Granites may be subdivided according to their intrusive settings into four main groups—ocean ridge granites (ORG), volcanic arc granites (VAG), within plate granites (WPG) and collision granites (COLG)—and the granites within each group may be further subdivided according to their precise settings and petrological characteristics. Using a data bank containing over 600 high quality trace element analyses of granites from known settings, it can be demonstrated using ORG-normalized geochemical patterns and element-SiO2 plots that most of these granite groups exhibit distinctive trace element characteristics. Discrimination of ORG, VAG, WPG and syn-COLG is most effective in Rb-Y-Nb and Rb-Yb-Ta space, particularly on projections of Y-Nb, Yb-Ta, Rb-(Y + Nb) and Rb—(Yb + Ta). Discrimination boundaries, though drawn empirically, can be shown by geochemical modelling to have a theoretical basis in the different petrogenetic histories of the various granite groups. Post-collision granites present the main problem of tectonic classification, since their characteristics depend on the thickness and composition of the lithosphere involved in the collision event and on the precise timing and location of magmatism. Provided they are coupled with a consideration of geological constraints, however, studies of trace element compositions in granites can clearly help in theelucidation of post-Archaean tectonic settings.

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Trace element discrimination diagrams for the tectonic interpretation of granitic rocks

A review of the geologic history of the Himalayan-Tibetan orogen suggests that at least 1400 km of north-south shortening has been absorbed by the orogen since the onset of the Indo-Asian collision at about 70 Ma. Significant crustal shortening, which leads to eventual construction of the Cenozoic Tibetan plateau, began more or less synchronously in the Eocene (50–40 Ma) in the Tethyan Himalaya in the south, and in the Kunlun Shan and the Qilian Shan some 1000–1400 km in the north. The Paleozoic and Mesozoic tectonic histories in the Himalayan-Tibetan orogen exerted a strong control over the Cenozoic strain history and strain distribution. The presence of widespread Triassic flysch complex in the Songpan-Ganzi-Hoh Xil and the Qiangtang terranes can be spatially correlated with Cenozoic volcanism and thrusting in central Tibet. The marked difference in seismic properties of the crust and the upper mantle between southern and central Tibet is a manifestation of both Mesozoic and Cenozoic tectonics. The form...

Geologic Evolution of the Himalayan-Tibetan Orogen

Granitoids as categorized by tectonic environment are (1) island arc granitoids (IAG), (2) continental arc granitoids (CAG), (3) continental collision granitoids (CCG), (4) postorogenic granitoids (POG), (5) rift-related granitoids (RRG), (6) continental epeirogenic uplift granitoids (CEUG), and (7) oceanic plagiogranites (OP). Of these, the IAG, CAG, CCG, and POG are considered orogenic granitoids, and the RRG, CEUG, and OP are considered anorogenic granitoids.

The discrimination of granitoids is based on the major-element chemistry. Various discrimination plots are presented which sequentially discriminate the different tectonic environments. OP are separated from all other granitoids on the K2O versus SiO2 plot. Discrimination between group I (IAG + CAG + CCG), group II (RRG + CEUG), and group III (POG) granitoids can be achieved by using plots of Al2O3 versus SiO2, FeO(T)/ [FeO(T) + MgO] versus SiO2, and AFM and ACF ternary diagrams. In the figures, group I and group II plot in individual fields. Identification of group III is different, in that group III does not have a unique field in which it plots. Group III is identified because it consistently displays characteristics of both group I and group II. Further discrimination within group I can be accomplished on the basis of Shand's index. Only CCG have A/CNK [AL2O3/(CaO + Na2O + K2O)] values greater than 1.15. It is not possible to discriminate between IAG and CAG. Further discrimination within group II is done using the TiO2 versus SiO2 plot.

The proposed discrimination scheme is applied to the Proterozoic granitoids of the midcontinent of the United States. It is shown that the Arbuckle granitoids are not anorogenic as previously thought.

Tectonic discrimination of granitoids

This geochemical classification of granitic rocks is based upon three INTRODUCTION variables. These are FeO/(FeO + MgO) = Fe-number [or Although granitoids are the most abundant rock types FeO/(FeO + MgO) = Fe∗], the modified alkali–lime index in the continental crust, no single classification scheme (MALI) (Na2O + K2O – CaO) and the aluminum saturation has achieved widespread use. Part of the problem in index (ASI) [Al/(Ca – 1·67P + Na + K)]. The Fe-number granite classification is that the same mineral assemblage, (or Fe∗) distinguishes ferroan granitoids, which manifest strong iron quartz and feldspars with a variety of ferromagnesian enrichment, from magnesian granitoids, which do not. The ferroan minerals, can be achieved by a number of processes. and magnesian granitoids can further be classified into alkalic, Granitoids can form from differentiation of any hyalkali–calcic, calc-alkalic, and calcic on the basis of the MALI persthene-normative melt and from partial melting of and subdivided on the basis of the ASI into peraluminous, metamany rock types. Furthermore, granitic melts may be luminous or peralkaline. Because alkalic rocks are not likely to be derived solely from crustal components, may form from peraluminous and calcic and calc-alkalic rocks are not likely to be evolved mantle-derived melts, or may be a mixture peralkaline, this classification leads to 16 possible groups of granitic of crustal and mantle-derived melts. Because of this rocks. In this classification most Cordilleran granitoids are magnesian complexity, petrologists have relied upon geochemical and calc-alkalic or calcic; both metaluminous and peraluminous classifications to distinguish between various types of types are present. A-type granitoids are ferroan alkali–calcic, although granitoids. Approximately 20 different schemes have evolved over the past 30 years [see Barbarin (1990, 1999) some are ferroan alkalic. Most are metaluminous although some are for a summary thereof]. Most of these schemes are either peraluminous. Caledonian post-orogenic granites are predominantly genetic or tectonic in nature. This paper is an attempt magnesian alkali–calcic. Those with <70 wt % SiO2 are domto present a non-genetic, non-tectonic geochemical clasinantly metaluminous, whereas more silica-rich varieties are comsification scheme that incorporates the best qualities of monly peraluminous. Peraluminous leucogranites may be either the previous schemes, and to explain the petrologic magnesian or ferroan and have a MALI that ranges from calcic to processes that makes this scheme work. alkalic.

/pdf/a-geochemical-classification-for-granitic-rocks-j2go5p9d9i.pdf

A geochemical classification for granitic rocks

Two end member models of how the high elevations in Tibet formed are (i) continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and (ii) time-dependent, localized shear between coherent lithospheric blocks. Recent studies of Cenozoic deformation, magmatism, and seismic structure lend support to the latter. Since India collided with Asia ∼55 million years ago, the rise of the high Tibetan plateau likely occurred in three main steps, by successive growth and uplift of 300- to 500-kilometer-wide crustal thrust-wedges. The crust thickened, while the mantle, decoupled beneath gently dipping shear zones, did not. Sediment infilling, bathtub-like, of dammed intermontane basins formed flat high plains at each step. The existence of magmatic belts younging northward implies that slabs of Asian mantle subducted one after another under ranges north of the Himalayas. Subduction was oblique and accompanied by extrusion along the left lateral strike-slip faults that slice Tibet's east side. These mechanisms, akin to plate tectonics hidden by thickening crust, with slip-partitioning, account for the dominant growth of the Tibet Plateau toward the east and northeast.

http://science.sciencemag.org/content/sci/294/5547/1671.full.pdf

Oblique Stepwise Rise and Growth of the Tibet Plateau

A detailed isotopic study of the Manaslu leucogranite was carried out. A U-Pb age of 25 Ma and a whole rock Rb-Sr age isochron of 18 Ma were obtained, suggesting that the magmatic activity lasted at least 7 Ma. Initial Sr isotopic ratios are very high (0.740 to 0.760) and initial Nd isotopic ratios are low (ɛ

 Nd
 in
 : −13 to −16), and they show the existence of large isotopic variations even at the metre scale. These are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes (e.g. fusion, mixing by convection). These results also support the crustal origin of this leucogranite. The Tibetan slab paragneisses, whose Sr and Nd isotopic ratios are very similar to those of the granite at an age of 20 Ma, are the most probable parental material. Nd model ages for both the leucogranite and the gneisses are in the range 1.5–2 Ga. A model of formation of the Manaslu granite by coalescence of different batches of magma is in agreement with the present data.

Isotopic study of the Manaslu granite (Himalaya, Nepal): inferences on the age and source of Himalayan leucogranites

The Manaslu pluton is one of 10 leucogranites that formed in the overthrusted Higher Himalaya after the Indo-Eurasian collision. Field and analytical data indicate that the underlying migmatites of the Tibetan Slab may be where the leucogranitic melts were generated. The Himalayan crustal thrusting of a hot slab over a rather cold volcano-sedimentary pile [Le Fort, 1975a] provides the necessary release of fluids. These fluids cross the Main Central Thrust (MCT), induce the partial anatexis of the overheated Tibetan Slab, and produce a leucogranitic magma. The emplacement of the magma at first is located along the main disharmonic plane above the MCT, between the infrastructure and the superstructure. There it generates a convective hydrothermal system extending very far laterally according to the stratification of permeabilities. Progressive emplacement of the granite proceeds as the convected fluids, including the fluids released by the saturated magma, dissolve the mainly calcareous host rocks of Tibetan sedimentaries. A frozen image of this ‘caving out’ progression is given by the extensive network of granitic dikes outside the pluton. The two quite independant fluid cycles of the generation and of the emplacement were triggered or guided by the tectonics due to collision. This two-fold model, dominated by fluid activity, may be of importance for other leucogranites and granites.

Manaslu leucogranite: A collision signature of the Himalaya: A model for its genesis and emplacement

The Four Plutonic Belts of the Transhimalaya-Himalaya: a Chemical, Mineralogical, Isotopic, and Chronological Synthesis along a Tibet-Nepal Section

duplex initiated at 9.8 ± 1.7 Ma, leading to an increase of uplift rate at front of the High Himalaya from 0.9 ± 0.31 to 3.05 ± 0.9 mm yr −1 . We also run 3‐D models by coupling PECUBE with a landscape evolution model (CASCADE). This modeling shows that the effectoftheevolvingtopographycanexplainafractionofthescatterobservedinthedatabut not all of it, suggesting that lateral variations of the kinematics of crustal deformation and exhumationarelikely.Ithasbeenarguedthatthesteepphysiographictransitionatthefootof the Greater Himalayan Sequence indicates OOS thrusting, but our results demonstrate that the best fit duplex model derived from the thermochronological and thermobarometric data reproduces the present morphology of the Nepal Himalaya equally well.

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Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography

decompression to achieve the same result appear to require extreme The Manaslu granite is the most studied of the dozen or so plutons conditions not permitted by available geological constraints. that make up the High Himalayan leucogranite belt. The inferred relationship of the Manaslu granite with important Himalayan tectonic structures has inspired repeated attempts to determine its crystallization age, but several geochemical controls have hampered these geochronological investigations. These limitations are circumvented by Th–Pb ion microprobe dating of monazite. We have

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Patrick Le Fort

Papers

Isotopic study of the Manaslu granite (Himalaya, Nepal): inferences on the age and source of Himalayan leucogranites

Manaslu leucogranite: A collision signature of the Himalaya: A model for its genesis and emplacement

The Four Plutonic Belts of the Transhimalaya-Himalaya: a Chemical, Mineralogical, Isotopic, and Chronological Synthesis along a Tibet-Nepal Section

Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography

Origin and Episodic Emplacement of the Manaslu Intrusive Complex, Central Himalaya