples collected from six widely separated areas of Southwest Japan and metamorphosed at temperatures from 165 to 655°C show systematic changes with metamorphic temperature that can be classified into four types: low-grade CM ( c. 150-280° C), medium-grade CM ( c. 280-400°C), high-grade CM ( c. 400-650°C), and well-crystallized graphite (> c. 650°C). The Raman spectra of low-grade CM exhibit features typical of amorphous carbon, in which several disordered bands (D-band) appear in the first-order region. In the Raman spectra of medium-grade CM, the graphite band ( G-band) can be recognized and several abrupt changes occur in the trends for several band parameters. The observed changes indicate that CM starts to transform from amorphous carbon to crystallized graphite at around 280°C, and this transformation continues until 400°C. The G-band becomes the most prominent peak at high-grade CM suggesting that the CM structure is close to that of well-crystallized graphite. In the highest temperature sample of 655°C, the Raman spectra of CM show a strong G-band with almost no recognizable D-band, implying the CM grain is well-crystallized graphite. In the Raman spectra of low- to medium-grade CM, comparisons of several band parameters with the known metamorphic temperature show inverse correlations between metamorphic temperature and the full width at half maximum ( FWHM) of the D1- and D2-bands. These correlations are calibrated as new Raman CM geothermometers, applicable in the range of c. 150-400°C. Details of the methodology for peak decomposition of Raman spectra from the low to medium temperature range are also discussed with the aim of establishing a robust and user-friendly geothermometer. [ABSTRACT FROM AUTHOR]

The convex form of subduction-stage pressure–temperature ( P–T) paths up to c. 2.0 GPa implies the Sambagawa high- P metamorphic belt, Japan, formed a few million years before ridge subduction. Additional compilation of P–T conditions for higher- P Sambagawa rocks ( c. 2.0–2.5 GPa) reveals that the thermal profile along the slab surface shows a remarkable high- T-ward warping at c. 2.0 GPa ( c. 65 km). Previous thermal models indicate that this warping corresponds to the onset of induced mantle flow towards the subducting slab. If a normal thickness continental crust of c. 30 km was present, this implies the hangingwall region between 30 and 65 km depth was occupied by serpentinized wedge mantle isolated from large-scale mantle flow. Subsequent arrival of the spreading ridge, reheating and dehydration of the serpentinized wedge probably supplied the water necessary for causing granitic magmatism in the Ryoke high- T metamorphic belt, which is paired with the Sambagawa belt. [ABSTRACT FROM AUTHOR]