Metaluminous Tonalite Research Articles

Petrogenesis of Mantle-Hosted Granitoids from the Samail Ophiolite

Abstract Granitoids with evolved chemical signatures, intruding both oceanic crustal rocks and mantle peridotite, are well known in the Samail supra-subduction ophiolite of Oman and United Arab Emirates. The former are metaluminous tonalites and trondhjemites have positive εNd, relatively low δ18O values, resemble ridge ‘tholeiitic’ granitoids with MORB-like affinity, and formed by fractionation of basalt/gabbro or anatexis of oceanic crust. In contrast, the mantle-hosted granitoids are peraluminous, with negative εNd and high δ18O values. They can be modeled as having formed by mixing between partial melts of metabasalt and metasedimentary rocks with minimal mantle contribution. The mantle-hosted granitoids resemble typical peraluminous S-type granitoids (e.g. Himalaya, Variscan, Lachlan) but are compositionally distinct in having notably scattered data values of K2O, Na2O, CaO, Rb/Sr, relatively higher silica and, lower (Al2O3 + FeO + MgO + TiO2 wt%) and Th/Yb as compared to typical S-types. The mantle-hosted granitoids mainly originate from the subducted slab, and mostly intruded mantle peridotite in the lithospheric mantle, with no apparent association with the overriding oceanic plate and little interaction (e.g. assimilation) with the mantle. While the specific controls on geochemical variations remain speculative, results from this study show that the Samail mantle-hosted S-type granitoids are compositionally distinct from classic S-type granitoids formed in orogenic belts. While global S-type granitoids may be mineralogically similar, the Samail mantle-hosted granitoids preserve different geochemical signatures, reflecting their formation in distinct tectonic setting, including the mantle.

Oxygen isotope ratios in zircon and garnet: A record of assimilation and fractional crystallization in the Dinkey Dome peraluminous granite, Sierra Nevada, California

Abstract The 119 Ma Dinkey Dome pluton in the central Sierra Nevada Batholith is a peraluminous granite and contains magmatic garnet and zircon that are complexly zoned with respect to oxygen isotope ratios. Intracrystalline SIMS analysis tests the relative importance of magmatic differentiation processes vs. partial melting of metasedimentary rocks. Whereas δ18O values of bulk zircon concentrates are uniform across the entire pluton (7.7‰ VSMOW), zircon crystals are zoned in δ18O by up to 1.8‰, and when compared to late garnet, show evidence of changing magma chemistry during multiple interactions of the magma with wall rock during crustal transit. The evolution from an early high-δ18O magma [δ18O(WR) = 9.8‰] toward lower values is shown by high-δ18O zircon cores (7.8‰) and lower δ 18O rims (6.8‰). Garnets from the northwest side of the pluton show a final increase in δ18O with rims reaching 8.1‰. In situ REE measurements show zircon is magmatic and grew before garnets. Additionally, δ18O in garnets from the western side of the pluton are consistently higher (avg = 7.3‰) relative to the west (avg = 5.9‰). These δ18O variations in zircon and garnet record different stages of assimilation and fractional crystallization whereby an initially high-δ18O magma partially melted low-δ18O wallrock and was subsequently contaminated near the current level of emplacement by higher δ18O melts. Collectively, the comparison of δ18O zoning in garnet and zircon shows how a peraluminous pluton can be constructed from multiple batches of variably contaminated melts, especially in early stages of arc magmatism where magmas encounter significant heterogeneity of wall-rock assemblages. Collectively, peraluminous magmas in the Sierran arc are limited to small <100 km2 plutons that are intimately associated with metasedimentary wall rocks and often surrounded by later and larger metaluminous tonalite and granodiorite plutons. The general associations suggest that early-stage arc magmas sample crustal heterogeneities in small melt batches, but that with progressive invigoration of the arc, such compositions are more effectively blended with mantle melts in source regions. Thus, peraluminous magmas provide important details of the nascent Sierran arc and pre-batholithic crustal structure.

Tracking rock sources of 3800-year-old burial goods using X-ray diffraction, electron microprobe analyses and X-ray mapping: a case study from Córdoba, Argentina

An axe head and a round bead were found at the burial site of an adult male (dated at 3770 ± 90 14C years BP) at Amboy, Cordoba (central Argentina). These artifacts were studied using electron microprobe, scanning electron microscopy and X-ray diffraction, for a complete characterization of the materials and to infer the source. The axe head is carved from very fine-grained tonalite (composed dominantly of zoned plagioclase, quartz, potassic feldspar, chlorite, magnesiohornblende, titanite and magnetite) with microporphyritic texture. The bead is mainly composed of muscovite, an amorphous Cu–Al hydrated silicate (similar to chrysocolla) and malachite. Based on the coincidence between textures, mineralogy and chemical compositions, the most likely source of the axe is dykes of metaluminous tonalite scattered in the Sierras Chicas Range. The provenance of the bead could not be ascertained, but it is probable that it was carved from material collected from a very localized outcrop. These artifacts associated with a funerary context represent one of the oldest evidences of such materials for hunter–gatherer groups in central Argentina.

Neoformation of magnetite during selective metasomatism controlling large-scale positive magnetic anomalies within the Brunovistulian unit (Bohemian Massif)

An airborne magnetometer survey together with field measurements of magnetic susceptibility have distinguished two main granitic suites within the eastern zone of the Brno batholith (the exposed part of the Brunovistulicum). Highly magnetic biotite-amphibole tonalites, forming widespread positive magnetic anomalies (up to 300 nT), have susceptibility (χ) regularly above 5 × 10−3. Less magnetic biotite granites display lower χ values on the order of 1 × 10−4. The metaluminous tonalites significantly differ in whole-rock chemistry (enriched in FeO, MgO, and depleted in SiO2) and degree of fractionation (decreased Rb/Sr and K/Ba ratios) reflecting the distinct microchemistry of Fe-rich mafic silicates (chamosite ~24 wt% FeO). Conversely, more felsic metaluminous to peraluminous biotite granites exhibiting a higher degree of fractionation characteristically contain mafic silicates depleted in Fe2+ (Mg-clinochlore ~20 wt% FeO). The geochemistry of granitoids and chemical composition of mafic phases suggest that increased contents of pure magnetite (TiO2 <0.15 wt%) along with diamond-shaped titanite originated as a result of the reaction between primary magmatic Fe-Ti oxides, annite and anorthite during late oxidation of granitic rocks. The subsolidus oxidation of granitoid has led to releasing of Fe2+ from mafic silicates representing a substantial source of Fe2+ for the formation of pure magnetite in an evolved and crystallized magmatic system under relatively oxidative conditions. The Curie temperature (585–601 °C) and the Verwey transition (-160 °C) demonstrate the absence of titanomagnetite in both types and the overall magnetic properties of granitoids dominated by pure magnetite slightly affected by oxidation. Nevertheless, the more extensive magnetite hematitization in biotite granites indicates a higher degree of oxidation caused by dissociation of a greater amount of water phase in the post-magmatic stage of evolution of a more fractionated felsic melt. The metasomatic transformation of paramagnetic (Fe-rich biotite and amphibole) to ferromagnetic (pure magnetite) phases, well described on granitoid varieties of the Brno batholith, seems to be responsible for the extraordinarily geomagnetic behavior of the entire eastern part of the Brunovistulicum (Slavkov terrane) mostly covered by Paleozoic sediments.

Post-collisional magmatism: Consequences of UHPM terrane exhumation and orogen collapse, N. Qaidam UHPM belt, NW China

Exhumation of subducted slabs and extensional collapse of orogens are the main tectonic processes in ancient and modern continental collisional zones. Magmatism during these two processes may play important roles in understanding reworking and growth of the continental crust We report here that a series of plutonic magmas, including intrusions of two-mica granite, tonalite, granodiorite, biotite monzogranite, porphyritic biotite granite and diorite, as well as contemporaneous mafic dykes, have been recognized in Dulan edogite-bearing terrane, the North Qaidam ultra-high pressure metamorphic (UHPM) belt The magmatism represented by these plutons is temporally similar to 20-30 million years (Mys) younger than the UHPM age, lasting for similar to 40 Mys and derived from different sources with different mechanisms. The magmatism was initiated by exhumation of UHPM terranes during which strongly-peraluminous two-mica granite and metaluminous tonalite were produced respectively by decompression melting of the exhumed UHPM upper and lower continental crust, respectively. The genesis of mafic magmatic enclave (MME)-hosting granodiorite with a clear hybrid signature and coeval biotite monzogranite reflected the upwelling of asthenospheric mantle by extension of lithosphere during the orogen collapse. It was induced by detachment of the subducted lithospheric mantle, which then brought heat and mantle material into continental crust and triggered the partial melting of the exhumed UHPM continental crust, and gave rise to mixing of crustal and mantle melts. Porphyritic biotite granite reflects a late melting event of continental crust Diorite marked by high magnesium content represents mantle melts with slight crustal contamination, which implies that the orogen has been unrooted and collapsed completely. The post-collisional magmatism of the North Qaidam belt provides an improved understanding for the late thermal and tectonic evolution of a UHPM continental collision zone. (C) 2014 Elsevier B.V. All rights reserved.

Early–Middle Paleozoic granitoids in Gorny Altai, Russia: Implications for continental crust history and magma sources

Major-element and trace-element compositions, as well as Nd isotope ratios and protolith model ages of Early to Middle Paleozoic granitoids from the Russian Gorny Altai are summarized and discussed in terms of the compositions and tectonic settings. The observed geographic variations of compositions in the granitoid assemblages correlate with three groups of late Neoproterozoic to Middle Paleozoic terranes corresponding to oceanic, island arc, and turbidite basin settings. Plutonic rocks in the oceanic and island arc terranes are commonly low-alkali metaluminous tonalite and trondhjemite that formed by melting of oceanic crust. The island arc terranes also contain tonalite–trondjemite–granodioritic and tonalite–granodiorite–granitic rocks, with higher alkali and potassium contents, which result from anatectic melting of volcanics and sediments. Granitoids in the turbidite basins are potassic peraluminous monzogranite produced by anatectic melting of upper crustal turbidites, as well as subalkaline tonalite, granodiorite, and granite derived from upper crustal metasediments and metamorphosed mafic oceanic crust underlying the turbidite basins. Alkaline granitoids emplaced during two major tectonomogamatic events (Early and Late Devonian) and are present in terranes of all three settings. Their source is of crustal origin, like in other groups of granitoids, but it was a mixture of crustal and mantle material. Thus the compositional trends of Early and Middle Paleozoic granitoids in the Gorny Altai record the history of continental crust.

A pseudo adakite derived from partial melting of tonalitic to granodioritic crust, Kyushu, southwest Japan arc

Cretaceous peraluminous granitic intrusions occur in Kyushu in the southwest Japan arc as batholiths or stocks accompanied by metaluminous tonalite to granodiorite. The peraluminous intrusions are classed as adakitic based on Sr/Y–Y (ppm) relationships, but they contain abundant K-feldspar, a phase not common in typical adakites. To consider their significance in the arc system, we investigated the Tsutsugatake stock as representative of the intrusions exposed in central Kyushu. The Tsutsugatake rocks are mainly garnet-bearing two-mica leuco-granitoids. Their Sr/Y ratios and rare earth element concentrations are appropriate for adakite, but K 2O/Na 2O ratios (0.53–1.43), and Al 2O 3 (13.7–15.0 wt.%), MgO (0.20–0.55 wt.%), Sr (207–462 ppm), Cr (≤ 12 ppm), and Ni (≤ 8 ppm) contents differ from those of typical adakite. Geochemical modeling shows that the petrogenesis of the Tsutsugatake stock can be explained by partial melting of arc-type tonalite or adakitic granodiorite under middle to lower crustal conditions, leaving amphibole and plagioclase as residual phases. The differences between the Tsutsugatake stock and classical adakites lie in their source rocks and melting conditions. The Tsutsugatake source was not basaltic, and the melting pressure was insufficient to stabilize garnet. The Tsutsugatake rocks are here designated “pseudo adakites”. Such “pseudo adakites” may be found in both modern and ancient arcs, and would play an important role in crustal evolution as a member of the recycled constituent.

Petrogenesis of Cretaceous Peraluminous Granite Suites with Low Initial Sr Isotopic Ratios, Kyushu Island, Southwest Japan Arc

Cretaceous granitic rocks are widely distributed in the northern to central parts of Kyushu Island, southwest Japan arc, and are divided into two types, granite and tonalite-granodiorite. These rocks are clearly defined by their mineral assemblage, that is, the granite contains mostly muscovite and/or garnet and does not have hornblende, whereas the tonalite-granodiorite is always observed with hornblende. Many workers suggest that most of the granite has low initial Sr isotopic ratios (SrI 90 Ma: 0.7052-0.7059) whose values are similar to those of tonalite-granodiorite (SrI 90 Ma: 0.7050-0.7056). The SiO 2 contents of the granite (67.9 to 78.0 wt.%) are higher than that of tonalite-granodiorite (54.2 to 68.2 wt.%). The granite is characterized by peraluminous compositions, while the tonalite-granodiorite shows metaluminous to peraluminous compositions. The petrogenetic relation between granite and tonalite-granodiorite is examined by a modal batch melting model, which referred to results of already published experiments. The result of the model suggests that the petrogenesis of the peraluminous granitic magma can be explained by an anatexis of metaluminous tonalite in the tonalite-granodiorite under lower to middle crustal conditions and variable fH 2O. Rocks of tonalite to granodiorite in a subduction system generally have low initial Sr isotopic ratios (<0.705), which imply that the magma derived from a highly metaluminous origin such as mafic magma, mafic lower crust and/or subducting slab. In this study, it is regarded that the peraluminous granitic magma can be directly produced by an anatexis of metaluminous tonalite. Therefore, peraluminous granite with low initial Sr isotopic ratios may imply to a first step of recycling of the granitic layer in an active plate margin.

Petrogenesis of the Pan-African El-Bula Igneous Suite, central Eastern Desert, Egypt

The Precambrian rocks of the El-Bula area include syn-orogenic intrusive granitoids associated with gneisses, metavolcanics and metagabbro-diorite complexes. The metavolcanics range in composition from basalt to basaltic andesite. They show tholeiite affinity and provide unfractionated, flat rare earth element patterns and have geochemical features suggesting their generation in a back-arc environment. Low and variable Mg# of the basic metavolcanics suggest that they were derived from the upper mantle with subsequent modification by fractional crystallisation. The least evolved basaltic sample could be generated by 25% batch melting of a spinel Iherzolite source followed by 35% fractionation of clinopyroxene, olivine and plagioclase in proportions of 45%, 30% and 25%, respectively. The most evolved basaltic rock could be generated by 55% fractional crystallisation (65% hornblende, 30% plagioclase and 5% titanomagnetite) of the least evolved basaltic sample liquid.The El-Bula syn-orogenic granitoids show a normal igneous evolution from metaluminous tonalites to weakly peraluminous granodiorites. They exhibit geochemical features of I-type calc-alkaline granites emplaced in a volcanic arc environment. The least evolved tonalitic sample could be generated by about 30% batch melting of the most evolved basic metavolcanic sample. The ElBula tonalites and granodiorites have geochemical and petrological characteristics indicating that they might be genetically related by fractional crystallisation. The chemical variation in the tonalites is dominantly controlled by about 45% fractional crystallisation of hornblende, plagioclase and K-feldspar in proportions of 55%, 44% and 1%, respectively. The least evolved granodioritic sample could be produced from the most evolved tonalitic sample by about 20% crystallisation of plagioclase, biotite, hornblende and K-feldspar phases in proportions of 54%, 43%, 2.5% and 0.5%, respectively. The variations in the granodiorites could be modelled by about 30% fractional crystallisation of plagioclase, K-feldspar and biotite (62%, 24% and 14%, respectively).

Isotopic signatures of Paleoproterozoic granitoids from the southern São Francisco Craton and implications for the evolution of the Transamazonian Orogeny

The southern São Francisco Craton, northeastern Brazil, consists of an Archean block surrounded by a Paleoproterozoic belt related to the Transamazonian Orogeny (ca. 2.0 Ga). A calc-alkaline plutonic arc developed within the belt and the granitoid plutons comprise two distinct groups. One group displays Archean T DM ages (3.07–2.62 Ga), ε Nd( t) values between −11.0 and −3.8 and high initial 87Sr/ 86Sr values, and it consists mainly of peraluminous granites. T DM ages for the other group are Paleoproterozoic (2.43–2.27 Ga), and ε Nd( t) values range between −2.8 and +1.3; the plutons are metaluminous tonalites (trondhjemites) to granodiorites. The Transamazonian granitoids can be related to contrasting source-regions, from mantle- to crust-derived ones. A number of them are probably derived from mixing of Paleoproterozoic juvenile material and variable proportions of Archean crust material. Magmatism related to deep faulting, during the compressional stages of the Transamazonian Orogeny, is a plausible model for granitoid generation. The contribution of mantle-derived material to the granitoid sources supports the idea that a significant episode of new crust formation occurred during the Transamazonian Orogeny.

Petrogenese kontrastierender Granitplutone in Westb�hmen (Tschechien)

Late-Variscan granitoid plutons in western Bohemia (Bor, Waidhaus-Rozvadov) have distinct petrographic, geochemical and isotopic features that suggest different magmatic evolutions. The Bor pluton comprises a suite of metaluminous tonalites and quartz diorites (Bor I), weakly peraluminous (monzo-)granites and granodiorites (Bor II) and medium-aluminous, late vein-forming leucomonzogranites (Bor III). The Waidhaus-Rozvadov pluton is strongly peraluminous, comprising a cordierite-biotite granitoid (CBG), the Rozvadov granite (ROG), the Barnau granite (BAG) and the subordinate, highly evolved Kreuzstein (Křižový kamen) granite (KG). Geochemical parameters and initial87Sr/86Sr ratios straddle the boundary between I- and S-type granites in the Bor pluton and are characteristic of purely S-type granites in the Waidhaus-Rozvadov pluton. The Bor II granitoids have been dated by the Rb-Sr whole-rock method at 341±17 Ma (ISr = 0.70724±0.00060). K-Ar biotite and muscovite ages of all units of the Bor pluton are mainly in the range 321-315 Ma. The K-Ar mineral ages are in good agreement with recently published U-Pb zircon data of these rocks. The different units of the Waidhaus-Rozvadov pluton have yielded less well-constrained Rb-Sr whole-rock ages, ranging from 313 to 300 Ma. However, the intrusion sequence is constrained by K-Ar muscovite ages (312-302 Ma), which define a systematic decrease towards the chemically more evolved granite types. Taken as a whole, it seems likely that the new radiometric ages characterize two temporally distinct periods of late-Variscan granitoid intrusion. The regional significance of these periods is emphasized by contemporaneous ages previously found in the adjacent northeastern Bavarian granitoids. The initial Sr and Nd isotope systematics indicate that the Bor and the WaidhausRozvadov plutons were derived from different source rocks. The Bor granitoids reflect the influence of less evolved crustal material which may have been similar to paragneisses of the Tepla-Barrandian region, including the Zone of ErbendorfVohenstraus (ZEV). The Waidhaus-Rozvadov granitoids probably resulted from anatexis of rocks resembling surrounding Moldanubian paragneisses or metapelites. In addition, the two plutons exhibit poorly defined, opposite trends of eNd(T) variation which are ascribed to assimilation processes.

Geochemistry and argon thermochronology of the Variscan Sila Batholith, southern Italy: source rocks and magma evolution

The Sila batholith is the largest granitic massif in the Calabria-Peloritan Arc of southern Italy, consisting of syn to post-tectonic, calc-alkaline and metaluminous tonalite to granodiorite, and post-tectonic, peraluminous and strongly peraluminous, two-mica±cordierite±Al silicate granodiorite to leucomonzogranite. Mineral 40Ar/39Ar thermochronologic analyses document Variscan emplacement and cooling of the intrusives (293–289 Ma). SiO2 content in the granitic rocks ranges from ∼57 to 77 wt%; cumulate gabbro enclaves have SiO2 as low as 42%. Variations in absolute abundances and ratios involving Hf, Ta, Th, Rb, and the REE, among others, identify genetically linked groups of granitic rocks in the batholith: (1) syn-tectonic biotite±amphibole-bearing tonalites to granodiorites, (2) post-tectonic two-mica±Al-silicate-bearing granodiorites to leucomonzogranites, and (3) post-tectonic biotite±hornblende tonalites to granodiorites. Chondrite-normalized REE patterns display variable values of Ce/Yb (up to ∼300) and generally small negative Eu anomalies. Degree of REE fractionation depends on whether the intrusives are syn- or post-tectonic, and on their mineralogy. High and variable values of Rb/Y (0.40–4.5), Th/Sm (0.1–3.6), Th/Ta (0–70), Ba/Nb (1–150), and Ba/Ta (∼50–2100), as well as low values of Nb/U (∼2–28) and La/Th (∼1–10) are consistent with a predominant and heterogeneous crustal contribution to the batholith. Whole rock δ18O ranges from ∼+8.2 to +11.7‰; the mafic cumulate enclaves have the lowest δ18O values and the two-mica granites have the highest values. δ18O values for biotite±honblende tonalitic and granodioritic rocks (9.1 to 10.8‰) overlap the values of the mafic enclaves and two-mica granodiorites and leucogranites (10.7 to 11.7‰). The initial Pb isotopic range of the granitic rocks (206Pb/204Pb ∼18.17–18.45, 207Pb/204Pb ∼15.58–15.77, 208Pb/204Pb ∼38.20–38.76) also indicates the predominance of a crustal source. Although the granitic groups cannot be uniquely distinguished on the basis of their Pb isotope compositions most of the post-tectonic tonalites to granodiorites as well as two-mica granites are somewhat less radiogenic than the syn-tetonic tonalites and granodiorites. Only a few of the mafic enclaves overlap the Pb isotope field of the granitic rocks and are consistent with a cogenetic origin. The Sila batholith was generated by mixing of material derived from at least two sources, mantle-derived and crustal, during the closing stages of plate collision and post-collision. The batholith ultimately owes its origin to the evolution of earlier, more mafic parental magmas, and to complex intractions of the fractionating mafic magmas with the crust. Hybrid rocks produced by mixing evolved primarily by crystal fractionation although a simple fractionation model cannot link all the granitic rocks, or explain the entire spectrum of compositions within each group of granites. Petrographic and geochemical features characterizing the Sila batholith have direct counterparts in all other granitic massifs in the Calabrian-Peloritan Arc. This implies that magmatic events in the Calabrian-Peloritan Arc produced a similar spectrum of granitic compositions and resulted in a distinctive type of granite magmatism consisting of coeval, mixed, strongly peraluminous and metaluminous granitic magmas.

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.