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Construction of oceanic crusts in ultraslow-spreading ridges are controlled by detachment faults, but the specific ways they are determined by detachments remain unclear. Abundant ophiolite massifs discovered along the Yarlung-Tsangpo Suture Zone, southern Tibet, are considered to represent ultraslow-spreading centers of the Cretaceous Neo-Tethys ocean. The presence of various mafic blocks within the ophiolitic mélange beneath the lithospheric mantle provides evidence of the entire processes involved in crustal generation. Therefore, in this study geochemical investigations on these mafic blocks from the southern Luqu (Xigaze) ophiolitic mélanges were conducted to shed new lights on the petrogenesis of mafic crust rocks in ultraslow-spreading settings. Geochronological results via zircon U-Pb dating gave evidence for their coeval formation to other Yarlung-Tsangpo ophiolitic crust rocks in the Early Cretaceous (ca. 120-130 Ma). Geochemistry and Sr-Nd-Hf isotopes document depleted and primitive characteristics, revealing that these mafic rocks were crystallized from residual melts experienced substantial fractional crystallization following along liquid lines of descent of H.sub.2O-poor magmas. However, amphiboles are pervasive throughout our samples, which is in contrast with expectations of clinopyroxene and plagioclase as dominant liquidus phases of anhydrous magma. Textures of replacement of clinopyroxene by magnesio-hornblende and their higher contents of trace elements and Eu negative anomalies support that the amphiboles crystallized during transition from dry magma to hydrous one, indicative of water additions to solidifying melts. Based on the tectonic settings of ultraslow-spreading centers, we proposed a model in which seawater-derived fluids induced through detachment faults could be responsible for the generation of ultraslow-spreading crust., Author(s): Chang Zhang [sup.1] [sup.2], Chuan-Zhou Liu [sup.1] [sup.2] [sup.3], Tong Liu [sup.1] [sup.2], Wen-Bin Ji [sup.4], Fu-Yuan Wu [sup.1] [sup.2] [sup.3] Author Affiliations: (1) grid.9227.e, 0000000119573309, State Key Laboratory [...]

Knowing the pressure of crystallization of peraluminous felsic rocks is essential to understanding their petrogenesis and implications for the tectonic histories of orogenic belts. Phase relations studies in the haplogranite system reveal that the stability field of quartz enlarges while the stability field of feldspars shrinks with increasing pressure. The result is that the compositions of the cotectic melts (saturated in quartz and feldspar) become less silicic with increasing pressure. Furthermore, the position of the quartz - feldspar cotectic is insensitive to water activity in the melt. This relationship provides an opportunity to estimate the pressure at which melt, quartz, and feldspar equilibrated. The accuracy of pressure estimation depends on accurately estimating the composition of the cotectic melt. However, additional components drive the composition of the cotectic melt away from that in the simple haplogranite system. For example, anorthite component (An) shifts the cotectic curve towards the Qz-Or join, and excess aluminum in the melt (i.e., peraluminous composition) drives the cotectic curve toward the Qz apex. A majority of felsic magmas (> ~ 70 wt% SiO.sub.2) are at least slightly peraluminous, and strongly peraluminous magmas are common in orogenic settings. A reasonable estimation of the pressures (depths) of crystallization of peraluminous magmas, which is critical for understanding accompanying tectonic processes, requires a better understanding of the effect of excess Al on the cotectic, and specifically on quartz saturation. We have compiled 32 experimental cotectic glass compositions in the felsic system, in which ASI (= Al.sub.2O.sub.3/(CaO + Na.sub.2O + K.sub.2O)(commonly defined identically to A/CNK), in mole content)) range from 1.10 to 2.13. The scheme of (Blundy and Cashman, Contrib Mineral Petrol 140:631-650, 2001) was chosen to correct for the effect of An on the position of the cotectic. The measured Qz of the experimental cotectic melts was compared to Qz calculated using the Blundy and Cashman correction at the experimental pressures. We assumed that the discrepancy between the corrected Qz content and the experimental Qz content was caused by excess aluminum in the melt. Based on this assumption, we can regress the relationship between excess aluminum (ASI-1) and 'excess' Qz content. The positive correlation between ASI-1 and increased Qz demonstrates that excess aluminum increases the cotectic Qz content, i.e. quartz solubility, in the peraluminous system. The deviation of Qz content systematically increases from 4 to 20 wt%, with ASI rising from 1.10 to 2.13; the increase is especially rapid and clearly defined between 1.1 and 1.5, the normal range for natural peraluminous melts. The departure from ASI = 1.00 (ASI-1) systematically affects normative Qz in melt at a given P. We calibrate this relationship as: [Formula omitted]where [delta]Qz is the deviation of normative Qz in quartz- + feldspar-saturated melts as ASI increases from 1.00. Uncertainty in our calibration is fairly high (~ 120 MPa (one sigma), based on a standard error from the regression). This uncertainty might be influenced by many factors: (1) the accuracy of the compiled experiments; (2) disequilibrium in the compiled experiments; (3) uncertainty in the estimation of quartz + feldspar-saturated melt compositions based on natural rock samples; (4) the effect of other elements on the position of the cotectic curve, such as F, P, B, Fe, Mg, Ti, Mn. The empirical barometer was applied to natural examples (compositions of peraluminous glasses and granite compositions that permit estimation of melt compositions) with other thermobarometric constraints (cordierite-saturation, GASP (garnet-Al.sub.2SiO.sub.5-quartz-plagioclase), GBPQ (garnet-biotite-plagioclase-quartz), TitaniQ barometers for estimated melts; host rock metamorphic conditions). We propose that, where natural samples provide good estimates of the compositions of peraluminous melts that were saturated in quartz + feldspars (ideally two feldspars), our calibration provides useful estimates of the pressure of equilibration. Note that whole-rock compositions of granites cannot be assumed to represent melt composition; hence, pressures calculated by our barometer may not be meaningful unless applied to rocks whose compositions are likely to represent melts on the quartz-feldspar cotectic. The most obvious candidates are rhyolite glasses that host quartz + feldspar phenocryst assemblages and phenocryst-poor, aplitic-textured granites with textures indicating simultaneous crystallization of quartz and feldspar. The pressure calculation can be easily performed with the user-friendly spreadsheet (online supplement Table S4). As anticipated, the calculated pressures for peraluminous compositions are significantly higher than current quartz-saturation-based barometers would otherwise suggest., Author(s): Lei Yang [sup.1] [sup.2], Calvin F. Miller [sup.3], Fu-Yuan Wu [sup.2] Author Affiliations: (1) grid.411288.6, 0000 0000 8846 0060, College of Earth Sciences, Chengdu University of Technology, , 610059, [...]

The nature (i.e., sub-oceanic, sub-arc or sub-continental) of ophiolitic mantle peridotites from the eastern Neo-Tethyan domain in southern Tibet has been hotly debated. This uncertainty limits our understanding of the history and evolution of the eastern Neo-Tethys Ocean. Here we present petrological, geochemical and Re-Os isotopic data for the mantle peridotites from the Dazhuqu ophiolite in the central segment of the Yarlung Zangbo suture zone, southern Tibet. Samples collected include both spinel lherzolites and spinel harzburgites. The lherzolites have spinel Cr# [Cr/(Cr + Al), ~ 0.3-0.4] comparable to those of typical abyssal peridotites. In contrast, the harzburgites have spinel Cr# (~ 0.3-0.7) overlapping with the ranges of both abyssal and fore-arc peridotites; two samples have spinel Cr# higher than 0.6, which is probably ascribed to intense melt-rock interactions. Clinopyroxene trace element modeling indicates that the Dazhuqu mantle peridotites have experienced 0-6% garnet-facies melting followed by 10-18% melting in the spinel stability field. This is similar to the degree of garnet-facies melting inferred for many abyssal peridotites and implies deep initial melting (> 85 km), which distinguishes the Dazhuqu mantle peridotites from fore-arc peridotites (commonly < 80 km in origin). The Dazhuqu peridotites have unradiogenic .sup.187Os/.sup.188Os of 0.11836-0.12922, which are commonly lower than the recommended value of primitive upper mantle (PUM). All but one samples yield relatively younger Re depletion ages (T.sub.RD = 0.06-0.81 Ga) with respect to the only one sample having an older T.sub.RD age of 1.66 Ga. Re-Os isotopes and highly siderophile element (HSE) compositions of the Dazhuqu peridotites are similar to those of abyssal peridotites and the Oman southern massifs but are distinct from non-cratonic sub-continental lithospheric mantle (SCLM) xenoliths and sub-arc mantle. We emphasize the similarity between the Dazhuqu and Oman ophiolites, both representing Neo-Tethyan oceanic lithosphere and implying ridge-trench collision., Author(s): Tong Liu [sup.1] [sup.2] , Fu-Yuan Wu [sup.1] [sup.2] [sup.3] , Chuan-Zhou Liu [sup.1] [sup.3] [sup.4] , Chang Zhang [sup.1] , Wen-Bin Ji [sup.1] , Yang Xu [sup.1] Author [...]

Ophiolites worldwide show striking diversities in their rock assemblage and structure (i.e., ophiolite diversity), raising a question whether ophiolites are originally similar before intense tectonic dismemberment. Comparison between ophiolites and oceanic lithospheres at modern mid-ocean ridges may provide key constraints on the origin of ophiolite diversity, because oceanic lithospheric structures are inherently controlled by spreading rates. Here, we present a case study of the Xigaze ophiolite in southern Tibet focusing on its gabbroic intrusions outcropping in three localities, i.e., Dazhuqu, Baigang and Jiding. Compared to the Jiding sequence, the Dazhuqu and Baigang gabbroic rocks are less evolved, characterized by higher Cr.sub.2O.sub.3 contents but lower contents of TiO.sub.2 and rare earth element in both clinopyroxene and bulk compositions. It is evident, hence, that the Xigaze ophiolite is characterized by variably evolved and discontinuously distributed gabbroic intrusions, rather by a continuous lower oceanic crust between the mantle and sheeted dike complex as the Penrose-type ophiolites. Our study, along with the identification in previous studies of oceanic detachment faults within the Xigaze ophiolite, demonstrates that the Xigaze ophiolite shows close similarities to oceanic lithospheres at modern slow- and ultraslow-spreading ridges. Hence, the significant structural distinctions between the Xigaze ophiolite and the Penrose-type ophiolites (e.g., the Oman ophiolite) may be inherently associated with different spreading rates of paleo-ridges. Considering the limited scale of the Xigaze gabbroic rocks, here we suggest the Xigaze ophiolite as a typical representative of fossil ultraslow-spreading ridges., Author(s): Tong Liu [sup.1] [sup.2] , Fu-Yuan Wu [sup.1] [sup.2] [sup.3] , Chuan-Zhou Liu [sup.1] [sup.2] [sup.3] [sup.4] , Riccardo Tribuzio [sup.5] , Wen-Bin Ji [sup.1] , Chang Zhang [sup.1] [...]

Ancient cratons are typically characterized by thick lithospheric-mantle roots that provide them with buoyancy and rigidity. Once formed, cratons, including their roots, move around Earth's surface as parts of rigid plates that are normally unaffected by later magmatism or tectonic activity. Considerable geophysical and geochemical evidence, however, suggests that the ancient lithospheric mantle beneath the Chinese portion of the eastern Sino-Korean craton was replaced by thinner, hotter, juvenile mantle during the Jurassic to Early Cretaceous. The extent and cause of the changes to this lithospheric mantle have been debated. Keys to deciphering the history of the changes include knowledge of both the lateral extent of the lithospheric modifications and the relative timing of change across the craton. Discerning the age and structure of lithospheric mantle underlying North Korea, the easternmost part of the Sino-Korean craton, is thus particularly important. Here we report mineral compositions and Re-Os isotopic data for peridotite xenoliths from North Korean Triassic kimberlite and Tertiary basalts. The data suggest that, from the Triassic to the present, North Korea has been underlain by young, hot and fertile lithospheric mantle, unlike typical cratonic lithospheric roots, but similar to the juvenile lithospheric mantle underlying Chinese portions of the craton. Given the generally ancient nature of the crust composing North Korea, our findings suggest that modification of underlying cratonic roots extended to the eastern edge of the craton. The Triassic eruption age of the kimberlite suggests that the lithospheric changes may have occurred earlier than in China, indicating that lithospheric removal evolved from east to west. These results are most consistent with the conclusion that lithospheric loss was initially triggered by extension that followed the collision between the Sino-Korean and Yangtze cratons. doi: 10.1130/G30944.1

Monazite is a key accessory mineral for metamorphic geochronology, but interpretation of its complex chemical and age zoning acquired during high-temperature metamorphism and anatexis remains a challenge. We investigate the petrology, pressure-temperature and timing of metamorphism in pelitic and psammitic granulites that contain monazite from the Greater Himalayan Crystalline Complex (GHC) in Dinggye, southern Tibet. These rocks underwent isothermal decompression from pressure of >10 kbar to ~5 kbar at temperatures of 750-830 °C, and recorded three metamorphic stages at kyanite (M.sub.1), sillimanite (M.sub.2) and cordierite-spinel grade (M.sub.3). Monazite and zircon crystals were dated by microbeam techniques either as grain separates or in thin sections. U-Th-Pb ages are linked to specific conditions of mineral growth on the basis of zoning patterns, trace element signatures, index mineral inclusions (melt inclusions, sillimanite and K-feldspar) in dated domains and textural relationships with co-existing minerals. The results show that inherited domains (500-400 Ma) are preserved in monazite even at granulite-facies conditions. Few monazites or zircon yield ages related to the M.sub.1-stage (~30-29 Ma), possibly corresponding to prograde melting by muscovite dehydration. During the early stage of isothermal decompression, inherited or prograde monazites in most samples were dissolved in the melt produced by biotite dehydration-melting. Most monazite grains crystallized from melt toward the end of decompression (M.sub.3-stage, 21-19 Ma) and are chemically related to garnet breakdown reactions. Another peak of monazite growth occurred at final melt crystallization (~15 Ma), and these monazite grains are unzoned and are homogeneous in composition. In a regional context, our pressure-temperature-time data constrains peak high-pressure metamorphism within the GHC to ~30-29 Ma in Dinggye Himalaya. Our results are in line with a melt-assisted exhumation of the GHC rocks., Author(s): Jia-Min Wang [sup.1] [sup.2] , Fu-Yuan Wu [sup.1] [sup.3] , Daniela Rubatto [sup.4] , Shi-Ran Liu [sup.5] , Jin-Jiang Zhang [sup.5] , Xiao-Chi Liu [sup.1] , Lei Yang [sup.1] [...]

The Liaonan metamorphic core complex formed during crustal extension in the Liaodong Peninsula, eastern North China craton, and consists of the Jinzhou detachment fault, Proterozoic-Paleozoic sedimentary rocks in its upper plate, and exhumed high-grade Archean metamorphic rocks and Early Cretaceous granitic plutons in the lower plate. Exhumation of its footwall from mid-crustal levels is evidenced in the detachment fault zone by the temporal transition from amphibolite facies mylonitization at depth, through retrograde chloritic shearing and brecciation, to brittle faulting during final uplift. The footwall mylonite zone is 2.5-3.5 km thick and includes Early Cretaceous (128-118 Ma) granitic rocks, together with older metamorphic rocks. The [sup.40]Ar/[sup.39]Ar ages of muscovite, hornblende, biotite, and K-feldspar from the mylonitic rocks record that the core complex cooled between ca. 120 and 107 Ma, from the time of initial crystallization of zircons (122--118 Ma) at 700-800[degrees]C in syntectonic leucocratic dikes and granitic rocks, to closure of argon diffusion in hornblende, micas, and K-feldspar at ~500 to ~200[degrees]C. Throughout the eastern North China craton, the synchroneity of cooling and exhumation of metamorphic core complexes, the formation of pull-apart basins, and regional alkaline igneous activity, reflects regional extensional tectonics in the Early Cretaceous. This accompanied lithospheric thinning, possibly resulting from the rollback of the subducted Pacific plate along the eastern Asian margin during the Early Cretaceous. Keywords: argon thermochronology, metamorphic core complex, crustal extension, Early Cretaceous, North China Craton, Liaodong Peninsula, Liaonan metamorphic core complex. doi: 10.1130/B26085.1

[1] India-Asia collision has resulted in the largest orogenic event on Earth since at least 500 Ma, and this has had a profound influence on the drainage patterns of Asia, on the Asian monsoon climate, on global oceanography, as well as on faunal extinctions. Despite this importance, understanding of the Paleogene evolution of this system is limited. U-Pb and Lu-Hf isotopic analyses of detrital zircons from the clastic sedimentary rocks of the Indus Molasse, located within the Indus Suture of the Ladakh Himalaya, India, show that most have Mesozoic-Cenozoic ages and juvenile Hf isotopic compositions. This suggests that the main source of these sediments is the Transhimalayan batholith, with minor flux from the Lhasa-Karakoram terrane and Indian plate. The lower Indus Molasse (Chogdo Formation) is inferred to have been deposited at 60-49 Ma, whereas the upper Choksti Formation was deposited later than 45-41 Ma and contains a greater contribution from the Lhasa terrane. The Transhimalayan batholith was eroded throughout Indus Molasse sedimentation, but increasing exhumation of Lhasa-Karakoram terrane indicates its progressive uplift after initial collision, and requires formation of a paleo-Indus River shortly after collision.

Byline: Jin-Hui Yang (1), Fu-Yuan Wu (1), Simon A. Wilde (2), Lie-Wen Xie (1), Yue-Heng Yang (1), Xiao-Ming Liu (3) Abstract: In situ zircon U--Pb and Hf-isotopic data have been determined for mafic microgranular enclaves and host granitoids from the Early Cretaceous Gudaoling batholith in the Liaodong Peninsula, NE China, in order to constrain the sources and petrogenesis of granites. The zircon U--Pb age of the enclaves (120 +- 1 Ma) is identical to that of the host monzogranite (120 +- 1 Ma), establishing that the mafic and felsic magmas were coeval. The Hf isotopic composition of the enclaves [[epsilon] .sub.Hf(t) = +4.5 to -6.2] is distinct from the host monzogranite [[epsilon] .sub.Hf(t) = -15.1 to -25.4], indicating that both depleted mantle and crustal sources contributed to their origin. The depleted mantle component was not previously revealed by geochemical and Nd and Sr isotopic studies, showing that zircon Hf isotopic data can be a powerful geochemical tracer with the potential to provide unique petrogenetic information. Some wall-rock contamination is indicated by inherited zircons with considerably older U--Pb ages and low initial Hf isotopic compositions. Hafnium isotopic variations in Early Cretaceous zircons rule-out simple crystal--liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, mixing of mantle-derived mafic magmas with crustal-derived felsic magmas, coupled with assimilation of wall rocks, is compatible with the data. Author Affiliation: (1) State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing, 100029, China (2) Department of Applied Geology, Curtin University of Technology, P.O. Box U1987, Perth, WA, 6845, Australia (3) Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an, 750069, China Article History: Registration Date: 24/08/2006 Received Date: 25/12/2005 Accepted Date: 17/08/2006 Online Date: 20/09/2006 Article note: Communicated by T.L. Grove. Electronic supplementary material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00410-006-0139-7 and is accessible for authorized users.

In situ zircon U-Pb and Hf-isotopic data have been determined for mafic microgranular enclaves and host granitoids from the Early Cretaceous Gudaoling batholith in the Liaodong Peninsula, NE China, in order to constrain the sources and petrogenesis of granites. The zircon U-Pb age of the enclaves (120 [+ or -] 1 Ma) is identical to that of the host monzogranite (120 [+ or -] 1 Ma), establishing that the mafic and felsic magmas were coeval. The Hf isotopic composition of the enclaves [[epsilon.sub.Hf](t) = +4.5 to -6.2] is distinct from the host monzogranite [[epsilon.sub.Hf](t) = -15.1 to -25.4], indicating that both depleted mantle and crustal sources contributed to their origin. The depleted mantle component was not previously revealed by geochemical and Nd and Sr isotopic studies, showing that zircon Hf isotopic data can be a powerful geochemical tracer with the potential to provide unique petrogenetic information. Some wallrock contamination is indicated by inherited zircons with considerably older U-Pb ages and low initial Hf isotopic compositions. Hafnium isotopic variations in Early Cretaceous zircons rule-out simple crystal-liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, mixing of mantle-derived mafic magmas with crustal-derived felsic magmas, coupled with assimilation of wall rocks, is compatible with the data.