271 research outputs found
Phase equilibrium modelling of the amphibolite to granulite facies transition in metabasic rocks (Ivrea Zone, NW Italy)
The development of thermodynamic models for tonalitic melt and the updated clinopyroxene and amphibole models now allow the use of phase equilibrium modelling to estimate PâT conditions and melt production for anatectic mafic and intermediate rock types at highâtemperature conditions.
The Permian midâlower crustal section of the Ivrea Zone preserves a metamorphic field gradient from mid amphibolite facies to granulite facies, and thus records the onset of partial melting in metabasic rocks. Interlayered metabasic and metapelitic rocks allows the direct comparison of PâT estimates and partial melting between both rock types with the same metamorphic evolution. Pseudosections for metabasic compositions calculated in the Na2OâCaOâK2OâFeOâMgOâAl2O3âSiO2âH2OâTiO2âO (NCKFMASHTO) system are presented and compared with those of metapelitic rocks calculated with consistent endmember data and aâx models. The results presented in this study show that PâT conditions obtained by phase equilibria modelling of both metabasic and metapelitic rocks give consistent results within uncertainties, allowing integration of results obtained for both rock types. In combination, the calculations for both metabasic and metapelitic rocks allows an updated and more precisely constrained metamorphic field gradient for Val Strona di Omegna to be defined. The new field gradient has a slightly lower dP/dT which is in better agreement with the onset of crustal thinning of the Adriatic margin during the Permian inferred in recent studies
Zircon UâPb ages and Hf isotopes of the Askot klippe, Kumaun, northwest India: Implications for Paleoproterozoic tectonics, basin evolution and associated metallogeny of the northern Indian cratonic margin
Exhumation history of the Higher Himalayan Crystalline along Dhauliganga-Goriganga river valleys, NW India: new constraints from fission track analysis
New apatite and zircon fission track data collected from two transects along the Dhauliganga and Goriganga rivers in the NW Himalaya document exhumation of the Higher Himalayan Crystalline units. Despite sharing the same structural configuration and rock types and being separated by only 60 km, the two study areas show very different patterns of exhumation. Fission track (FT) data from the Dhauliganga section show systematic changes in age (individual apatite FT ages range from 0.9 Âą 0.3 to 3.6 Âą 0.5 Ma, r 2 = 0.82) that record faster exhumation across a zone that extends from the Main Central Thrust to north of the Vaikrita thrust. By contrast, FT results from the Goriganga Valley show a stepwise change in ages across the Vaikrita thrust that suggests Quaternary thrust sense displacement. Footwall samples yield a weighted mean apatite age of 1.6 Âą 0.1 Ma compared to 0.7 Âą 0.04 Ma in the hanging wall. A constant zircon fission track age of 1.8 Âą 0.4 Ma across both the footwall and hanging wall shows the 0.9 Ma difference in apatite ages is due to movement on the Vaikrita thrust that initiated soon after âź1.8 Ma. The Goriganga section provides clear evidence for >1 Ma of tectonic deformation in the brittle crust that contrasts with previous exhumation studies in other areas of the high Himalaya ranges; these studies have been unable to decouple the role of climate erosion from tectonics. One possibility why there is a clear tectonic signal in the Goriganga Valley is that climate erosion has not yet fully adjusted to the tectonic perturbation
Deformational temperatures across the Lesser Himalayan Sequence in eastern Bhutan and their implications for the deformation history of the Main Central Thrust
We postulate that the inverted metamorphic sequence in the Lesser Himalayan Sequence of the Himalayan orogen is a finite product of its deformation and temperature history. To explain the formation of this inverted metamorphic sequence across the Lesser Himalayan Sequence with a focus on the Main Central Thrust (MCT) in eastern Bhutan, we determined the metamorphic peak temperatures by Raman spectroscopy of carbonaceous material and established the deformation temperatures by Ti-in-quartz thermobarometry and quartz c axis textures. These data were combined with thermochronology, including new and published Ar-40/Ar-39 ages of muscovite and published apatite fission track, and apatite and zircon (U-Th)/He ages. To obtain accurate metamorphic, deformation, and closure temperatures of thermochronological systems, pressures and cooling rates for the period of interest were derived by inverse modeling of multiple thermochronological data sets, and temperatures were determined by iterative calculations. The Raman spectroscopy of carbonaceous material results indicate two temperature sequences separated by a thrust. In the external sequence, peak temperatures are constant across the structural strike, consistent with the observed hinterland-dipping duplex system. In the internal temperature sequence associated with the MCT shear zone, each geothermometer yields an apparent inverted temperature gradient although with different temperature ranges, and all temperatures appear to be retrograde. These observations are consistent with the quartz microfabrics. Further, all thermochronometers indicate upward younging across the MCT. We interpret our data as a composite peak and deformation temperature sequence that formed successively and reflects the broadening and narrowing of the MCT shear zone in which the ductile deformation lasted until similar to 11 Ma.Peer reviewe
Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya
Geochemical and geochronological analyses provide quantitative evidence about the origin, development and motion along ductile faults, where kinematic structures have been overprinted. The Main Central Thrust is a key structure in the Himalaya that accommodated substantial amounts of the IndiaââŹâAsia convergence. This structure juxtaposes two isotopically distinct rock packages across a zone of ductile deformation. Structural analysis, whole-rock Nd isotopes, and UââŹâPb zircon geochronology reveal that the hanging wall is characterized by detrital zircon peaks at c. 800ââŹâ1000ââŹâ°Ma, 1500ââŹâ1700ââŹâ°Ma and 2300ââŹâ2500ââŹâ°Ma and an ĂÂľNd(0) signature of ââŹâ18.3 to ââŹâ12.1, and is intruded by c. 800ââŹâ°Ma and c. 500ââŹâ600ââŹâ°Ma granites. In contrast, the footwall has a prominent detrital zircon peak at c. 1800ââŹâ1900ââŹâ°Ma, with older populations spanning 1900ââŹâ3600ââŹâ°Ma, and an ĂÂľNd(0) signature of ââŹâ27.7 to ââŹâ23.4, intruded by c. 1830ââŹâ°Ma granites. The data reveal a c. 5ââŹâ°km thick zone of tectonic imbrication, where isotopically out-of-sequence packages are interleaved. The rocks became imbricated as the once proximal and distal rocks of the Indian margin were juxtaposed by Cenozoic movement along the Main Central Thrust. Geochronological and isotopic characterization allows for correlation along the Himalayan orogen and could be applied to other cryptic ductile shear zones
Distribution, Abundance and Molecular Analysis of Genus Barbadocladius Cranston & Krosch (Diptera, Chironomidae) in Tropical, High Altitude Andean Streams and Rivers
The distribution of the genus Barbadocladius Cranston & Krosch (Diptera: Chironomidae), previously reported from Chile to Bolivia, has extended northwards. Larvae, pupae and pupal exuviae of this genus have been found in the high mountain tropical streams of Peru to 9°22â˛56âł, but are restricted to very high altitude streams (altitudes over 3,278 m asl) compared to the lower altitude streams (below 1,100 m asl) in which the genus is reported in Chile and Argentina. Based on morphological studies, both described species in the genus, Barbadocladius andinus Cranston & Krosch and Barbadocladius limay Cranston & Krosch, have been found in Peru as pupae or pupal exuviae. Morphological analysis of the larvae and pupae revealed no differences between the two described species from Patagonia and Peru, which are of similar size and with a similar armament of hooklets and spines in pupal tergites and sternites. However, molecular analysis of larvae and pupae revealed that in Peru, there are at least two different evolutionary lines, one distributed widely and another restricted to one site. Phylogenetic analysis (using cox1 mitochondrial sequences) of all available sequences of Barbadocladius shows that the Chilean and Argentinean material differs from that of Peru. Therefore, a total of four molecular segregates are identified, although morphologically, neither larvae nor the pupae may be differentiated
Boron isotopic composition of olivine-hosted melt inclusions from Gorgona komatiites, Colombia : new evidence supporting wet komatiite origin
Author Posting. Š The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 312 (2011): 201â212, doi:10.1016/j.epsl.2011.09.033.A fundamental question in the genesis of komatiites is whether 30 these rocks originate
from partial melting of dry and hot mantle, 400â500°C hotter than typical sources of MORB and
OIB magmas, or if they were produced by hydrous melting of the source at much lower
temperatures, similar or only moderately higher than those known today. Gorgona Island,
Colombia, is a unique place where Phanerozoic komatiites occur and whose origin is directly
connected to the formation of the Caribbean Large Igneous Province. The genesis of Gorgona
komatiites remains controversial, mostly because of the uncertain origin of volatile components
which they appear to contain. These volatiles could equally result from shallow level magma
contamination, melting of a âdampâ mantle or fluid-induced partial melting of the source due to
devolatilization of the ancient subducting plate. We have analyzed boron isotopes of olivine40
hosted melt inclusions from the Gorgona komatiites. These inclusions are characterized by
relatively high contents of volatile components and boron (0.2â1.0 wt.% H2O, 0.05â0.08 wt.%
S, 0.02â0.03 wt.% Cl, 0.6â2.0 Îźg/g B), displaying positive anomalies in the overall depleted,
primitive mantle (PM) normalized trace element and REE spectra ([La/Sm]n = 0.16â0.35;
[H2O/Nb]n = 8â44; [Cl/Nb]n = 27â68; [B/Nb]n = 9-30, assuming 300 Îźg/g H2O, 8 Îźg/g Cl and
0.1 Îźg/g B in PM; Kamenetsky et al., 2010. Composition and temperature of komatiite melts
from Gorgona Island constrained from olivine-hosted melt inclusions. Geology 38, 1003â1006).
The inclusions range in δ11B values from â11.5 to +15.6 Âą 2.2â° (1 SE), forming two distinct
trends in a δ11B vs. B-concentration diagram. Direct assimilation of seawater, seawater-derived
components, altered oceanic crust or marine sediments by ascending komatiite magma cannot
readily account for the volatile contents and B isotope variations. Alternatively, injection of <3%
of a 11B enriched fluid to the mantle source could be a plausible explanation for the δ11B range
that also may explain the H2O, Cl and B excess.Financial support
to AAG during data acquisition and manuscript preparation was provided by Northeast National
Ion Microprobe Facility (Woods Hole Oceanographic Institution, USA) and the Centre de
Recherches PĂŠtrographiqueset GĂŠochimiques (France). This research was also supported by the
Australian Research Council (Research Fellowship and Discovery grants to VSK). We
acknowledge partial support of the Alexander von Humboldt Foundation, Germany (F.W. Bessel
Award to VSK and Wolfgang Paul Award to A.V. Sobolev who provided access to the electron
microprobe at the Max Planck Institute, Mainz, Germany
Reconciling Himalayan midcrustal discontinuities: The Main Central thrust system
The occurrence of thrust-sense tectonometamorphic discontinuities within the exhumed Himalayan metamorphic core can be explained as part of the Main Central thrust system. This imbricate thrust structure, which significantly thickened the orogenic midcrustal core, comprises a series of thrust-sense faults that all merge into a single detachment. The existence of these various structures, and their potential for complex overprinting along the main detachment, may help explain the contention surrounding the definition, mapping, and interpretation of the Main Central thrust. The unique evolution of specific segments of the Main Central thrust system along the orogen is interpreted to be a reflection of the inherent basement structure and ramp position, and structural level of exposure of the mid-crust. This helps explain the variation in the timing and structural position of tectonometamorphic discontinuities along the length of the mountain belt
PlioâQuaternary exhumation history of the central Nepalese Himalaya: 2. Thermokinematic and thermochronometer age prediction model
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95647/1/tect1875.pd
The Neotethyan SanandajâSirjan zone of Iran as an archetype for passive marginâarc transitions
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