Peraluminous Two-mica Granite Research Articles

Tracing the history from Rodinia break-up to the Gondwana amalgamation in the Embu Terrane, southern Ribeira Belt, Brazil

The Mantiqueira Province (MP) is the major tectonic unit of southern Brazil, resulting from the closure of the Adamastor Ocean during Gondwana assembly. The Ribeira Belt (RB), the largest domain of the MP, has developed in several episodes of convergence during the Brasiliano-Pananafricano Orogeny and is subdivided into several terranes. One of them is the Embu Terrane (ET) that hosts metasedimentary successions and granitic bodies of different tectonic contexts. A combined study of petrography, geochemistry, LA–ICP–MS zircon U—Pb dating, and Hf isotope geology was carried out on the Juquiá (JG) and Sete Barras (SBG) granites of the southeastern ET, hosted by metasupracrustal rocks. The JG yielded U—Pb zircon ages of 799 ± 5 Ma and 755 ± 3 Ma, while the SBG U—Pb zircon and monazite ages of 602 ± 2 Ma and 598 ± 2 Ma respectively. Both granites have Neoarchean/Paleoproterozoic Hf(TDM) model ages and strongly negative εHf(t) values in zircon. Whole rock isotopic data support an evolved crustal signature, pointing to sources of long-lived crustal residence. The JG is a ferroan calc-alkalic peraluminous two-mica granite, while the SBG is a ferroan alkali-calcic peraluminous two-mica granite. The acquired data disclose two distinct magmatic episodes in ET: the Tonian Juquiá magmatism took place during the final stages of the Rodinia break-up or initial stages of expansion of one of the hypothetical oceans in the region and the Ediacaran Sete Barras magmatism might have been formed at the local transtensional area during final West Gondwana amalgamation. The data presented here and the wide-ranging geochronological data assemblage from the RB allowed a new paleogeographic reconstruction of the West Gondwana assembly.

Experimental and geologic evaluation of monazite (U–Th)/He thermochronometry: Catnip Sill, Catalina Core Complex, Tucson, AZ

Monazite is a petrologically important and analytically promising target for (U–Th)/He thermochronology. Previous studies have reported highly variable He diffusion results from monazite from a single sample and demonstrated that composition can significantly affect He diffusion parameters. In this study, we performed incremental heating of single monazite grains to experimentally determine the 4He diffusion properties of reference monazite ‘554’ that occurs within a peraluminous two-mica granite from the Catnip Sill within the Catalina Core complex, Arizona. Assuming that the grain size defines the diffusion geometry, the six experiments yielded Ea values of 212 to 238±5 kJmol−1 (1σ) and Do values of 15.7 to 103 cm2s−1 with one value of 784 cm2s−1. Monazite (U–Th)/He data from five grains yielded closure temperatures of 291 to 262 °C (± c. 15 °C) and ages of 23.8–20.3(±∼1.2;2σ) Ma; the weighted mean age is 21.8±0.73 (MSWD=1.83, n=5) and the weighted mean closure temperature is 282±6°C (MSWD=0.96, n=5;1σ). We tested the accuracy of these results by comparing our monazite thermochronology data with monazite Th/Pb depth profiling results, the 40Ar/39Ar thermal history for the Catnip Sill constrained using coexisting muscovite, biotite, and K-feldspar, and published regional zircon and apatite fission track results. The monazite Th/Pb data indicate emplacement of the Catnip Sill at ∼45 Ma. The 40Ar/39Ar muscovite and biotite data indicate cooling from 460 to 350 °C from 27 to 26 Ma. K-feldspar MDD modeling suggests cooling from 360 to 240 °C from 26 to 24 Ma. Zircon fission track data indicate cooling through 250 °C between 29 and 20 Ma. Additional cooling through 110 °C is recorded by apatite fission track ages of 19–16 Ma. Because the monazite thermochronology results are reproducible and consistent with the thermal history constrained by the other chronometers, our results 1) confirm the accuracy of the 4He diffusion kinetics from monazite ‘554,’ and 2) demonstrate that monazite can be an effective (U–Th)/He thermochronometer, provided that He diffusion properties are either measured or reasonably assumed for each sample.

Multiple-aged granitoids and related tungsten-tin mineralization in the Nanling Range, South China

The Nanling metallogenic belt in South China is characterized by well-developed tungsten-tin mineralization related to multiple-aged granitoids. This belt is one of the 5 key prospecting and exploration areas among the 19 important metallogenic targets in China. Important progress has been made in recent years in understanding the Nanling granitoids and associated mineralization, and this paper introduces the latest major findings as follows: (1) there exists a series of Caledonian, Indosinian, and Yanshanian W-Sn-bearing granites; (2) the Sn-bearing Yanshanian granites in the Nanling Range form an NE-SW trending aluminous A-type granite belt that stretches over 350 km. The granites typically belong to the magnetite series, and dioritic micro-granular enclaves with mingling features are very common; (3) the Early Yanshanian Sn- and W-bearing granites possess different petrological and geochemical features to each other: most Sn-bearing granites are metaluminous to weakly peraluminous biotite (hornblende) granites, with zircon ɛHf(t) values of ca. −2 to −8, whereas most W-bearing granites are peraluminous two-mica granites or muscovite granites with ɛHf(t) values of ca. −8 to −12; (4) based on the petrology and geochemistry of the W-Sn-bearing granites, mineralogical studies have shown that common minerals such as titanite, magnetite, and biotite may be used as indicators for discriminating the mineralizing potential of the Sn-bearing granites. Similarly, W-bearing minerals such as wolframite may indicate the mineralizing potential of the W-bearing granites. Future studies should be focused on examining the internal relationships between the multiple-aged granites in composite bodies, the metallogenic peculiarities of multiple-aged W-Sn-bearing granites, the links between melt evolution and highly evolved ore-bearing felsic dykes, and the connections between granite domes and mineralization.

A unique sequential melting mechanism for the generation of anatectic granitic rocks from the Penafiel area, northern Portugal

Granitic rocks derived by sequential partial melting are very uncommon occurrences worldwide with only three known cases in Portugal (Tourém, Guarda–Sabugal and Penafiel) and one in Argentina (Achiras complex). The suite documented in this paper comprises two of the three main phases of a batholith in the Penafiel area. These Variscan peraluminous two-mica granite and granodiorites formed at 310 and 305Ma during and near the end of the Variscan D3 deformation and have the characteristics of S-type granites. None of the metamorphic country rocks have an adequate Sr isotopic composition to be a protolith. It is argued that progressive isotopic re-equilibration of crustal material, probably a metagreywacke for G1 and G3 with also some metapelitic contribution for G2, during granulization of the lower Variscan crust explains the difference in isotopic composition between outcropping granitic rocks and metamorphic country rocks. Biotite>muscovite granite G1 and biotite>muscovite granodiorite G3 have similar (87Sr/86Sr)i and εNdt values and were derived by sequential partial melting of the same source material. This is indicated by coherent trends of the whole rock major and trace element compositions, the compositions of feldspar, biotite, muscovite and tourmaline, the presence and type of xenocrystic mineral phases and xenoliths, and whole rock oxygen isotope compositions. This mechanism was the result of post-thickening Variscan extension and mantle upwelling. Another member of the same batholith is a biotite≈muscovite granodiorite (G2), with an intermediate age of 307Ma, but with geochemical characteristics suggesting that it corresponds to a chemically distinct pulse of magma, although likely stemming from a comparable, though more metapelitic, lower crustal source.

The Miocene granitoid rocks of Mt. Bukulja (central Serbia): evidence for pannonian extension-related granitoid magmatism in the northern Dinarides

The study presents evidence about the origin and evolution of the Miocene (20–17 Ma) granitoid pluton of Mt. Bukulja, situated within the southern Pannonian/northern Dinarides region (central Serbia, south-central Europe). The pluton is composed of slightly peraluminous two-mica granite (TMG), metaluminous hornblende-biotite and biotite-bearing (H-BG) granite and rare aplite granite. A lamprophyre dyke (BLD) similar in composition and age to other Serbian primitive minettes has been found in the vicinity of Mt. Bukulja. The available and newly determined radiometric age suggests that the TMG was emplaced around 20 Ma whereas the age of the H-BG is inadequately constrained. TMG and H-BG show similar petrographic characteristics but evidence of open system magma processes is found only in the H-BG. In comparison to the H-BG, the TMGs are less enriched in most trace elements, including REE, and have a more fractionated REE-pattern and stronger negative Eu-anomaly. The TMGs display a wider range of initial Sr-Nd isotope ratios (87Sr/86Sr20 Ma = 0.70652–0.71368 and 143Nd/144Nd20 Ma = 0.51223–0.51283) than the H-BG (87Sr/86Sr20 Ma = 0.70768–0.70781 and 143Nd/144Nd20 Ma = 0.51242–0.51256). Geochemical modeling suggests that the H-BG could have derived from a BLD-like melt by mixing plus fractionation processes assuming a batch of TMG-like magma as the acid end-member. On the other hand, the geochemical variability of the TMG is reproduced by an AFC model with an assimilation/fractionation ratio ( r ) of 0.5 and with high amount of crustal component (~20–50 %) starting from the least evolved TMG rocks. In the modeling, the average composition of the least evolved TMG samples was used to represent the parental magma composition whereas the composition of adjacent metamorphic rocks was adopted as possible contaminant. The composition of the least evolved TMG implies that the TMG parental magma likely originated by melting of a mafic lithology such as earlier basalts underplating in the lower crust. The high proportions of crustal assimilation along with other geochemical and geological evidence suggest that the Mt. Bukulja TMG originated within the same geotectonic setting as acid volcanics of the north Pannonian Basin. The results of this study support the hypothesis that the Mt. Bukulja pluton is related to tectonomagmatic events controlled by the early extensional phases in the opening of the Pannonian basin.

Emplacement of ellipsoid-shaped (diapiric?) granite: Structural and gravimetric analysis of the Oulmès granite (Variscan Meseta, Morocco)

The Oulmès granite is a NE–SW elongated stock intruded in Ordovician metasedimentary rocks which crop out at the core of a regional anticlinorium of the Moroccan Variscan Meseta. The stock, dated at around 300 Ma, is made up of peraluminous two-mica granite, with subordinate amounts of muscovite leucogranite. Mineral composition and textural features of both the granite and the thermal aureole enable us to constrain the P–T conditions during magma intrusion in ≈250–300 MPa and ≈600 °C at the contact between the granite and its host rock. Emplacement of the Oulmès granite postdates the main regional structures and is coeval with the late stages of Variscan compression. Prior to granite intrusion, the host rocks were affected by a main deformation phase responsible for the development of a penetrative slaty cleavage and regional-scale folds. Granite emplacement gave way to a 2 km-thick strain aureole, which includes both the uppermost part of the stock and the surrounding host rocks. The strain aureole is characterized by a gently-dipping planar-linear fabric with a generally weak N–S oriented stretching lineation. In the granite, the fabric of the strain aureole consists of a solid-state, locally mylonitic, foliation. In the host rocks, a secondary foliation develops which crenulates the older regional foliation. Magmatic structures, mainly consisting of steeply dipping N–S oriented magmatic foliation, are preserved in the central part of the pluton. The 3D shape of the granite has been modelled from gravity data and, together with the observed magmatic flow along subvertical planes and the strain recorded in the aureole, leads us to envisage a diapiric-dominated mechanism for the emplacement of the Oulmès granite. Drawing on the petrogenesis of peraluminous granites, the diapiric ascent of the granite probably has not exceeded 10 km.

The Nevoria Gold Skarn Deposit, Southern Cross Greenstone Belt, Western Australia: II. Pressure-Temperature-Time Path and Relationship to Postorogenic Granites

The Nevoria deposit (production 11.88 t Au, 0.41 t Ag) is one of more than 20 gold skarns mined in the 3.1 to 3.0 Ga Southern Cross greenstone belt. The belt constitutes part of the continental foreland of the 2.7 to 2.6 Ga Norseman-Wiluna fold belt, the youngest Archean orogen of the Yilgarn craton. The Nevoria skarns are confined to iron formations at the limb of a regional F 1 anticline bent around the Ghooli orthogneiss dome. The refolding (F 2 ) took place at 2775 to 2724 Ma during batholith emplacement and contact metamorphism at estimated P-T conditions of 610° ± 50°C and 400 ± 100 MPa (14-km burial depth). This structural setting differs from that of the mezozonal lode gold deposits in the adjacent fold belt, which are controlled by fault-vein arrays in crustal-scale shear zones. The Nevoria orebodies (6–7 g/t Au), composed of pyrrhotite-rich hedenbergite-actinolite and almandine-hornblende skarns, are dated by a concordant U-Pb age (2635.7 ± 1.2 Ma) and by a less precise Pb-Pb errorchron age (2630 ± 13 Ma, MSWD = 5.9, n = 7), both defined by allanite-bearing almandine and by cogenetic scheelite. These ages demonstrate that the skarns formed 90 m.y. after amphibolite-facies metamorphism in the Southern Cross greenstone belt and at least 20 m.y. after transpressional faulting related to late-orogenic deformation in the Norseman-Wiluna fold belt. The peak fluid temperature during skarn formation is estimated at 550° to 600°C, based on the Fe-Mg exchange thermometry of almandine-biotite and almandine-hornblende pairs. Calcic-potassic mineral assemblages in skarn hosted by amphibolite constrain the pressure to 300 to 400 MPa, based on the reaction of biotite + cummingtonite + anorthite to almandine + grossular + biotite. The pressure estimate confirms that the Nevoria deposit formed at considerable depth (11–14 km) in a midcrustal environment. Substantial uplift did not take place until 2565 ± 25 Ma, when biotite in the associated granite closed to Rb-Sr diffusion at 300°C ambient temperature. The Nevoria orebodies are cut by pegmatite dikes and are underlain by a 500-m-thick pluton of peraluminous two-mica granite. The concordant zircon U-Pb age (2634 ± 4 Ma) of the granite is indistinguishable from the age of the skarn. The abundance of pegmatite in the main intrusion, increasing upward to a more than 80-m-thick roof zone, indicates that an aqueous fluid separated from the melt. The enrichment of aluminum, Au, As, Bi, Cu, Nb, W, and Zn in the upper pegmatite suggests that these elements were transported to the roof by an early magmatic fluid (700°–600°C), prior to the development of disseminated muscovite-epidote-carbonate alteration at a lower temperature (400°C). The major and trace elements elevated in the pegmatite are also enriched in the ore skarn, implicating the granite as the fluid source. Fluid inclusion data indicate that the magmatic fluid was CO 2 bearing, aqueous, and of moderate salinity (about 10 wt % CaCl 2 + NaCl equiv). Gold skarn formed in the iron formations above when the infiltrating fluid became progressively reduced during the replacement of grunerite. Other gold skarn deposits in the continental foreland of the Norseman-Wiluna fold belt show a similar space-time relationship to differentiated I-type granites (Thornton-Tuttle index >90). These granites (2.66–2.62 Ga) postdate the compressional deformation in the fold belt and are thus classified as postorogenic. The scheelite-bearing Archean gold skarns share characteristic features such as gangue-sulfide mineralogy and granite association with the deep-seated (150–250 MPa), reduced tungsten skarns located on the continental side of the North American Cordillera. Cretaceous deposits such as MacTung contain up to 2 g/t gold, locally.

Late Eburnean granitization and tectonics along the western and northwestern margin of the Archean Kénéma–Man domain (Guinea, West African Craton)

Recent mapping of the eastern part of Guinea has revealed a vast plutonic belt that rims the margin of the Archean Kénéma–Man craton and that is made up of a variety of granitic rocks (granodiorite, biotite granite, monzogranite, two-mica granite), granodiorite, with common clinopyroxene, being the most abundant. Single-zircon dating by stepwise Pb-evaporation carried out on characteristic rock types (granodiorite, monzogranite, two-mica granite) reveals their age to be late Eburnean (c. 2090–2070 Ma), and indicates that their emplacement occurred within a relatively short time interval (⩽20 Ma). The granodiorite, biotite granite and monzogranite of the plutonic belt all present a calc-alkaline and highly potassic nature and show numerous analogies with the Neogene calc-alkaline rocks of the central Andes; there is evidence to suggest strong contamination from the Archean crust. These analogies make it possible to envisage the emplacement of the plutonic belt directly above a subduction zone. The occurrence of peraluminous two-mica granite at the northern extension of the plutonic belt in the Siguiri Basin may suggest that the convergence here was associated with the local melting of metasedimentary rocks in the deep part of the basin. Contemporaneously with its emplacement, the late Eburnean plutonic belt accommodated regional convergence by major WNW–ESE sinistral movements along the northwestern margin of the Archean block. Horizontal shortening towards the WSW is envisaged at the scale of eastern Guinea in relation with these kinematics. The late Eburnean plutonic belt of eastern Guinea is a key element of the Archean/Proterozoic transition zone in the southern part of West Africa. Evidence along the western and northwestern border of the Archean Kénéma–Man domain leads us to interpret this transition zone as an active margin, along which strike–slip tectonism accommodated regional convergence towards the end of the Eburnean cycle.

Polyphase deformation and metamorphism at the Kalahari Craton — Mozambique Belt boundary

AbstractThe rocks of the Kalahari Craton in central western Mozambique have crystallization ages of between c. 2300 and 3400 Ma and comprise dominantly granite-greenstones, peraluminous two-mica granites, subordinate younger mafic and granitic intrusions of uncertain age and cover sedimentary rocks. The rocks of the Mozambique Belt comprise c. 1100 Ma intrusive granitoids as well as mafic intrusives and supracrustal migmatite gneisses of uncertain age. The boundary zone between and including these two crustal provinces is characterized by a strong N-S penetrative planar and migmatitic fabric. Sparse kinematic indicators suggest a sinistral sense of displacement along this shear zone. The metamorphic gradient increases from west to east from low grade on the Kalahari Craton to high-grade in the east, characterized by two generations of anatectic migmatization. 40Ar/39Ar thermochronology on mica suggests that the Kalahari Craton lithologies have experienced heating above at least c. 300°C during the c. 1100 Ma Grenville age orogeny and again at c. 530 Ma during the Pan-African Orogeny, possibly related to the collisional amalgamation of East and West Gondwana. The Mozambique Belt lithologies record a c. 550 Ma thermal overprint with the lithologies in the vicinity of the N-S shear zone recording thermal reactivation at c. 470 Ma. Comparisons of the new data with that from western Dronning Maud Land, which was adjacent to the study area prior to Gondwana fragmentation, yield many similarities.

A three stage fluid flow model for Variscan gold metallogenesis in northern Portugal

Mineralogical, fluid inclusion and geochemical studies were made on two intra-granitic gold deposits (Grovelas and Penedono), together with a deposit linked to sub-vertical structures in silicified metasediments at Três-Minas, and several intra-metamorphic occurrences at Vila Pouca de Aguiar. They all possess similar mineral assemblages, deformational state, fluid flow characteristics, ore fluid composition and have comparable P–T conditions. Three successive crystallisation stages are recorded during the formation of gold-bearing structures independent of their location or host rocks (granites or metasediments). They are:Stage 1 — the development of milky quartz veins that formed primarily after the emplacement of peraluminous two-mica granites (315–310Ma) at P–T conditions reflecting high temperature and low pressure. They are similar to those from pluton induced metamorphism (P=300–350MPa and T=500–550°C). No clear evidence was found for gold deposition during this stage.Stage 2 — during orogenic uplift and repeated tectonic reactivation a clear quartz was deposited in the early milky quartz veins (Stage 1) at P–T conditions between 100 and 300MPa and 300 and 450°C. Local sulphide deposition (arsenopyrite II and pyrite II) occurred in clear quartz, but was never massive. The fluids percolating within the granite were mainly aqueous-carbonic and reflect equilibrium with the metamorphic host rocks. They are very similar to those found in metamorphic environments. No evidence for the involvement of magmatic fluids was found.Stage 3 — intense microfissuring of the earlier vein infillings occurred, associated with the main episode of gold deposition. The P–T conditions were <100MPa and <300°C based on aqueous fluid inclusions. Native gold and electrum crystallised together with sulphides (galena, chalcopyrite and bismuthinite), native Bi and sulphosalts (Pb–Bi–Ag dominated). The fractures frequently contain chlorite (± sericite) especially where they crosscut earlier sulphides (arsenopyrite).These processes and fluid types are similar in both the granites and metamorphic host rocks. Therefore, the gold ores appear to be the result of successive periods of fluid circulation, in this case related to the uplift of the Variscan basement in response to high heat flow and the intrusion of granites. Without exception, these fluids have been re-equilibrated with the metamorphic rocks. However magmatic fluids are absent; the granites thus act passively as heat engines for fluid circulation.

Generation and evolution of subduction-related batholiths from the central Urals: constraints on the P-T history of the Uralian orogen

Batholiths from the accreted terranes in the Urals were generated by repeated episodes of melting and intrusion. Verkhisetsk, the largest and one of the most complex subduction-related batholiths from the Urals, comprises an outer envelope of older tonalites, trondhjemites and granodiorites dated at 315–320 Ma and equilibrated at 6 kbar, intruded by an inner core of younger granodiorites, adamellites and granites dated at 275–290 Ma and equilibrated at 4 kbar. Older rocks have a high-A1 TTD/adakite chemistry, ϵ 320Ma chur(ND) ≈ 2–5, and initial 87Sr 86 Sr ≈ 0.7043 , with pronounced lateral zoning marked by the increase of LREE/HREE, Cr, Ni, and Mg eastwards. They were originated by melting of metabasalts in a subducted slab of young hot oceanic lithosphere with a temperature/depth trajectory that intersected the garnet-in univariant at ≈ 1050°C and 13 kbar, thus causing the lateral zoning. We believe that such a ‘warm’ trajectory can be attributed to the oblique subduction of a young lithosphere. Younger rocks range from Na-rich metaluminous granodiorites to K-rich peraluminous two-mica granites with relict epidote crystals, and have almost the same isotopic signature as older rocks, from which they were generated by anatexis as a consequence of a melting event that occurred throughout the Urals at 275–290 Ma and produced most of the huge continental-type batholiths on the eastern side. This melting event is tentatively supposed to be the result of underplating by mafic magmas, which also caused the growth of the Urals crust from the Moho downwards until it reached the increased thickness revealed by seismological studies.

Implications of UPb ages of columbite-tantalites from granitic pegmatites for the Palaeoproterozoic accretion of 1.90–1.85 Ga magmatic arcs to the Baltic Shield

The Palaeoproterozoic growth of the Baltic Shield involved the accretion of several ∼1.90–1.85 Ga old magmatic arcs to the southwest of the Archaean craton and the deformation and migmatization of sedimentary basins located between the arcs. During crustal thickening after the collision of the arcs, the sedimentary basin fill became intruded by peraluminous two-mica granites. Locally, columbite-bearing pegmatites are genetically associated with these granites. Columbites from lithium-cesium-tantalum-type (LCT-type) pegmatites from the Stockholm area (Sörmland gneisses) yield UPb ages at 1815–1820 Ma, while niobium-yttrium-fluorine-type (NYF-type) pegmatites from the same area are younger (1795±2 Ma, 2σ). Farther to the north, LCT-type pegmatites from the central Bothnian Basin, that correspond geochemically and mineralogically to those of the Stockholm area, yield UPb columbite ages at 1795–1800 Ma, while LCT-type pegmatites in the sedimentary basin between Skellefteå and Luleå yield less well constrained UPb columbite ages at 1765–1775 Ma. LCT-type pegmatites are mainly associated with crustal melts that form during postcollisional thickening of continental crust. They represent markers for the time when the Palaeoproterozoic Baltic Shield suffered sufficient thickening to yield voluminous anatectic melts. The UPb columbite ages from the LCT-type pegmatites indicate that comparable phases of post-collisional crustal thickening of the Svecofennian area of the Baltic Shield occurred later to the north.

Magmatically zoned muscovite from the peraluminous two-mica granites of the Leinster batholith, southeast Ireland

Research Article| May 01, 1991 Magmatically zoned muscovite from the peraluminous two-mica granites of the Leinster batholith, southeast Ireland Patrick Roycroft Patrick Roycroft 1University College, Belfield, Dublin 4, Ireland Search for other works by this author on: GSW Google Scholar Author and Article Information Patrick Roycroft 1University College, Belfield, Dublin 4, Ireland Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (5): 437–440. https://doi.org/10.1130/0091-7613(1991)019<0437:MZMFTP>2.3.CO;2 Article history First Online: 02 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 Patrick Roycroft; Magmatically zoned muscovite from the peraluminous two-mica granites of the Leinster batholith, southeast Ireland. Geology 1991;; 19 (5): 437–440. doi: https://doi.org/10.1130/0091-7613(1991)019<0437:MZMFTP>2.3.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 SocietyGeology Search Advanced Search Abstract Euhedral muscovites from the granites of the Leinster batholith, Ireland, have zone textures that clearly demonstrate their magmatic origin. These textures include corrosion, synneusis, development of transient faces, zone thickening, face widening, and rational and irrational growth in response to inclusions. Inclusions of biotite, zircon, and sillimanite are zonally controlled, whereas those of quartz, apatite, and monazite are not. Muscovite nucleation over a wide span of magmatic evolution from a strongly peraluminous melt under conditions of enhanced muscovite stability is implied. These micas provide and validate criteria that distinguish primary from hydrothermal muscovites. The observations here significantly alter previous models for the evolution of the Leinster granites and provide a potentially powerful means of discriminating primary from hydrothermal muscovites in other two-mica granites. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Plutonic rocks of the 1985 Tibet Geotraverse, Lhasa to Golmud

Two large east-trending granitic batholiths are exposed on the plateau of Central Tibet. In the southern Lhasa Terrane , north of the Zangbo Suture, the Gangdise Belt is a calc-alkaline composite batholith dominated by monzodiorites, tonalites, granodiorites and monzo granites. Trace elements indicate that strongly fractionated melts were emplaced at an active continental margin; deeper crustal levels of the batholith are exposed in the crustally -derived Nyainqentanglha orthogneiss. A long the northern edge o f the plateau , a syn-tectonic calcic to calc-alkaline suite of tonalites, granodiorites and monzo granites forms the Kunlun batholith with post-tectonic granites emplaced to the south. The Kunlun intrusions are derived from anatexis of a garnet-bearing source at in termediate crustal depths above an active or recently active continental margin . Between these two batholiths, a bimodal suite of metaluminous tonalite -grano-diorite and peraluminous two-mica granite is exposed in the northern Lhasa Terrane, indicative of melting both in the upper crust and at deeper levels in the crust or upper mantle. This association suggests a post-collision setting.