ALBERT

Description: Gneiss domes in the Pamir (Central Asia) and the Himalaya provide key data on mid- to deep-crustal processes operating during the India-Asia collision. Laser ablation split-stream inductively coupled plasma–mass spectrometry (LASS-ICP-MS) data from monazite in these domes yield a time record from U/Th-Pb dates and a petrologic record from rare earth element (REE) abundances. Seven samples from the Pamir and six samples from the north Himalayan gneiss domes yield almost identical monazite dates of ca. 28–15 Ma. Most monazite has invariant heavy REE (HREE) abundances; two samples, however, have older monazite that records progressive HREE depletion and two samples have younger monazite that records progressive HREE enrichment. These variations in HREE are compatible with increasing garnet abundance—prograde metamorphism—until ca. 20 Ma, and decreasing garnet abundance thereafter. The change from HREE depletion to enrichment may record a transition from crustal thickening and heating to dome exhumation and cooling. This documentation of synchronous Barrovian metamorphism within domes of Indian crust along the margin of the orogen (Himalaya) and within domes of Asian crust within the core of the orogen (Pamir) is best explained by a plate-scale driving force rather than by local events. We propose that widespread, synchronous thickening was initiated by the resumption of Indian subduction following slab breakoff and then terminated by a second slab-tearing event—both plate-scale events inferred from tomography.

Description: Monazite ([LREE]PO 4 , where LREE stands for light rare earth element) and xenotime (Y[HREE]PO 4 , where HREE stands for heavy rare earth element) occur in ore-grade concentrations within the Pinto Gneiss in the Music Valley region of southern California. However, both the age and petrogenesis of this potentially economically significant rare earth element (REE) deposit remain uncertain. New petrologic and geochronologic data enable assessment of the textural and temporal relationships between REE-bearing minerals and the host rock and development of a petrogenetic model for REE mineralization. Ore-forming monazite and xenotime are typically restricted to biotite folia within the host Pinto Gneiss, with greatest modal abundances occurring within a few meters of contacts between the host gneiss and a metadiorite intrusive unit, the latter of which is crosscut by pegmatite veins generated by partial melting of the Pinto Gneiss. Ore-forming monazite and xenotime preserve complex internal elemental zonation defining two distinct textures: (1) oscillatory zoning interpreted to represent primary crystallization, overprinted by (2) irregular embayed textures inferred to be the result of fluid-mediated dissolution re-precipitation reactions. The altered domains in monazite consist of primary monazite replaced by secondary monazite along with xenotime and uranothorite [(U,Th)SiO 4 ] inclusions. Similarly, primary xenotime is replaced by secondary xenotime with monazite and uranothorite inclusions. Localized breakdown of monazite, anorthite, and biotite to apatite and allanite provides further evidence for postmineralization metasomatism of the ore bodies. In situ monazite and xenotime U-Pb geochronology constrains the timing of primary REE mineralization to ca. 1.71 Ga, consistent with zircon dates obtained from the Pinto Gneiss. Based on the similarity in ages of monazite, xenotime, and zircon in the Pinto Gneiss, along with relict "igneous" zoning in the ore-bearing phosphate minerals, REE mineralization is inferred to have occurred during crystallization of the igneous protolith to the Pinto Gneiss. Metadiorite emplacement at ca. 1.4 Ga and pegmatite genesis at ca. 165 Ma both postdate the main phase of REE mineralization but likely played a role in fluid-assisted alteration, breakdown, and partial resetting of monazite and xenotime U-Pb systematics in the Pinto Gneiss.

Description: Integrated pseudosection modeling and monazite petrochronology of paragneiss from the Kanchenjunga region of northeastern Nepal reveal the presence of cryptic tectonometamorphic discontinuities within the Himalayan metamorphic core. These new data outline a series of thrust-sense structures that juxtapose rocks that generally record a protracted history of early Eocene to latest Oligocene–early Miocene (ca. 41–23 Ma) prograde metamorphism and lateral extrusion against others that typically record short prograde (between 2 and 7 m.y.) and retrograde (between 3 and 6 m.y.) histories variably spanning the middle Oligocene to the middle Miocene (ca. 31–12 Ma). Retrograde metamorphism in the hanging wall of the thrust faults is typically coeval with prograde metamorphism in the footwall, indicating that overthrusting/underthrusting accommodated crustal shortening and drove metamorphic processes. The structures and juxtaposed panels were cut by early Miocene (ca. 20–18 Ma) out-of-sequence thrusting coincident with the previously mapped High Himal thrust. The resulting kinematic model for the evolution of the Himalayan metamorphic core in the Kanchenjunga area demonstrates that the Himalayan metamorphic core was dominated by underplating from at least the Oligocene through to present, and that the internal structure of the exhumed metamorphic core is significantly more complex than has been documented previously.

Description: The exhumed Himalayan midcrustal core exposed in the Likhu Khola region of east-central Nepal includes upper-greenschist- to upper-amphibolite-grade metamorphic rocks that record pervasive, top-to-the-south sense deformation. Metamorphic temperature estimates are within error across the mapped area ranging from 772 ± 37 °C in the structurally lower, southern part of the study area to 853 ± 58 °C in the structurally higher, northern area. Estimated metamorphic pressures are relatively constant at lower structural levels, but they decrease from 11.8 ± 1.4 kbar to a minimum of 6.5 ± 1.3 kbar up structural section. The decrease in pressures coincides with an abrupt change in pressure estimates up structural section that is interpreted to mark a tectonometamorphic discontinuity that separates two domains with distinct structural, thermal, and metamorphic histories. In situ laser-ablation split-stream monazite U-Th/Pb and rare earth element (REE) petrochronology outlines dates ranging from ca. 27.8 Ma to 15.1 Ma in the hanging wall of the interpreted discontinuity; monazite REE data indicate the spread in ages is the result of a protracted metamorphic history and late-stage anatexis. Metamorphic and petrochronologic data from the Likhu Khola are consistent with a kinematic model in which material structurally above the discontinuity was metamorphosed in the deep hinterland of the orogen and was subsequently incorporated into the foreland of the orogen. The transition from hinterland- to foreland-style processes was marked by a shift to discrete deformation and the development of the discontinuity. Movement across the discontinuity is interpreted to have driven metamorphism and deformation of the rocks structurally below at or after 15 Ma. Discontinuities similar to that identified in this study are being identified and described across the orogen, indicating they are important, orogenwide features.

Description: A laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) study of seven thorite and uranothorite [(Th,U)SiO 4 ] samples ranging in age from ~13 to ~500 Ma provides new insights into the U-Th/Pb isotope systematics of these geologically significant, high-Th mineral species. Despite extreme actinide enrichment and complex intra-crystal zonation in actinides and rare earth elements, this study demonstrates that radiogenic-lead loss and/or metamictization is minimal and restricted to domains that have undergone significant hydration. Dating of four igneous uranothorites yields ages that are concordant in U/Pb and Th/Pb space, consistent with other high-temperature chronometers, and are inferred to accurately reflect the timing of crystallization of each rock. Similarly, Th/Pb ages of three thorite and/or huttonite-bearing samples yield geologically plausible dates consistent with other mineral chronometers. No evidence of isotopic inheritance was observed in any of the samples. Data presented here demonstrate for the first time the feasibility of extracting accurate and precise U/Pb and Th/Pb ages from Phanerozoic thorite, uranothorite and huttonite using LA-MC-ICPMS at the 5 μm spatial resolution. These phases have the potential to be robust chronometers in igneous and metamorphic rocks as well as to provide important provenance information complementary to more widely used minerals such as detrital zircon.

Description: Granitoids associated with the Neoproterozoic–early Paleozoic Ross orogeny are extensively exposed in the Dry Valleys region of southern Victoria Land, Antarctica, affording an exceptional opportunity to gain insight into the temporal and spatial scales of continental arc magmatism. Samples spanning 150 km along strike and 50 km across strike were selected for isotopic and geochemical analysis. Zircon U-Pb geochronology and the first Hf isotope data for Dry Valleys granitoids, coupled with whole-rock elemental data, reveal mixing between enriched lithospheric mantle and Precambrian crustal components and indicate that the principal phase of magmatism in the Dry Valleys area was restricted to a period of 23 m.y., from ca. 515 to 492 Ma. This relatively short period of magmatism contrasts with other segments of the Ross orogen, in which magmatism spanned greater than 100 m.y. Most calc-alkaline intrusions spanned 515–500 Ma, while postkinematic granitoids with alkali-calcic geochemical signatures spanned 505–492 Ma, indicating a transitional shift to an overall extensional tectonic regime. Zircon Hf(i) values range between –0.3 and –7.2, with two-stage depleted-mantle model ages ranging from 1.5 to 1.9 Ga. Low Hf(i) values in mafic samples are consistent with derivation from an enriched subcontinental lithospheric mantle source, while the large-volume granitic intrusions show evidence for increasing assimilation of old crust over time. A broadening of the Hf(i) range to more negative values in the younger intrusions may reflect crustal thickening or underplating of fertile continental material into the source region of the arc.

Description: 〈p〉Recognition and subsequent study of the syn-convergent low-angle normal faults and shear zones – the South Tibetan Detachment System (STDS) – that form the upper boundary of the Himalayan mid-crust fundamentally changed views of how the Himalayan orogenic system developed. This paper reviews the past four decades of discovery and major advances in our understanding of the detachment system. Significantly conflicting maps of the fault trace, as well as proposed extensions of the detachment system up to hundreds of kilometres both up and down dip of the main fault trace, call for a unifying definition of the detachment system based on structural criteria. The different proposed models for the formation of the STDS during tectonic evolution of the Himalayan orogen are compared. Finally, critical outstanding questions about the origin, extent and character of the detachment system are identified and point to future directions for research.〈/p〉

Description: 〈p〉Gneiss domes in the Himalaya and southern Tibet record processes of crustal thickening, metamorphism, melting, deformation and exhumation during the convergence between the Indian and Eurasian plates. We review two types of gneiss domes: North Himalayan gneiss domes (NHGD) and later domes formed by orogen-parallel extension. Located in the southern Tibetan Plateau, the NHGD are cored by granite and gneiss, and mantled by the Tethyan sedimentary sequence. The footwall of these were extruded southwards from beneath the Tibetan Plateau and subsequently warped into a domal shape. The second class of domes were formed during displacement on normal-sense shear zones and detachments that accommodated orogen-parallel extension during the Late Miocene. In some cases, formation of these domes involved an early stage of southwards-directed extrusion prior to doming. We review evidence for orogen-parallel extension to provide context for the formation of these gneiss domes. Compilations of pressure–temperature–time–deformation data and temperature–time paths indicate differences between dome types, and we accordingly propose new terminology. Type 1 domes are characterized by doming as an artefact of post-high-temperature exhumation processes in the Middle Miocene. Type 2 domes formed in response to exhumation during orogen-parallel extension in the Late Miocene that potentially post-dates south-directed extrusion.〈/p〉

Description: A newly identified and dated segment of the South Tibetan detachment in the Karnali klippe, western Nepal Himalaya, constrains initiation of mid-crustal tectonically driven exhumation to the early Oligocene. The folded top-to-the-northeast high-temperature (~600 °C) shear zone separates amphibolite-facies rocks with a ca. 36–30 Ma prograde metamorphic history in the footwall from weakly to non-metamorphosed upper crustal rocks in the hanging wall. In situ dating of syn-kinematic–post-metamorphic peak monazite indicates that the base of the shear zone was active from ca. 30–29 to 〈24 Ma, and a post-deformation muscovite cooling age implies that ductile shearing had ceased by ca. 19 Ma. Deformation along the South Tibetan detachment in western Nepal was thus synchronous with thrust-sense shearing along the lower boundary of a zone of migmatitic rocks, compatible with tectonic models involving mid-crustal channelized flow during the Oligocene. Along with other published data from the Himalayan range, this suggests that the South Tibetan detachment actively exhumed the middle crust for almost 20 m.y.