Hybrid Granites Research Articles

Geochronology and petrogenesis of granitoids from the Bas Draa inlier (Western Anti-Atlas, Morocco): Revived debate on the tectonic regime operating during early Paleoproterozoic at the NW edge of the West African Craton

Proterozoic terrains of the Anti-Atlas belt (Morocco) represent one of the key areas for studying the Precambrian geodynamics of the northwestern West African craton (WAC). Voluminous Paleoproterozoic granitoids, which outcrop in the basement of the Precambrian inliers of the southern Anti-Atlas, are conventionally regarded as vestiges of a magmatic arc constructed during incipient subduction on Earth. However, except for their calc-alkaline, arc-like signatures, little evidence supports the existence of this arc system and the sources and tectonic context of these granitoids remain elusive. In this study, we present new whole-rock major- and trace-element, and Sr-Nd isotope composition, and in situ zircon U-Pb ages of the composite granitoid intrusions from the Bas Draa inlier. We obtained Rhyacian ages for the quartz-diorite (~2059 ± 6 Ma) and leucogranite (~2051 ± 6 Ma), and Orosirian age for the biotite-bearing granite (~2022 ± 7 Ma). The studied magmatic rocks are classified into three groups according to their petrographic features and chemical composition: a) layered, sanukitoid quartz-diorites, b) biotite-muscovite (i.e., two-mica) leucogranites, and c) biotite-bearing, hybrid granites. All three groups of magmatic rocks share a common geochemical composition with late Archean granites worldwide, including a high Mg# (52.9–64), and Ni (average 40 ppm), Cr (10–250 ppm) and V (139-238 ppm) content. The identification of mantle-derived sanukitoids and hybrid granites, combined with the absence of first-order evidence for subduction, obduction, and collision, as commonly observed in Neoproterozoic and Phanerozoic orogens worldwide (i.e., blueschist facies metamorphics, ophiolites, and accretionary prism units), contests the theory for the onset of plate tectonics during Early Paleoproterozoic in the Anti-Atlas. We instead propose that the available data rather suggests that vertical tectonics, characteristic of the Archean, still prevailed in this domain throughout the Early Proterozoic times.

Hybrid granite magmatism during orogenic collapse in the Eastern Desert of Egypt: Inferences from whole-rock geochemistry and zircon U–Pb–Hf isotopes

Granites can provide detailed insights into crust formation and reworking during the different stages of orogenic cycles. New field and petrographic observations are integrated with whole-rock geochemical and zircon U-Pb and Hf isotopic data of granitic and subvolcanic felsite intrusions associated with the Atalla Shear Zone (ASZ). Formation of the ASZ is attributed to regional oblique convergence, late in the tectono-magmatic evolution of the Central Eastern Desert of Egypt. The new data reveal three stages of post-collisional felsic magmatism, with a change in magma type from I- to A-type granites. The evolution commenced with formation of quartz-monzodiorite and granodiorite at 639 ± 2 Ma (stage 1), followed by a suite of granodiorite to syenogranite intrusions between 619 ± 2 and 612 ± 2 Ma (stage 2), and ceased with the Atalla felsites at 607 ± 2 Ma (stage 3). During stages 1 and 2, the ASZ was intruded by hybrid granites as a result of mixing of lower crust material with mantle-derived mafic sanukitoid-like melts, in addition to nearly pure crustal melts during stage 2. The Atalla felsites, with distinct A-type affinities, are interpreted to have resulted from fractional crystallization of hybrid magmas in shallow crustal levels prior to emplacement. The identical initial Hf isotope compositions of all ASZ-associated magmatic rocks imply that the crust and mantle reservoirs had similar Hf isotopic composition at ~640–610 Ma (εHf(t) = 8 ± 1). This implies that mantle enrichment was controlled largely by subducted Neoproterozoic oceanic crust, volcanic arc, and syn-collisional rocks. The new U-Pb ages combined with the geochemical characteristics of the ASZ granitic rocks are consistent with a prolonged period of decompression melting of hitherto enriched mantle and lower crust. Melting was likely triggered by rejuvenation of thoroughgoing transcurrent shearing and shear-driven asthenospheric mantle upwelling during the orogenic collapse stage.

Petrogenesis of the Kanker Granites From the Bastar Craton: Implications for Crustal Growth and Evolution During the Archean-Proterozoic Transition

Archean-Proterozoic boundary represents a significant transitional phase in the Earth’s history. Bastar Craton is one of the major Archean cratons in the Indian subcontinent with voluminous granites, supracrustal rocks and tectonic belts. Malanjkhand, Dongargarh, and Kanker are the three major granitic plutons emplaced during the Archean-Proterozoic transition in the Bastar Craton, and this study is confined to the granites of Kanker pluton. Based on geochemical systematics, the Kanker granites are classified into sanukitoids, biotite and two-mica granites and hybrid granites. The compositional diversity of the Kanker granites is attributed to two end-member sources, i.e., the enriched mantle and an older felsic crust, and the interactions between them. The sanukitoids were derived from an enriched mantle source that was metasomatized by the subducted sediments. Heat supplied by the sanukitoid magmas induced the crustal melting to form the biotite and two-mica granites. The interaction between these two mutually end-member sources, i.e., the enriched mantle and an older felsic crust, resulted in the formation of hybrid granites. The evolution of the Kanker granites can be accounted for a transitional geodynamic environment, involving subduction and collisional tectonics during the Archean-Proterozoic transition.

Geology and geochemical constraints on the origin of the Mesoarchean granitoids from Carajás province, Amazonian craton

The Carajás province (CP) is the largest preserved Archean core of the Amazonian craton. In this paper we present a detailed work about the Tucumã area located in the northwestern portion of the Rio Maria domain of the CP, where more detailed studies had not yet been carried out. The new geological and geochemical data obtained allowed the identification of Mesoarchean granitoids of different natures that resemble typical late-Archean granitoids (TTG, leucogranites, sanukitoids and hybrid granites) identified in several cratons around the world. The most extensive unit in the Tucumã area is the Leucomonzogranitic batholith which is intrusive in a tonalite crust of TTG affinity, that exhibits an older N-S structural pattern commonly found in oldest rocks (greenstone belt sequence). The batholith is coeval with small granitoid intrusions that occur as lenses controlled by NE-SW and E-W direction anastomosed shear zones. Apart from the tonalite, these granitoids show affinity with the calc-alkaline series and are individualized in four units. The porphyritic granodiorite is a magnesium-rich unit that has moderate K2O/Na2O ratio and higher levels of Ba, K and Th, indicating similarities with the Transitional TTG suites (Yilgarn craton). Equigranular granodiorites are equivalent of sanukitoid suites; they show enrichment in Sr and compatible elements (Mg, Cr, and Ni) and depletion in HREE. Hybrid granitoids were identified as High-HFSE granites (medium- and high-Ba) that share some geochemical affinities with Closepet granite of the Dharwar craton. The Leucomonzogranite is enriched in LILE and exhibit negative Eu anomalies, which indicate crustal reworking processes. This granitoids are syntectonic bodies that underwent a progressive deformation processes that started before the magma crystallization was complete, under high temperatures (>500 °C) conditions compatible with those of the amphibolite facies. Two phases of magmatism are recorded in the RMD, which would be responsible for the formation of these granitoids. The first phase (2.98-2.92 Ga) is related to formation of a thick TTG crust from parcial melting of an oceanic plateau or a thickened mafic crust. The second magmatism phase (~2.87 Ga) initiated due to thermal events in a collisional setting, where both crustal rework processes and generation of juvenile magmatism occurred at the same time, resulting in the formation of heterogeneous plutons, at different crustal levels that were emplaced by mechanisms analogous to diapirs.

Crustal growth of the Eastern Dharwar Craton: a Neoarchean collisional orogeny?

Abstract The Eastern Dharwar Craton (EDC) is predominantly made of Neoarchean potassic granitoids with subordinate linear greenstone belts. Available geochemical and isotopic systematics of these granitoids suggest variations in the source and petrogenetic mechanisms. By compiling the available geochemical data, these granitoids can be classified into four groups, namely: TTGs (tonalite–trondhjemite–granodiorite); sanukitoids; biotite and two-mica granites; and hybrid granites. This classification scheme is in line with the global classification of Neoarchean granites, and enables the sources and petrogenetic mechanisms of these variants to be distinguished. Available geochemical, isotopic and geochronological datasets of these granitoids are integrated and the existing tectonic models for the Neoarchean EDC are reviewed. The variability of the EDC granitoids is ascribed to crustal reworking associated with the collision of two continental blocks. The tectonomagmatic evolution of the EDC is analogous to the development of the Himalayan Orogeny. Based on the evolutionary history of the Dharwar Craton, it can be concluded that convergent margin tectonics were operational in the Indian Shield from at least c. 3.3 Ga and continued into the Phanerozoic. However, the nature and style of plate tectonics could be different with time.

Unravelling the record of Archaean crustal evolution of the Bundelkhand Craton, northern India using U–Pb zircon–monazite ages, Lu–Hf isotope systematics, and whole-rock geochemistry of granitoids

Combined whole-rock, U–Pb zircon and monazite dating, in situ zircon (MC)-SF-ICP-MS Lu–Hf-isotope data are reported for more than 20 Archaean granitoid samples to constrain magmatic events and their sources in the Bundelkhand Craton, northern India. The results are crucial for understanding the extent of reworking vs juvenile crustal growth during Palaeoarchaean–Neoarchaean and also provide new information on the temporal gap between Palaeoarchaean TTG and Neoarchaean granodiorite–granite magmatism. These data reveal that the Bundelkhand Craton was affected by at least four discrete episodes of Palaeoarchaean TTG magmatism between 3.55Ga and 3.20Ga at regular intervals of 100–150 Myr. After a period of about 500 Myr of near magmatic shutdown, it was followed by a relatively short-lived, but more voluminous granitoid emplacement event during the Neoarchaean at 2.58–2.50Ga, during which minor granodioritic gneisses and voluminous granites intruded. The new data also provide evidence for a metamorphic overprint at ca. 3.20Ga, and a Pan-African tectonothermal overprint at 0.50–0.60Ga.Hf-isotope and whole-rock geochemical data of the TTGs suggest a complex process of crust formation and stabilisation during the Palaeoarchaean, mainly by the reworking of older mafic and felsic crust between 3.55 and 3.20Ga. The CHUR parallel array for the TTGs may indicate that these rocks were formed by partial melting of an oceanic crust that was derived from chondritic mantle sources, shortly before formation of TTG. This interpretation is inconsistent with Hf-isotope data from worldwide sources, which provide evidence for a successive mantle depletion since at least 4.0Ga, resulting in εHft of about +5.0 at 3.0Ga. Intense melting of the heterogeneous TTG crust during the Neoarchaean resulted in the coeval formation of granodioritic gneisses and voluminous granites, and subordinate hybrid granites with a minor contribution of melts from an enriched mantle source, affected by subduction-zone processes prior to 2.55Ga.

REINTERPRETAÇÃO DO COMPLEXO INTRUSIVO LAVRAS DO SUL, RS, DE ACORDO COM OS SISTEMAS VULCANO-PLUTÔNICOS DE SUBSIDÊNSIA. PARTE 2: QUÍMICA MINERAL, GEOQUÍMICA E ISÓTOPOS DE PB-SR-ND

The post-collisional Lavras do Sul intrusive complex- LSIC, southern Brazil, is associated with basic-to-intermediate rocks derived from two lithospheric mantle segments. All rocks show a geochemical subduction signature, with negative Nb anomalies. Minettes occurring as dykes are potassic, alkalic rocks containing olivine (F 091-86 ), Cr-diopsideand Ba-rich Ti-phlogopite, whose chemistry is only slightly modified by crustal processes. They are enriched in LREE, LILE and HFSE, and have high radiogenic isotopic ratios, so they derived from an enriched mantle previously modified in subduction events. We conclude that they register the change to an extensional regime that had occurred during the later magmatic LSIC-events. The others rocks are more evolved and have a shoshonitic composition, including trachyandesites, spessartites and the Arroio do Jacques monzodiorite (AJM). They have augite as the main mafic phase, and variable trace element sand Pb-Sr-Nd isotopic ratios. They derived from the mantle wedge produced during Neoproterozoic orogenies, but with a more prominent role of open-system differentiation processes. The interaction with minette magma is proposed for AJM that has intermediate trace element content sand isotopic ratios. The two groups of LSIC-granitoids were similarly originated from two parental magmas. Most have a shoshonitic affiliation, including the Tapera monzonite-TM in the north, and the core facies of the southern body (granodiorite and hybrid granites). Alkaline granites (syenogranite and perthite granite) form semicircular stocks around this core. Contrasts in the emplacement depth of these rocks, revealed by Al t -in amphibole geobarometer, imply in more than one cycle of subsidence-resurgence. Granites from the core, classified as low temperature l-type granites, had a major crustal contribution. They formed during the resurgence (4-2 kbar) at ca. 600 Ma. Alkaline granites were emplaced at shallow levels (minor or equal to 2 kbar) through successive episodes, between 598 and 586 Ma, that apparently affected the border of those from the core. They involved the interaction of two parental magmas, but with increasing contribution of the alkalic type.

Origin and evolution of Pliocene–Pleistocene granites from the Larderello geothermal field (Tuscan Magmatic Province, Italy)

Extensive, mainly acidic peraluminous magmatism affected the Tuscan Archipelago and the Tuscan mainland since late Miocene, building up the Tuscan Magmatic Province (TMP) as the Northern Apennine fold belt was progressively thinned, heated and intruded by mafic magmas. Between 3.8 and 1.3 Ma an intrusive complex was built on Larderello area (Tuscan mainland) by emplacement of multiple intrusions of isotopically and geochemically distinct granite magmas. Geochemical and isotopic investigations were carried out on granites cored during drilling exploration activity on the Larderello geothermal field. With respect to the other TMP granites the Larderello intrusives can be classified as two-mica granites due to the ubiquitous presence of small to moderate amounts of F-rich magmatic muscovite. They closely resemble the almost pure crustal TMP acidic rocks and do not show any of the typical petrographic features commonly observed in the TMP hybrid granites (enclaves, patchy zoning of plagioclase, amphibole clots). On the basis of major and trace elements, as well as REE patterns, two groups of granites were proposed: LAR-1 granites (3.8–2.3 Ma) originated by biotite-muscovite breakdown, and LAR-2 granites (2.3–1.3 Ma) generated by muscovite breakdown. At least three main crustal sources (at 14–23 km depth), characterized by distinct ε Nd( t) and 87Sr/ 86Sr values, were involved at different times, and the magmas produced were randomly emplaced at shallow levels (3–6 km depth) throughout the entire field. The partial melting of a biotite-muscovite-rich source with low ε Nd( t) value (about −10.5) produced the oldest intrusions (about 3.8–2.5 Ma). Afterwards (2.5–2.3 Ma), new magmas were generated by another biotite-rich source having a distinctly higher ε Nd( t) value (−7.9). Finally, a muscovite-rich source with high ε Nd( t) (about −8.9) gave origin to the younger group of granites (2.3–1.0 Ma). The significant Sr isotope disequilibrium recorded by granites belonging to the same intrusion is interpreted, as due to the short residence time of magmas in the source region followed by their rapid transfer to the emplacement level. Partial melting was probably triggered by multiple, small-sized mafic intrusions, distributed over the last 3.8 Ma that allowed temporary overstepping of biotite- and muscovite-dehydration melting reactions into an already pre-heated crust. Dilution in time of the magmatic activity probably prevented melt mingling and homogenization at depth, as well as the formation of a single, homogeneous, hybrid pluton at the emplacement level. Moreover the high concentrations of fluxing elements (B, F, Li) estimated for the LAR granites modified melt properties by reducing solidus temperatures, decreasing viscosity and increasing H 2O solubility in granite melts. The consequences were a more efficient, fast, magma extraction and transfer from the source, and a prolonged time of crystallization at the emplacement level. These key factors explain the long-lived hydrothermal activity recorded in this area by both fossil (Plio-Quaternary ore deposits) and active (Larderello geothermal field) systems.

Petrographic, Geochemical and Zircon Typology of Neoproterozoic Granitoids: Evidence of Continental Thickening in Saghro Arc (Eastern Anti-Atlas, Morocco)

Saghro is considered as a volcanic arc of the Neoproterozoic. The magmatic activity during this period is characterized by the emplacment of granitic plutons North-East from Bou-Azzer El Graara's suture zone. Sidi Flah-Bouskour buttonhole represents the near part of Saghro to this zone and Ougnat hills the far part. The buttonhole of Sidi Flah-Bouskour is cut by several successive granitic intrusions of Neoproterozoic. The earliest suite, composed of gabbro, quartz diorite and amphibole granodiorite constitute the Pan-African B1 phase related to pre- collision domain, and shows a calc-alkaline chemistry. The second phase includes the biotite granite of Bouskour which belong to the potassic calc-alkali suite emplaced during the Pan- African B2 phase in a syn-collisional environment. The granodioritic and granitic trends show separate evolution in CaO-Na,O-K,O diagram. Quartz diorite and amphibole granodiorite have moderate content of Sr (420-40ppm), Rb (133-88ppm), Cr (177-21ppm), Zr (131-67ppm) and V(470- 30ppm) and hight content of Ba (1156-368ppm). Garnet and biotite have low content of Sr (3lppm), Rb (104ppm), Cr (42ppm), V (93ppm), Zr (93ppm) and Ba (450ppm). REE normalised to MORB show that the Sidi Flah-Bouskour granitoids are enriched in LREE and LILE and relatively depleted in HFSE and Nb.The zircon populations of the quartz diorite and amphibole granodiorite show a strong frequency in the sub-type S17-18 with low @ index and high T index implying a temperature of 750 - 800°C. In addition to the T.E.T, the average points (0, T) placed in the (LA, 1.T) typological diagram permit to classify these rock into the hybrid granites domain which belong to the calc-alkali granodioritic affinity. The biotite granite shows zircon populations with dominance of the S19 sub-type implying high 0 index with apparition of the types P, D and J, and high T index. The average points (0, T) and the T.E.T. permit to classify the biotite granite among the hybrid granites implying a alkali-rich calc-alkali trend. Quartz diorite, amphibole granodiorite and garnet granite are emplaced in Ougnat hill, far separated from the suture zone. Petrographic, geochemical and zircon typology study of these rocks shows existence of two magmatic groups. The quartz diorite and the amphibole granodiorite represents a medium-K calc-alkaline trend and shows alkali enrichment (Bl). CaO-Na,O- KzO relationship shows that quartz diorite and granodiorite evolved separately from that of the garnet granite. Petrographic, mineralogical (zircon typology) and geochemical (overlap of garnet granite REE pattern on calculated crustal partial melt pattern) features of garnet granite are similar to those of crustal anatectic granites (B2). The occurrence of granodiorite very rich in biotite and mantled garnet grains identical to those in the crustal garnet granite indicates processes of assimilation and magma contamination during the emplacement of Ougnat granitoids. Biotite granite emplaced in Sidi Flah-Bouskour, adjacent to Saghro near the suture zone shows strong crustal participation and granitoids in Iknioun show high-K affinities. Away from the suture zone' in Ougnat' garnet granites of crustal origin occur. Isotopic data show variation of ENdi between biotite granite in Sidi Flah-Boukour (ENd= -1.8-2.4) and garnet granite in Ougnat (ENdi= -4.5). This is interpreted to indicate an increase in incorporation of crustal component while moving North-East. The contrast in isotopic data agree well with the spatial zonation observed in Saghro. This zonation might have evolved by crustal thickening toward the axis of Bou-Azzer El Graara's suture which is considered as a paleo-subduction zone during Neoproterozoic according to the model of Meen (1987).

Zircon and granite petrology

The typologic study of zircon populations from granitic rocks lead to the proposition of a genetic classification with three main divisions: (1) granites of crustal or mainly crustal origin [(sub) autochthonous and aluminous granites)]; (2) granites of crustal+mantle origin, hybrid granites (calc-alkaline and sub-alkaline series granites); (3) granites of mantle or mainly mantle origin (alkaline and tholeiitic series granites). In detail, there are many petrogenetic variants of each of the following granitic rocks: granodiorite, monzogranite and alkaline granite. The variations observed with zircon typology are accompanied petrographically by modifications of associations of other main and accessory minerals, and on the field by the presence or absence of basic microgranular xenoliths, associated microgranites, rhyolites or basic rocks. In the typologic diagram, some endogenous non granitic rocks (i.e. migmatites, tonalites, rhyolites ...) show a logical distribution with regard to different genetic stocks of granitic rocks.