The Tama Kosi/Rolwaling area of east-central Nepal is underlain by the exhumed mid-crustal core of the Himalaya. The geology of the area consists of Greater Himalayan sequence phyllitic schist, paragneiss, and orthogneiss that generally increase in metamorphic grade from biotite ± garnet assemblages to sillimanite-grade migmatite up structural section. All metamorphic rocks are pervasively deformed and commonly record top-to-the-south sense shear. The top of the Greater Himalayan sequence in the mapped area is marked by an undeformed, pegmatitic leucogranite stock. Relationships in adjacent areas constrain the age of the leucogranite and the deformation structures it crosscuts, including the top-to-the-south sense deformation, to be older than middle Miocene. The lower portion of the exhumed midcrustal package has been subject to late-stage folding during the formation of the Tama Kosi window, a structural culmination that may reflect out-of-sequence adjustment of the orogenic wedge. The geology of the mapped area appears similar to that observed in the adjacent, better-studied Everest region.
Abstract:The role of continent collisional belts in the global carbon budget remains controversial. Collisional orogens have traditionally been considered a net carbon sink, but recent studies have highlighted significant CO2 fluxes. This study carried out comprehensive field mapping, petrography, pressure-temperature determination, geochemistry, and geochronological data along three transects of the South Tibetan Detachment System (STDS) in the Mount Everest and Nyalam regions of the central Himalaya. The results outline a previously unrecognized carbon source in the Himalaya: Buchan-type metamorphic decarbonation of carbonate-bearing lithologies in the migrating hanging-wall of the STDS driven by the juxtaposition against hot migmatite/magma in the footwall of the structure. Specifically, calc-silicates and schists incorporated into the active STDS underwent Buchan-type metamorphic overprinting at P-T conditions of 630–400°C and 5–3 kbar (36–48°C/km) compared to Barrovian-type metamorphism (28°C/km) in the footwall. Monazite and titanite U(-Th)-Pb petrochronology indicate that metamorphism within the STDS occurred between ca. 23 and 19 Ma, which is contemporaneous with deformation along the STDS evidenced by the ages of mylonitized leucogranites. Activity along the STDS sustained to 17–16 Ma, causing resetting of titanite U-Pb ages in some calc-silicates. Detrital zircon geochronology shows that the Yellow Band and North Col Formation in the STDS have an affinity to the Tethyan Himalayan Sequence and were involved in the shear zone during its upward expansion into hanging-wall rocks. Based on decarbonation reactions, protolith restoration, and decarbonation efficiency studies, the metamorphic CO2 degassing from the metamorphism of calc-silicate rocks is quantified to be ~0.8 Mt C/yr during 23–19 Ma. The quantification of upward expansion of the STDS, the resulting juxtaposition of underlying hot migmatites/magma with cold hanging wall-rocks, and the proposed metamorphic decarbonation phenomenon are crucial to understanding the development process of orogen-scale low-angle normal-sense faulting and the resulting carbon sources during Himalayan orogenesis.
Magmatic and tectonic processes can transport large volumes of magma generated in the deep crust as discrete pulses to shallower crustal depths, resulting in the incremental construction of large, composite batholiths over thousands to tens of millions of years. The Silurian to Early Devonian Donegal composite batholith in Ireland is a classic example of which regional geological syntheses and lithogeochemical data show that emplacement was syn- and post-kinematic with respect to the terminal phases (ca. 437−415 Ma) of the Caledonian orogeny. We used U-Pb dating of zircon and titanite to investigate the construction of the batholith over time. Imaging of these minerals reveals complex, zoned grains with distinct autocrystic (growth during pluton emplacement) and antecrystic (growth during lower crustal incubation) domains as well as xenocrysts (incorporated from wall rocks). To determine the ages of emplacement and of inherited domains, discrete growth zones were targeted for dating using laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS). Taken together, the zircon and titanite U-Pb isotopic data indicate that magmatism occurred over at least 30 m.y., between ca. 430 Ma and 400 Ma. Batholith emplacement is bracketed by the ca. 427−423 Ma Ardara pluton and the latest phases in the Main Donegal and Trawenagh Bay plutons (ca. 400 Ma). Although apparently volumetrically minor, U-Pb data from spatially associated mafic rocks (appinite suite, lamprophyre dikes, and mafic enclaves in granitoid plutons) yield ages ranging from ca. 431−416 Ma, which indicates ongoing mafic magmatism during emplacement of much of the Donegal composite batholith.
Abstract The continental collision between India and Asia has been ongoing since early Eocene time, but the orogenic record is typically dominated by Miocene and younger deformation and metamorphism that largely overprinted earlier Eocene‐Oligocene events. This hinders our understanding of how crustal thickening responds to initial collision and when the Himalayan mountains initially rise. The advancement of spatially precise petrochronology techniques, however, has provided the means to see through the Miocene overprint and enabled the characterization of Eocene metamorphism in different parts of the Himalaya. The current study presents new monazite petrochronology and paired thermobarometry from the Kathmandu klippe in the central Nepalese Himalaya. These data reveal Eocene prograde metamorphism (44‐38 Ma) and partial melting (38‐35 Ma) under peak P‐T conditions of 730 °C–760 °C and up to 10.5 kbar. The migmatites within the Kathmandu klippe is equivalent to the Upper or Uppermost Greater Himalayan Crystallines and should have been exhumed during Eocene‐Oligocene. The new evidence of Eocene metamorphism and anatexis presented herein adds to a growing body of data detailing initial crustal thickening during the early continent collision. The mid‐Eocene crustal thickening event indicates that the Himalayan felsic crust was thickened to a depth of ∼35 km shortly within 10–20 Myr of the initial collision, which was probably responsible for the initial topographic rise of the Himalayan proto‐mountains. Characterizing the effects of this early orogenesis is critical in understanding the Himalayan architecture prior to the better‐preserved Miocene metamorphism and anatexis record and how the orogen may have been preconditioned for the younger stage.
Abstract. Re-examination of International Ocean Discovery Program (IODP) sediment samples collected from the Bay of Bengal via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) Rb–Sr geochronology demonstrates the viability of the biotite Rb–Sr system for use as a detrital chronometer. The age population defined by the Rb–Sr dates essentially reproduces that previously published for detrital 40Ar/39Ar dates. The effect of unknown/assumed initial 87Sr/86Sr on the calculated population can be ameliorated by filtering for higher 87Rb/86Sr ratios. Such filtering, however, could introduce bias toward more radiogenic populations, especially in younger material that has not had time to accumulate radiogenic product (e.g. limiting the effect of initial 87Sr/86Sr to ∼ <5 % requires filtering of 87Rb/86Sr >500 at 250 Ma and 87Rb/86Sr >50 at 2500 Ma). Finally, Ti-in-biotite temperatures calculated based on element concentration data collected during LA-ICP-MS overlap with those calculated for the same material based on electron probe microanalyzer data, demonstrating the potential for in situ biotite petrochronology based on the Rb–Sr system.
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