Nd–Sr isotopic relationship in granitoid rocks and continental crust development: a chemical approach to orogenesis

Role of Avalonia in the development of tectonic paradigms

The geological evolution of Avalonia was fundamental to the first application of plate tectonic principles to the pre-Mesozoic world. Four tectonic phases have now been identified. The oldest phase (760–660 Ma) produced a series of oceanic arcs, some possibly underlain by thin slivers of Baltica crust, which accreted to the northern margin of Gondwana between 670 and 650 Ma. Their accretion to Gondwana may be geodynamically related to the break-up of Rodinia. After accretion, subduction zones stepped outboard, producing the main phase (640–570 Ma) of arc-related magmatism and basin formation that was coeval with the amalgamation of Gondwana. Arc magmatism terminated diachronously between 600 and 540 Ma by the propagation of a San Andreas style transform fault, followed by the Early Paleozoic platformal succession used by Wilson to define the eastern flank of the proto-Atlantic (Iapetus) Ocean. This implies the ocean outboard from the northern Gondwanan margin survived into the Cambrian. Avalonia is one of several terranes distributed obliquely with respect to the adjacent cratonic provinces of Gondwana and Baltica, implying that these terranes evolved on different cratonic basements. As a result, their ages and differing isotopic signatures can be used to reconstruct their respective locations along the ancient continental margin.

Assessing effective provenance methods for fluvial sediment in the South China Sea

Sediment is delivered by the rivers of SE Asia to the South China Sea where it provides an archive of continental environmental conditions since the Eocene. Interpreting this archive is complicated because sediment may be derived from a number of unique sources and the rivers themselves have experienced headwater capture that also affects their composition. A number of methods exist to constrain provenance, but not all work well in this area. Anthropogenic impacts, most notably agriculture, mean that the modern rivers contain more weathered materials than they did up until about 3000 years ago. The rivers have also changed their bulk chemistry and clay mineralogy in response to climate change, so that these proxies, as well as Sr isotopes, are generally unreliable provenance indicators. Nd isotopes resolve influx from Luzon, but many other sources in SE Asia have similar values and clear resolution of end members can be difficult. Instead, thermochronology methods are best suited, especially apatite fission track, which shows more diversity in the sources than either U–Pb zircon or Ar/Ar muscovite dating. Nonetheless, even fission track is best used as part of a multiproxy approach if a robust quantitative budget is desired.

Regional framework and geodynamic evolution of the Indus-Tsangpo suture zone in the Ladakh Himalayas

The Indus-Tsangpo suture and its adjoining tectonic zones are well displayed in the Ladakh Himalayas where four tectonic zones have been distinguished, viz. the Zanskar, Indus suture, Shyok suture and Karakoram zones. The Zanskar zone is made up of Precambrian basement of the Zanskar crystalline complex and overlying Phanerozic sediments including Upper Palaeozoic volcanic rocks of the Zanskar Supergroup; they form the northern margin of the Indian plate. The Indus suture zone consists of a remnant of tectonised oceanic lithosphere represented by the Shergol melange and the Nidar complex with a former volcanic arc indicated by the volcanogenic Dras and Khardung formations and the Ladakh plutonic complex. The Shyok suture zone does not represent a tectonic repetition of the Indus suture; it is interpreted as a relic of a back-arc basin. The Karakoram plutonic complex appears to be genetically related to the Ladakh plutonic complex; both were generated from the subducting Indian oceanic plate. It is proposed that the boundary between the Indian and Eurasian plates does not lie along the Indus and Shyok sutures, but is located further N at the junction of Central Pamir (Alpine-Himalayan) and North Pamir (Hercynian).

Field relations, geochemistry, origin and emplacement of the Baltoro granite, Central Karakoram

The Miocene Baltoro granite forms a massive plutonic unit within the Karakoram batholith, and is composed of comagmatic monzogranites and leucogranites with a mineralogy consisting of quartz-K-feldspar-plagioclase-biotite ± muscovite ± garnet, with accessory sphene, zircon, monazite and opaques. Geochemically the Baltoro granites are mildly peraluminous, and show a calc-alkaline trend on trace-element normalised diagrams with high LIL/HFS element ratios and negative Nb, P and Ti anomalies. REE are strongly fractionated with little or no Eu anomaly. Leucogranites are depleted in most elements compared to monzogranites with notable exceptions being Rb, K and the HREEs. Initial 87Sr/86Sr ratios are 0·7072-0·7128, considerably lower than High Himalayan leucogranites (0·74-0·79), and are indicative of a lower continental crust source. The probable petrogenesis of the Baltoro granite involves dehydration melting of a biotite-rich pelite to produce a voluminous, hot, water-undersaturated magma which could then separate from its source and intrude through an already thickened and still hot crust. Fractional crystallisation of the monzogranites produced the leucogranites and a pegmatite dyke swarm. A suite of lamprophyre dykes including amphibolerich vogesites and biotite-rich minettes intrude the country rock, dominantly to the north, around the Baltoro granite. These calc-alkaline shoshonitic lamprophyres are volatile-rich mantle-derived melts intruded around the same time as the granite, indicating simultaneous melting of the mantle and lower crust beneath the Karakoram during the Miocene, approximately 30 Ma after the India-Asia collision which initially caused the crustal thickening. Intrusion of mantle melts provided heat to promote crustal melting and may have selectively contaminated the granite magma.

The Baltoro granite intrudes sillimanite gneisses with melt pods along the southern margin indicating temperatures above 700°C at the time of intrusion. Locally, internal fabrics and numerous aligned xenoliths along the southern margin in the Biafo glacier region indicate steep, southward-directed thrusting during emplacement. Along the northern contact, the Baltoro granite intrudes anchimetamorphic to greenschist facies metasedimentary rocks with an andalusite-bearing contact aureole. Northward-directed culmination collapse normal faulting during Miocene emplacement is inferred, in order to explain the P-T differences either side of the pluton. This also provided an extensional stress regime in the upper crust to accommodate the rising magma.

Stratigraphy, structure and evolution of the Tibetan–Tethys zone in Zanskar and the Indus suture zone in the Ladakh Himalaya

The Tibetan–Tethys zone of the Zanskar Himalaya shows a complete Mesozoic shelf carbonate sequence overlying metamorphic basement of the Central crystalline complex and Palaeozoic sedimentary rocks. Continental rifting in the Permian produced the alkaline and basaltic Panjal volcanic rocks and by Triassic time a small ocean basin was developed in the Indus-Tsangpo zone. Stable sedimentation continued until the Middle-Late Cretaceous when a thick sequence of tholeiitic to andesitic island arc lavas (Dras arc) were erupted in the basin above a N-dipping subduction zone. The Spontang ophiolite was emplaced southwards onto the Zanskar shelf edge during latest Cretaceous or earliest Tertiary times.

Following emplacement of the Spontang ophiolite, deep-sea sedimentation ended abruptly with initial collision between the Indian plate and the Dras island arc. Emplacement of the massive Ladakh (Trans-Himalayan) batholith along the southern margin of Tibet in late Cretaceous-Eocene time occurred by crustal melting as a result of northward subduction of Mesozoic oceanic crust along the Indus subduction zone. Southward-directed thrusting in both Zanskar and Indus zones accompanied ocean closure during the late Cretaceous–Eocene. Late Tertiary compression caused intense folding, overturning and a phase of northward-directed thrusting along the Indus suture zone and the northern margin of the Tibetan–Tethys zone, resulting in a large amount of crustal shortening.

Growth and Deformation of the Ladakh Batholith, Northwest Himalayas: Implications for Timing of Continental Collision and Origin of Calc-Alkaline Batholiths

The calc-alkaline Ladakh batholith (NW Himalayas) was dated to constrain the timing of continental collision and subsequent deformation. Batholith growth ended when collision disrupted subduction of the Tethyan oceanic lithosphere, and thus the youngest magmatic pulse indirectly dates the collision. Both U-Pb ages on zircons from three samples of the Ladakh batholith and K-Ar from one subvolcanic dike sample were determined. Magmatic activity near Leh (the capital of Ladakh) occurred between 70 and 50 Ma, with the last major magmatic pulse crystallizing at ca. 49.8+/-0.8 Ma (2sigma). This was followed by rapid and generalized cooling to lower greenschist facies temperatures within a few million years, and minor dike intrusion took place at 46+/-1 Ma. Field observations, the lack of inherited prebatholith zircons, and other isotopic evidence suggest that the batholith is mantle derived with negligible crustal influence, that it evolved through input of fresh magma from the mantle and remelting of previously emplaced mantle magmatic rocks. The sedmimentary record indicates that collision in NW Himalaya occurred around 52-50 Ma. If this is so, the magmatic system driven by subduction of Tethys ended immediately on collision. The thermal history of one sample from within the Thanglasgo Shear Zone (TSZ) was determined by Ar-Ar method to constrain timing of batholith internal deformation. This is a wide dextral shear zone within the batholith, parallel to the dextral, N 30 degrees W-striking crustal-scale Karakoram Fault. Internal deformation of the batholith, taken up partly by this shear zone, has caused it to deviate from it regional WNW-ESE trend to parallel the Karakoram Fault. Microstructures and cooling history of a sample from the TSZ indicate that shearing took place before 22 Ma, implying that (1) the history of dextral shearing on NW-striking planes in northern Ladakh started at least 7 m.yr. before the <15 Ma Karakoram Fault, (2) shearing was responsible for deviation of the regional trend of the Ladakh batholith, and (3) dextral shearing occured within a zone apporximately 100 km wide that includes the Ladakh batholith and portions of the younger Karakoram batholith.

Tracing the origins of the western Himalaya: an isotopic comparison of the Nanga Parbat massif and Zanskar Himalaya

New Sr and Nd isotope data for basement gneisses and leucogranites are presented from two contrasting areas of the western Himalaya; the Nanga Parbat-Haramosh massif (NPHM) and Zanskar. Sr-isotope systematics of metapelites and anatectic migmatites from the Zanskar Himalaya are characterized by εSr of 515–930, typical of the High Himalayan Crystalline unit as exposed for more than 2000 km along strike. Moreover, Zanskar leucogranites are typical of the belt of Early Miocene granites intruding the High Himalayan Crystallines across the orogen (mean εSr = 834). In contrast, the NPHM leucogranites show an elevated average εSr of 2400, and basement samples show a wide range in εSr from 1850 to 8150. Errorchrons for the metasedimentary gneisses indicate isotopic homogenization of the basement at c. 500 Ma for the Zanskar samples compared with c. 1800 Ma from the NPHM, confirming that the two terrains have experienced contrasting pre-Himalayan histories.

Nd isotopic data from the NPHM indicate model ages from 2300 to 2800 Ma, indicating the mean crustal formation ages of the protoliths from which the sediments were derived. A compilation of published Nd data from the Himalaya indicates average protolith formation ages of 2640 ± 220 Ma for the Lesser Himalaya lithologies, compared with 1940 ± 270 Ma for the High Himalaya unit.

Gneissic lithologies from Zanskar and the NPHM have previously been correlated with the High Himalayan Crystalline Series, since both display high-grade Himalayan metamorphism and are intruded by syn- to post-tectonic tourmaline-bearing leucogranites. Isotopic systematics in the Zanskar region confirm this correlation. In contrast, the NPHM basement rocks are better correlated with Lesser Himalayan lithologies, exposed south of the Main Central Thrust. We conclude that the NPHM represents either a lower structural level of the Lesser Himalaya Series, or its protolith.

Isotope geochemistry of the 1985 Tibet Geotraverse, Lhasa to Golmud

Geochronological data from the Golmud —Lhasa section across the Tibetan Plateau indicate progressively younger periods of magmatism from north to south associated with successively younger ocean closures. Pre -collision Eocene magmatism (50—4 0 Ma) exposed along the southern margin of the Lhasa Terrane in the Gangdise Belt resulted from anatexis of mid -Proterozoic crust (~ 1000 Ma) at depths greater than 10 km, but at higher crustal levels subduction-related intrusions were predominantly mantle-derived with ~ 30 % crustal assimilation . Intrusions from the northern Lhasa Terrane are early Cretaceous in age (130 —110 Ma). These form a bimodal suite comprised of two-mica granites derived from anatex is of Mid -Proterozoic crust and of biotite -hornblende granodiorites from about 60 % crustal assimilation by mantle magmas above a post-collision subduction zone. They place a minimum constraint on collision between the Lhasa and Qiangtang Terranes of 130 Ma . Granite magmatism from the Kunlun Mountains is late P ermian -early Jurassic in age (260—190 Ma). The Kunlun batholith represents reworked mid-Proterozoic crust (1400 —1000 Ma) at an active continental margin from 260 —2 4 0 M a . Post-tectonic granites were emplaced in a post-collision setting (200 -190M a). Collision between the Qiangtang and Kunlun Terranes is dated as end -Triassic. Nd model ages of sediments from across the plateau record up lift and erosion of young source regions throughout the Phanerozoic confirming that the Tibetan Plateau is the site of multiple continental collision through time. Phanerozoic magmagenesis throughout the plateau requires considerable crustal reworking and limited crustal growth which suggests thickened continental crust in the region may predate the most recent Eocene collision.

Origin of the Sudbury Complex by Meteoritic Impact: Neodymium Isotopic Evidence

Samarium-neodymium isotopic data on whole rocks and minerals of the Sudbury Complex in Canada gave an igneous crystallization age of 1840 +/- 21 x 10(6) years. The initial epsilon neodymium values for 15 whole rocks are similar to those for average upper continental crust, falling on the crustal trend of neodymium isotopic evolution as defined by shales. The rare earth element concentration patterns of Sudbury rocks are also similar to upper crustal averages. These data suggest that the Sudbury Complex formed from melts generated in the upper crust and are consistent with a meteoritic impact.

Eastern Indian 3800-Million-Year-Old Crust and Early Mantle Differentiation

Samarium-neodymium data for nine granitic and tonalite gneisses occurring as remnants within the Singhbhum granite batholith in eastern India define an isochron of age 3775 +/- 89 x 10(6) years with an initial (143)Nd/(144)Nd ratio of 0.50798 +/- 0.00007. This age contrasts with the rubidium-strontium age of 3200 x 10(6) years for the same suite of rocks. On the basis of the new samarium-neodynium data, field data, and petrologic data, a scheme of evolution is proposed for the Archean crust in eastern India. The isotopic data provide evidence that parts of the earth's mantle were already differentiated with respect to the chondritic samarium-neodymium ratio 3800 x l0(6) years ago.