Leucogranite Dikes Research Articles

Late Cretaceous coeval acidic and basic magmatism, Karacaali Magmatic Complex, Central Anatolia, Turkey

The Central Anatolia Crystalline Complex (CACC) is characterized by Late Cretaceous high-temperature metamorphic rocks intruded by S-, I-, and A-type granitoids. Coeval basic plutonic and volcanic rocks also crop out in the complex. The NE–SW-trending Karacaali Magmatic Complex (KMC) represents a clear example of synchronous basic and acidic magmatic associations. We present new data on this coeval magmatism. The KMC plutonic rocks mainly consist of monzonite, granite, and gabbro, whereas the associated volcanic rocks are chiefly of basalt and rhyolite. All of the units have been cut by quartz, quartz-tourmaline, and calcite veins and by porphyritic leucogranite, aplitic, and basaltic dikes. The rhyolitic, basaltic, and gabbroic samples yield well-defined 40Ar/39Ar plateau ages of 69.1 ± 1.3, 58 ± 10, and 66.4 ± 1 million years, respectively; these data indicate that a younger multiphase basic magma was injected into a partially crystallized monzonitic magma chamber. The basic intrusions added heat to the system and gave rise to the re-fusion of the already crystallized parts of the monzonitic melt, forming the younger leucogranitic magma. The gradational contacts, cross-cutting relationships, trace element contents, trace element patterns, rare-earth element (REE) patterns, and 40Ar/39Ar geochronological data of the studied igneous suite clearly demonstrate that the acidic and basic rocks of the KMC were contemporaneous and are produced by partial melting of distinct sources rather than by fractional crystallization of a single source.

Chronology (SHRIMP II) of magmatism in the Kalguty rare-metal-tungsten-molybdenum ore-magmatic system, Gorny Altai, Russia

The Kalguty ore-magmatic system (OMS) contains economic greisen and vein rare-metal-tungsten-molybdenum mineralization. The data on U-Pb zircon (SHRIMP II) age of nine samples of igneous rocks from the Kalguty OMS are accompanied by chemical, ICP-MS, and ICP-AES analyses of the main rock varieties. Porphyritic biotite granite of the main phase of the Kalguty pluton is characterized by the concordant age of 207.5 ± 1.7 Ma (MSWD = 0.034). The concordant age of the leucogranitic dikes pertaining to the East Kalguty Complex is 204 ± 2 Ma (MSWD = 0.65) for elvan and 200.8 ± 1.1 Ma (MSWD = 0.72) for ultrapotassic rhyolite porphyry. The two-mica and muscovite leucogranite of the Eastern stock is significantly younger: 195 ± 2.7 Ma (MSWD = 0.076) and 193.1 ± 2.1 Ma (MSWD = 0.0009). Thus, the Chingadatui Complex (the main phase of the Kalguty pluton) and dikes of the East Kalguty Complex are Late Triassic in age. The two-mica leucogranites of the Eastern, Zhumaly, and other stocks are most likely correlated to the Early Jurassic Alacha Complex of rare-metal granites. The superposition of greisen mineralization on elvan and ongonite dikes may be related to the emplacement of younger, Early Jurassic ore-forming two-mica and muscovite granites. Judging from zircon xenocrysts in granites, the Mesoproterozoic igneous rocks dated at 1.5 Ga and products of erosion of the rocks dated at 1.7 and 2.5 Ga occur in the basement of the Kalguty volcanic-tectonic structure. This is sound evidence for the occurrence of ancient continental crustal blocks in the southwestern part of the Altai-Sayan region.

Middle Paleozoic and Early Mesozoic anorogenic magmatism of the South Yenisei Ridge: first geochemical and geochronological data

In this paper we present complex geological, petrographic and geochronological data of the study of intermediate and acid composition intrusive and volcanogenic rocks from the Porozhnaya massif of the South Yenisei Ridge. For the first time in the Yenisei Ridge Devonian and Triassic U-Pb age values (SHRIMP method) have been obtained for leucogranites—387 ± 5 Ma and alkaline trachytes—240 ± 3 Ma, which allows us to attribute them to two different complexes, despite the fact that these rocks were formed within the same Severnaya riftogenic structure. Geochronological Ar-Ar data (392–387 Ma) for micas from paragneisses and leucogranitic dikes of the Yenisei suture zone on whose extension the Severnaya riftogenic structure is located are also given in this study. These data on Devonian tectonic-magmatic events in the South Yenisei Ridge agree well with coeval events of continental rifting—the formation of intrusive and volcanogenic rocks of the Agul graben in the Prisayan region and the Minusa basin in the Altai-Sayan folded area. The forming of alkaline trachytes and alkaline syenites of the Severnaya riftogenic structure, for which an age of 240 ± 3 Ma has been established, is related to the trap magmatism of the Siberian platform.

Pleistocene melting and rapid exhumation of the Nanga Parbat massif, Pakistan: Age and P– T conditions of accessory mineral growth in migmatite and leucogranite

Rapid Pleistocene exhumation of the core of the Nanga Parbat massif (northwestern Himalayan syntaxis) is inferred by combining U-Pb dates from monazite, xenotime, and zircon from migmatitic rocks and a leucogranite dike with pressure estimates that are closely linked to dated events in the melting and crystallization history. Exhumation rates of ∼ 11–13 mm/a were calculated from (i) migmatitic rocks that were produced at ∼ 1.7 Ma and 5.0 kbar by dehydration melting of biotite on decompression and (ii) veins with garnet and cordierite that crystallized at ∼ 1.0 Ma and 3.5 kbar. Tourmaline-bearing leucogranitic dikes separated from the source and ascended to crystallize near their solidus at ∼ 0.7 Ma. Modeling of Th/U in the leucogranite magma based on Th/U and U-Pb data from monazite, xenotime, and zircon shows a decrease from 1.1 to 0.2 over a span of 0.15 Ma. The implied acceleration of exhumation at ∼ 1.7 Ma may be linked to mid-crust flow as the evolving thermal structure of the Neogene metamorphism encountered the biotite dehydration–melting reaction. The rapid exhumation may have resulted from significant lowering of the effective viscosity of mid-crustal rocks, leading to vertical channel flow into the core of the Nanga Parbat massif along bounding shear zones.

LA‐ICP‐MS U‐Pb zircon ages from Mesozoic plutonic rocks in eastern Fiordland, New Zealand

Laser ablation inductively coupled plasma mass‐spectrometry (LA‐ICP‐MS) 206Pb‐238U zircon dates are reported for eight plutonic rocks from eastern Fiordland. The oldest samples are Jurassic (granodiorite, 160.6 ±1.4 Ma; quartz diorite, 153.0 ± 0.8 Ma; granodiorite, 152.5 ± 0.7 Ma; errors at 2 a) and collectively belong to the Hunter Intrusives member of the Darran/Median Suite. The LA‐ICP‐MS age for the younger granodiorite agrees with a concordant multigrain TIMS U‐Pb zircon age previously obtained from the same sample. Xenoliths of Hunter Intrusives occur within the Early Cretaceous (127.9 ± 1.1 Ma) granitic Fowler Pluton, also of the Darran/Median Suite. There was a short lull in plutonism until emplacement of leucogranite dikes (123.5 ± 1.2 Ma) and granitoids of the Refrigerator Orthogneiss (120.7 ± 1.1 Ma), Puteketeke Pluton (120.8 ± 0.9 Ma), and West Arm Leucogranite (116.3 ± 1.2 Ma), all of which are members of the Separation Point Suite.

Structure and geochronology of the southern Xainza-Dinggye rift and its relationship to the south Tibetan detachment system

The Xainza-Dinggye rift is one of several north-south trending rifts in central and southern Tibet created by Cenozoic east-west extension during Indo-Asian convergence. The southern part of the rift cuts through the Tethyan and High Himalayas. In the Tethyan Himalaya, this rift consists of an early domal structure and a late normal fault developed during the progressive deformation. The dome is cored by leucogranitic plutons that intruded during extension. Muscovite 40Ar/ 39Ar ages of the mylonitic leucogranite indicate that extension in the Tethyan Himalaya began at ∼8 Ma or before. In the High Himalaya, the rift is controlled by a normal fault dipping to the southeast. This fault has a structural constitution similar to a detachment fault. Its lower block is made up of mylonitic High Himalayan gneiss, intruded by early mylonitic leucogranite sills and late less-deformed biotite-bearing leucogranite dikes. Mica 40Ar/ 39Ar ages of these leucogranites and the retrograded metamorphosed gneiss of the lower block range from ∼13 to ∼10 Ma. In the study area, the south Tibetan detachment system (STDS) is a ductile shear zone composed of mylonitic leucogranite that is intruded by less-deformed leucogranite and overlain by low grade metamorphic rocks. Mica 40Ar/ 39Ar ages of leucogranites in the shear zone and schist from the detachment hanging wall indicate a protracted deformation history of the STDS from ∼19 to ∼13 Ma. The Xainza-Dinggye rift is younger than the STDS because it offsets the STDS; this north-south trending rift belongs to a different tectonic system from the east-west striking STDS, and may be caused by geological process related to India–Asia convergence. This temporal and spatial relationship of the STDS to the rift may indicate an important change in tectonic regime at ∼13 Ma in the building of the plateau.

40Ar/ 39Ar thermochronology of the Kampa Dome, southern Tibet: Implications for tectonic evolution of the North Himalayan gneiss domes

Structural and thermochronological studies of the Kampa Dome provide constraints on timing and mechanisms of gneiss dome formation in southern Tibet. The core of Kampa Dome contains the Kampa Granite, a Cambrian orthogneiss that was deformed under high temperature (sub-solidus) conditions during Himalayan orogenesis. The Kampa Granite is intruded by syn-tectonic leucogranite dikes and sills of probable Oligocene to Miocene age. Overlying Paleozoic to Mesozoic metasedimentary rocks decrease in peak metamorphic grade from kyanite + staurolite grade at the base of the sequence to unmetamorphosed at the top. The Kampa Shear Zone traverses the Kampa Granite — metasediment contact and contains evidence for high-temperature to low-temperature ductile deformation and brittle faulting. The shear zone is interpreted to represent an exhumed portion of the South Tibetan Detachment System. Biotite and muscovite 40Ar/ 39Ar thermochronology from the metasedimentary sequence yields disturbed spectra with 14.22 ± 0.18 to 15.54 ± 0.39 Ma cooling ages and concordant spectra with 14.64 ± 0.15 to 14.68 ± 0.07 Ma cooling ages. Petrographic investigations suggest disturbed samples are associated with excess argon, intracrystalline deformation, mineral and fluid inclusions and/or chloritization that led to variations in argon systematics. We conclude that the entire metasedimentary sequence cooled rapidly through mica closure temperatures at ∼ 14.6 Ma. The Kampa Granite yields the youngest biotite 40Ar/ 39Ar ages of ∼ 13.7 Ma immediately below the granite–metasediment contact. We suggest that this age variation reflects either varying mica closure temperatures, re-heating of the Kampa Granite biotites above closure temperatures between 14.6 Ma and 13.7 Ma, or juxtaposition of rocks with different thermal histories. Our data do not corroborate the “inverse” mica cooling gradient observed in adjacent North Himalayan gneiss domes. Instead, we infer that mica cooling occurred in response to exhumation and conduction related to top-to-north normal faulting in the overlying sequence, top-to-south thrusting at depth, and coeval surface denudation.

SHRIMP study of zircons from Early Archean rocks in the Minnesota River Valley: Implications for the tectonic history of the Superior Province

Research Article| January 01, 2006 SHRIMP study of zircons from Early Archean rocks in the Minnesota River Valley: Implications for the tectonic history of the Superior Province M.E. Bickford; M.E. Bickford 1Department of Earth Sciences, Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070, USA Search for other works by this author on: GSW Google Scholar J.L. Wooden; J.L. Wooden 2U.S. Geological Survey, Stanford–U.S. Geological Survey Ion Probe Facility, Stanford University, Stanford, California 94305-2220, USA Search for other works by this author on: GSW Google Scholar R.L. Bauer R.L. Bauer 3Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2006) 118 (1-2): 94–108. https://doi.org/10.1130/B25741.1 Article history received: 12 Oct 2004 rev-recd: 05 May 2005 accepted: 30 Jun 2005 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M.E. Bickford, J.L. Wooden, R.L. Bauer; SHRIMP study of zircons from Early Archean rocks in the Minnesota River Valley: Implications for the tectonic history of the Superior Province. GSA Bulletin 2006;; 118 (1-2): 94–108. doi: https://doi.org/10.1130/B25741.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Interest in Paleoarchean to early Meso-archean crust in North America has been sparked by the recent identification of ca. 3800–3500 Ma rocks on the northern margin of the Superior craton in the Assean Lake region of northern Manitoba and the Porpoise Cove terrane in northern Quebec. It has long been known that similarly ancient gneisses are exposed on the southern margin of the Superior craton in the Minnesota River Valley and in northern Michigan, but the ages of these rocks have been poorly constrained, because methods applied in the 1960s through late 1970s were inadequate to unravel the complexities of their thermotectonic history. Rocks exposed in the Minnesota River Valley include a complex of mig-matitic granitic gneisses, schistose to gneissic amphibolite, metagabbro, and paragneisses. The best-known units are the Morton Gneiss and the Montevideo Gneiss. The complex of ancient gneisses is intruded by a major younger, weakly deformed granite body, the Sacred Heart granite. Regional geophysical anomalies that extend across the Minnesota River Valley have been interpreted as defining boundaries between distinct blocks containing the various gneissic units.New sensitive high-resolution ion micro-probe (SHRIMP) U-Pb data from complex zircons yielded the following ages: Montevideo Gneiss near Montevideo, 3485 ± 10 Ma, granodiorite intrusion, 3385 ± 8 Ma; Montevideo Gneiss at Granite Falls, 3497 ± 9 Ma, metamorphic event, 3300–3350 Ma, mafic intrusion, 3141 ± 2 Ma, metamorphic overprint (rims), 2606 ± 4 Ma; Morton Gneiss: 3524 ± 9 Ma, granodiorite intrusion, 3370 ± 8 Ma, metamorphic overprints (growth of rims), 3140 ± 2 Ma and 2595 ± 4 Ma; biotite-garnet paragneiss, 2619 ± 20 Ma; and Sacred Heart granite, 2604 ± 4 Ma. Zircons from a cordierite-bearing feldspar-biotite schist overlying the Morton Gneiss yielded well-defined age peaks at 3520, 3480, 3380, and 3140 Ma, showing detrital input from most of the older rock units; 2600 Ma rims on these zircons indicate metamorphism at this time. Zircons from a hypersthene-bearing biotite-garnet paragneiss, overlying the Montevideo Gneiss near Granite Falls, yielded ca. 2600 Ma ages, indicating zircon growth during high-grade metamorphism at this time. Despite some differences in the intensity of the 2600 Ma event between the Morton and Montevideo blocks, both blocks display similar thermochronologic relationships and ages, suggesting that their boundary is not a fundamental suture between two distinct Paleoarchean terranes.Previously obtained zircon age data from the tonalitic gneiss at Watersmeet Dome in northern Michigan indicated formation at ca. 3500 Ma, whereas a granite body near Thayer was dated at 2745 ± 65 Ma and leucogranite dikes are ca. 2600 Ma. Thus, these rocks and those in the Minnesota River Valley were formed in the late Paleoarchean and show a history of igneous activity and metamorphism in the Mesoarchean and Neoarchean. The occurrence of ancient crustal rocks on both the northern and southern margins of the ca. 2900–2700 Superior craton suggests that they are remnants of once more-extensive Paleoarchean crust that existed prior to formation of the Neoarchean Superior craton. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains

Research Article| November 01, 2001 Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains James E. Fraser; James E. Fraser 1Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar Michael P. Searle; Michael P. Searle 1Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar Randall R. Parrish; Randall R. Parrish 2Department of Geology, University of Leicester, and Natural Environment Research Council Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar Stephen R. Noble Stephen R. Noble 3Natural Environment Research Council Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar GSA Bulletin (2001) 113 (11): 1443–1455. https://doi.org/10.1130/0016-7606(2001)113<1443:CODMAM>2.0.CO;2 Article history received: 03 Dec 1999 rev-recd: 09 Aug 2000 accepted: 16 May 2001 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James E. Fraser, Michael P. Searle, Randall R. Parrish, Stephen R. Noble; Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains. GSA Bulletin 2001;; 113 (11): 1443–1455. doi: https://doi.org/10.1130/0016-7606(2001)113<1443:CODMAM>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract U-Pb dating of metamorphic and igneous rocks from the Hunza Valley and Baltoro regions of the Karakoram Mountains in northern Pakistan addresses the thermal and magmatic evolution of the thickened Asian plate crust before, during, and after the collision of the Kohistan arc and the Indian plate. Crustal thickening and high- temperature, sillimanite-grade metamorphism in the southern Karakoram Mountains followed the collision and accretion of the Kohistan arc during the Late Cretaceous. U-Pb ages of metamorphic monazites from sillimanite gneisses in the Hunza Valley are 63.3 ± 0.4 Ma, ca. 50–52 Ma, and 44.0 ± 2.0 Ma, and monazites from a kyanite-grade schist from the Baltoro region are 28.0 ± 0.5 Ma. Metamorphic monazites from a highly graphitic garnet + staurolite schist from the Hunza Valley yield a crystallization age of 16.0 ± 1.0 Ma. Sillimanite gneisses from the Dassu dome have magmatic zircons of 1855 ± 11 Ma, reflecting a Proterozoic continental crustal source, and metamorphic monazites of 5.4 ± 0.2 Ma. Magmatism was also sporadic; early granodiorite, monzogranite, and leucogranite dikes yield zircon, monazite, and uraninite ages of 50–52 Ma and 35.0 ± 1.0 Ma. Widespread lower crustal melting during the latest Oligocene–early Miocene culminated with emplacement of the Baltoro Plutonic Unit in the Karakoram batholith that cuts deformation fabrics in the high-grade gneisses to the south. The youngest magmatic phase dated is the 9.3 ± 0.2 Ma Sumayar leucogranite pluton. On the basis of detailed structural field studies combined with U-Pb geochronology, sillimanite-grade metamorphism was either a protracted event lasting as long as 20 m.y. (64–44 Ma) or peaked at different times within the lower crust following collision of first, the Kohistan arc, and later, the Indian plate. We also present evidence for southward propagation of peak metamorphism and postmetamorphic thrusting and folding of isograds within the past 5 m.y. Detailed geochronology shows that deformation, metamorphism, and magmatism in the middle and lower crust of the south Asian margin has been occurring within the Karakoram metamorphic complex for more than 60 m.y. Similar processes may also have affected the unplumbed depths of the south Tibetan crust. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Contrasting behaviour of boron during crustal anatexis

The study of volatile elements in crustal anatectic terranes may provide information about the role of fluid flow and fluid pathways during high temperature metamorphism. We have studied the distribution of Li and B in two migmatitic areas: the Peña Negra Anatectic Complex (central Spain) and the migmatites and leucogranites associated with the Ronda peridotites (southern Spain). They represent two contrasting cases in the behaviour of volatiles, particularly B, during anatexis. The Peña Negra migmatites are dominated by diatexites where it is possible to distinguish granitic leucosomes, melanosomes, metric to decimetric leucogranitic segregates, and decimetric to metric sillimanite-rich restitic enclaves. All lithologies are characterized by low B concentrations (mean values <30 ppm), and B is enriched in melanosomes/restites with respect to leucosomes/leucogranites. This may be due to the fact that a notable proportion of B can reside in restitic sillimanite. B concentrations in leucosomes and leucogranites can be explained by closed system anatexis. At Ronda, metatexitic and diatexitic migmatites underlie the Ronda peridotites, and diatexite and leucogranite dikes intrude the peridotites. Diatexites underlying the peridotites are characterized by cordierite+biotite±garnet as ferromagnesian phases, with rare or absent tourmaline, low B contents (<10 ppm), and cordierite±garnet leucosomes. Leucogranite dikes have high B concentrations (mean value≈400 ppm), with tourmaline being a common and, sometimes, the most abundant ferromagnesian phase. A genetic relationship between diatexites and leucogranites is strongly suggested by the presence of dikes of diatexites intruding the bottom of the peridotites. We conclude that B in the leucogranites may stem from the dissolution of tourmaline originally present in the metasediments, or infiltration of B-rich aqueous fluids into the anatectic zone. According to recent experimental work, textures in some tourmaline-rich Ronda leucogranites suggest initial B 2O 3 contents in the melt higher than the present measured concentrations. Preliminary experimental results presented in this paper propose that this difference is not due to water activities in the melt. Rapid crystallization of the melts following undercooling may explain the existence of abundant euhedral tourmaline in leucogranites with low aluminium saturation index (ASI=molar Al 2O 3/(CaO+Na 2O+K 2O)) and relatively low B contents.

Mazeno Pass Pluton and Jutial Pluton, Pakistan Himalaya: age and implications for entrapment mechanisms of two granites in the Himalaya

Zircon and monazite U-(Th)-Pb ion microprobe analysis were performed on the Mazeno Pass pluton and the Jutial pluton, two leucogranite bodies within the Nanga Parbat-Haramosh massif (NPHM), Pakistan Himalaya. Zircon rim ages and monazite ages indicate the Mazeno Pass pluton in southwest NPHM intruded at 1.40 ± 0.05 Ma; the Jutial pluton, to the north, similarly yields concordant zircon and monazite ages suggesting crystallization at 9.45 ± 0.06 Ma. The Jutial pluton was subsequently intruded by leucogranite dikes at 5.3 Ma, as revealed by monazite ages. Concordancy of U-Pb and Th-Pb accessory mineral ages demonstrates the robustness of the technique on young rocks. Both plutons, some of the youngest in the Himalaya, have a general association with nearby shear zones that we interpret to have played an integral role in granite evolution and emplacement setting (`deformation enhanced ascent'). Together with new field observations, these results provide an insight on the spatial and temporal relationship between plutonism and deformation relating to the development of the massif.

Strontium Isotope Systematics of a Himalayan Glacial Chronosequence

Chemical weathering of silicate minerals consumes atmospheric CO2, and therefore, changes in global silicate weathering rates may act as a mechanism to drive major fluctuations in the Earth's climate and long-term global carbon cycle. Recent studies suggest that steep increases in the Cenozoic marine Sr isotope record correlate with periods of global cooling and enhanced silicate weathering rates, initiated by the onset of glaciation in the uplifting Himalayan mountains (Raymo and Ruddiman, 1992). However, due to the Himalaya's unique orogenic history, strontium with high 87Sr/86Sr ratios may reside in easily weathered carbonate minerals that do not act as a net sip& for atmospheric CO2 when undergoing chemical weathering (Edmond, 1992; Blum et al., 1998). Thus, the dissolved flux of Sr from the Himalayan Mountains may not be an ideal proxy for silicate weathering and atmospheric CO2 consumption. By studying the major element and Sr isotope chemistry of soils developed on a Himalayan glacial chronosequence, we are investigating how the dissolved flux of ions derived from carbonate versus silicate mineral weathering varies as a function of time following the exposure of fresh rock surfaces to the agents of chemical weathering. Geologic setting and methods. The -200 km 2 Raikhot Valley, located at an average elevation of ~4000 m on the northern slope of the Nanga ParbatHarimosh Massif (NPHM) within the Pakistani Himalayas, contains both the ~50 km 2 Raikhot Glacier as well as a clearly visible glacial chronosequence. Four moraines, thought to be roughly Little Ice Age to neoglacial in age, are nested near the modem terminus of the glacier, while a fifth moraine, believed to be last glacial maximum in age, forms a bench about 200 m from the glacial terminus. The bedrock of the watershed, considered to be representative of the High Himalayan Crystalline Series (HHCS) which stretches for 2000 km across the length of the Himalayan Mountains, primarily consists of quartzofeldspathic biotite gneisses, biotite schists, leucogranite dikes, and granitic stocks, as well as minor amounts (~1%) of interbedded amphibolite and calc-silicate schists. In order of increasing relative age, the sample numbers 96PK1, 96PK2, 96PK3, and 96PK4 refer to soil profiles collected from the four young nested moraines, while the sample numbers 96PK5 and 96PK5a refer to 2 separate soil profiles developed on the fifth moraine. This report presents results obtained from the analysis of the major element (Ca, Mg, Na, K, and Si) concentrations, minor element (Sr, Ti, and Zr) concentrations, and the Sr isotope ratios (87Sr/86Sr) contained in the bulk and exchangeable fractions of these soils. Ca to Sr ratios. The analysis of Ca/(1000*Sr) ratios provides a useful mechanism for characterizing the mineral provenance of the bulk soil and exchangeable ions. The Ca/(1000*Sr) ratios of the bulk soil and exchangeable ions represent a mixture between the average calcium carbonate and silicate endmembers for the watershed, as determined by sequentially leaching and digesting modem riverbed sediment with 4N acetic acid and HF-HC104, respectively (Blum et al., 1998). The average Ca/(1000*Sr) ratio of the exchangeable fraction in profiles 96PK1, 96PK2, 96PK3, and 96PK4 appears similar to the calcium carbonate endmember for the watershed (1.4), while the Ca/(1000*Sr) ratios contained in the entire suite of bulk soils as well as the exchangeable fraction in the C-horizons of profiles 96PK5 and 96PK5a appears more similar to the silicate endmember (0.19). These observations suggest that even though calcium carbonate represents only -1% of the rock in the watershed, calcium carbonate is the primary source of Ca and Sr provided to the dissolved groundwater flux during the early stages of weathering. In addition, the Ca/(1000*Sr) ratios in the C-horizons of profiles 96PK5 and 96PK5a demonstrate that the calcium carbonate reservoir in the soils is nearly depleted within the -15,000 to 20,000 years during which the soils have developed on the moraines. However, a reversal toward carbonate-like Ca/(1000*Sr) ratios in the uppermost horizons of the older profiles indicates that the continual addition of fresh weatherable material,

Grenville Magmatism in West Texas: Petrology and Geochemistry of the Red Bluff Granitic Suite

zone of regional extension, as suggested by recent geophysical and The Red Bluff granitic suite (RBGS) underlies part of the Franklin petrological studies. Mountains in west Texas. It was emplaced into a Middle Proterozoic shelf sequence at 1120±35 Ma. The suite is predominantly metaluminous; it consists of early granitic to quartz syenitic sills, a main mass of alkali feldspar granite, small bodies of alkali feldspar syenite, leucogranitic dikes, and late-stage peralkaline

On preparatory causal factors, initiating the prehistoric Tsergo Ri landslide (Langthang Himal, Nepal)

The most probable series of causal factors and their complexity make analysis and synthesis of the Tsergo Ri landslide (Langthang Himal, central-north Nepal) restricted, especially in this case of a prehistoric event with a spread of age data. Interpreting morphologic, lithologic, structural and engineering-geologic aspects in relation to preparatory causal factors and subsequent processes will therefore be only an approach.The solid basement and the broken crest of the landslide, as well as the surrounding areas of the upper section of Langthang valley, are to be included in the High Himalayan Crystalline: leucogranitic dikes (dipping towards the southwest) intruded gneisses and migmatites (primary foliation towards the northeast). Formation of these dikes caused one of many factors or preexisting structures, leading to the development of the landslide. In addition, subsequent tectonic movements and related seismicity forced specific deformation of metamorphic rocks: ultramylonites, two generations of faults with striated slickensides, and pseudotachylites degraded to substantial horizons of weakness. The spatial orientation enabled predetermination of subsequent morphologic processes (e.g., slope destabilising forces, trend and dip of sliding).In association with leucogranitic intrusions, a disseminated mineralized, extensive sulphidic ore structure outcrops at the broken crest of the landslide. Due to brittle rheologic behaviour and poor weathering resistance this ore-bearing horizon (parallel to the main sliding plane) also made the slopes susceptible to sliding. Material displaced from the scarp by landsliding, was mixed up with rocks from undisturbed ground and subsequently cemented by secondary ore mineralization, thus forming ore-breccias at the top of the surface of rupture.Neotectonic structures, caused by stress release associated with erosional processes and similarly oriented as the existing zones of weakness, may have acted as a preparatory and/or triggering causal factor for slope destabilisation too.Hyalomylonite from primary and secondary sliding surfaces, as well as gneiss from the solid basement of the landslide, analysed by means of scanning electron microscopy and energy-dispersive X-ray analysis, indicate deformational patterns attributed to tectonics and landslide mechanisms. The deformation feature is distinguished clearly from those created by shock-wave events (i.e. impact-triggered Köfels landslide, Tyrolean Alps, Austria). Thus, seismic activity might have been the triggering causal factor for the Tsergo Ri landslide event.

Metamorphism and Melting of the Lithosphere Due to Rapid Denudation, Nanga Parbat Massif Himalaya

The Nanga Parbat-Haramosh Massif of the western Himalaya has undergone a complex metamorphic and denudational history over the past 1.8 Ga. Metamorphism in the Tato region is characterized by a Barrovian type metamorphism followed by partial melting and recrystallization along shear zones during nearly isothermal decompression while the massif was still at temperatures >550°C. Mineralogical and textural evidence for this path includes garnet zoning patterns, late-stage migmatization, abundant cordierite both in schists and leucogranite dikes, replacement of high-pressure assemblages by low-pressure assemblages, and abundant low-density inclusions. Thermobarometry on the nonmigmatized gneisses reveals conditions of 540-740°C at 7.1-13.1 kbar. Thermobarometry in the migmat-ized rocks reveals final equilibration at 608-675°C at 3.9-6.8 kbar. Early fluid inclusions occur in quartz inclusions within garnet porphyroblasts that grew during decompression. The last fluid inclusions to be trapped occur in microfractures that locally crosscut several grain boundaries and, hence, record conditions after quartz grain boundary migration ceased. Examination of all the petrologic and fluid inclusions data conclusively show a very rapid denudation event late in the collisional process. Rapid denudation resulted in the advection of isotherms to shallow crustal levels causing an elevated geotherm (~60°C/km) in the upper crust, metamorphism along shear zones, and partial melting of the crust at depths of ca. 22 km.

Documentation of Neogene regional metamorphism in the Himalayas of Pakistan using U-Pb in monazite

In the Himalayas, investigators have constrained the timing of metamorphism associated with the Eocene orogeny through the use of cooling ages on a number of minerals. The majority of such studies give minimum metamorphic ages of ∼ 30–40 Ma [ 1–6], largely confirming theoretical studies predicting peak metamorphic conditions occurring tens of millions of years after large-scale collision [e.g. 7]. In northwestern Pakistan, however, the Nanga Parbat-Haramosh Massif (NPHM) exhibits much more recent activity, with rapid and accelerating cooling over the past 10 Ma [ 1,2]. The NPHM also exhibits unexpectedly recent igneous activity: three leucogranite dikes give minimum zircon U-Pb ages of 7, 5 and 2.3 Ma [ 8]. The problem we address in this paper is whether the thermal structure suggested by recent leucogranite production at depth is discernible in the shallower metamorphic rocks of the NPHM. To date metamorphism, we have used the U-(Th)-Pb system in monazite, a common accessory mineral in high-grade metamorphic rocks which can be a viable metamorphic geochronometer, dating the formation of the mineral under moderate metamorphic conditions [ 9,10]. Monazite separated from schists and gneisses of the NPHM gives U-Pb ages of 4–11 Ma. We interpret these young ages as evidence for Neogene metamorphism affecting the NPHM, an event heretofore unsuspected in the greater Himalaya and only intimated in the NPHM. This recent metamorphism roughly coincided with leucogranite production at depth [ 8] and with the onset of high denudation rates [ 1,2]. As such, our conclusions offer important constraints on further modeling of similarly anomalous behavior within tectonically active regions, suggesting that any reasonable model should strive to explain the relationship between recent, rapid uplift and coincident recent metamorphism.

Constraints on the Tectonic Evolution of the Northwestern Himalaya from Geochronologic and Petrologic Studies of Babusar Pass, Pakistan

Petrologic, geochronologic, and structural studies of Indian-plate rocks in the footwall of the Main Mantle Thrust indicate that peak metamorphism occurred in thrust imbricates well before the final assembly of the Kohistan island-arc with the Indian plate. As the northern margin of the Indian plate was thickened by this early thrusting, the metamorphic rocks evolved along increasing pressure-temperature paths and, in some regions, reached temperatures sufficient for partial melting. It is difficult to constrain the timing of this early deformation and metamorphism, but $$^{40}Ar/^{39}Ar$$ hornblende ages and U/Pb ages of crosscutting leucogranite dikes suggest that in most areas peak metamorphism was earlier than ~40 Ma and in some areas earlier than 50 Ma. After peak metamorphism, in the interval from ~40 to 25 Ma, rocks of the Indian plate continued to be thrust southward, creating sharp metamorphic discontinuities at thrust boundaries. From 25 to 20 ma rocks of the Indian plate underwent rapid cooling, and data suggest that this rapid cooling occurred over large regions of the northern margin of the Indian plate, along as much as 300 km of the Main Mantle Trust. We tentatively suggest that the rapid cooling resulted from tectonic denudation of the Indian plate as Kohistan slid northward along later normal faults. Between 15 and 20 Ma, faulting ceased and the terranes were locked into their present structural position; since then these terranes have simply undergone uplift and erosion. North of the Babusar region, in the vicinity of Nanga Parbat, tectonic activity along the Main Mantle Thrust continued to the present. In this region, the Indian plate rocks are presently being thrust over the adjacent Kohistan terrane.

Petrogenetic and tectonic significance of young leucogranites from the northwestern Himalaya, Pakistan

In the Himalaya of northwestern Pakistan, small discordant bodies of leucogranite intrude metamorphosed basement of the Indian crust. As the first step in assessing the tectonic significance of these granites, we have used the SHRIMP ion microprobe to obtain U‐Pb dates on zircon. Zircons of igneous appearance are scarce in the leucogranites, and with one exception are associated with xenocrystic components, which occur as discrete grains and as cores rimmed by young zircon. Zircons of igneous appearance are high in U [5000 to 40,000 ppm], low in Th/U [0.01 to 0.05], and frequently colored a distinct blue. Xenocrystic components give a range of Proterozoic ages consistent with ages previously reported for Indian crust. In sharp contrast, zircons of igneous appearance yield clusters of concordant Tertiary ages. Two leucogranite dikes from southern localities yield rather old emplacement ages of about 35 Ma (Swat) and about 50 Ma (Naran). Together with previously determined 40Ar/39Ar and fission track cooling ages, these ages document immediate and rapid metamorphism and denudation following Eocene collision. Three leucogranite dikes from northern localities within the Nanga Parbat‐Haramosh Massif are very young, having emplacement ages of about 2.3 Ma, 5 Ma, and 7 Ma. The formation and emplacement of these leucogranites during the rapid late Tertiary denudation of the Nanga Parbat region suggests to us that decompression melting may be a viable mechanism for leucogranite genesis.

Petrogénesis del plutón monzogranítico peralumínico de Santa Eufemia (Batolito de los Pedroches, Córdoba)

The Santa Eufemia pluton is part of the composite magmatic assoclatlon of Los Pedroches batholith, located in the southern branch of the Central Iberian Zone of the Hercynian orogenic belt of Spain. This high-Ievel postkynematic batholith consists of three main types of plutonic rocks: boitite±amphibole granodiorites, biotite-cordierite porphiritic monzogranites and cordierite leucogranites. The Santa Eufemia pluton consists of biotite-cordierite monzogranites, and in a lesser extent of cordierite leucogranites. Scarce enclaves of different type are found in the monzogranites, biotiteplagioclase- eordierite and fine grained porphiritic monzogranite enclaves being the most common. The petrographic, structural and geochemical (major and trace elements) data suggests a compiex evolutionary history of a metasedimentary-source monzogranitic magma with no genetical relationships with more basic magmas such as the one which is now represented by the granodiorite facies of the batholith. Non-minimum melt and reequilibrated source material (secondary restites) mixing during the ortomagmatic evolution of the magma accounts for the compositional heterogenities of the monzogranites. The ultimate product of this evolution is the segregation of a fluid-saturated residual liquid in the final level of intrusion of the magma, leading to the emplacement of leucogranite masses and dikes and the exsolution of a supercritical fluid phase related with the late magmatic recrystallization of sorne mineral phases (muscovite, albite, quartz, tourmaline, topaz), the geochemical removilization of sorne incompatible trace elements (Rb, Cs, Li, Be, Sn, Nb) due to fluid-rock interactions and the formation of periplutonic wolframite-arsenopyritequartz mineralizations.