Gabbro-diorite Complex Research Articles

Petrogenesis of the late Tonian arc-related Um Balad gabbro-diorite complex (Egypt) and insight into its spatially related orogenic gold mineralization

ABSTRACT The gabbro-diorite complex of the Um Balad prospect hosts lode gold mineralization. The complex is dated at 723 ± 4 Ma using the LA-ICP-MS U-Pb zircon method and is correlated with the late Tonian-early Cryogenian subduction-related magmatic stage during the evolution of the Arabian-Nubian Shield. The gabbro-diorite complex evolved through the crystallization of a calc-alkaline magma and the subduction signature of this magma is verified by primitive mantle-normalized trace element patterns that show enrichment in large ion lithophile elements, U, and Th relative to high field strength elements as well as negative Nb and Ti anomalies. The high La/Ybcn (3.1–9.4) and Ta/Yb (0.12–0.41) ratios are consistent with a continental arc rather than an oceanic arc system. Amphibole chemistry indicates that this complex might have crystallized under a moderately oxidizing condition from a hydrous magma (>6 wt% water content) at temperature and pressure estimates of about 800°C and 3 kbar, respectively. Gold mineralization in the Um Balad prospect is confined to structurally controlled massive quartz±carbonate veins and surrounding alteration halos. Alteration in the prospect is represented by localized sericitization and carbonation as well as pervasive chlorite-sericite alteration. The alteration halos are characterized by enrichment in K and Rb and depletion in Ca and Sr compared to their host rocks. The veins of the prospect are related to lower order extensional fractures associated with the regional first order transpressional Najd Fault System. The high Fe contents of the gabbro-diorite complex represent a suitable chemical trap for gold through sulfidation of the host rocks. Supergene alteration resulted in the formation of goethite in association with atacamite and chrysocolla. Free mill gold is associated with these supergene phases, which were deposited in near neutral to slightly alkaline conditions.

The Atud gabbro–diorite complex: glimpse of the Cryogenian mixing, assimilation, storage and homogenization zone beneath the Eastern Desert of Egypt

We analysed gabbroic and dioritic rocks from the Atud igneous complex in the Eastern Desert of Egypt to understand better the formation of juvenile continental crust of the Arabian–Nubian Shield. Our results show that the rocks are the same age (U–Pb zircon ages of 694.5 ± 2.1 Ma for two diorites and 695.3 ± 3.4 Ma for one gabbronorite). These are partial melts of the mantle and related fractionates (εNd 690 = +4.2 to +7.3, 87 Sr/ 86 Sr i = 0.70246–0.70268, zircon δ 18 O ∼ +5‰). Trace element patterns indicate that Atud magmas formed above a subduction zone as part of a large and long-lived ( c . 60 myr) convergent margin. Atud complex igneous rocks belong to a larger metagabbro–epidiorite–diorite complex that formed as a deep crustal mush into which new pulses of mafic magma were periodically emplaced, incorporated and evolved. The petrological evolution can be explained by fractional crystallization of mafic magma plus variable plagioclase accumulation in a mid- to lower crustal MASH zone. The Atud igneous complex shows that mantle partial melting and fractional crystallization and plagioclase accumulation were important for Cryogenian crust formation in this part of the Arabian–Nubian Shield. Supplementary material: Analytical methods and data, calculated equilibrium mineral temperatures, results of petrogenetic modeling, and cathodluminesence images of zircons can be found at https://doi.org/10.6084/m9.figshare.c.4958822

Identification of a new source for the Triassic Langjiexue Group: Evidence from a gabbro-diorite complex in the Gangdese magmatic belt and zircon microstructures from sandstones in the Tethyan Himalaya, southern Tibet

AbstractConsiderable debate persists as to the Triassic paleogeographic framework of the Neotethys and the origin of the Late Triassic Langjiexue Group in the Tethyan Himalaya. Triassic magmatic rocks in the Gangdese belt and Late Triassic Langjiexue sediments play a pivotal role in addressing these issues. Geochronological, petrological, and geochemical analyses have been performed on the Middle Triassic gabbro-diorite complex (with crystallization ages of ca. 244–238 Ma) from the Gangdese belt. These plutonic rocks are characterized by relatively low MgO and high Al2O3 contents, calc-alkaline trends, and depletion of Nb, Ta, and Ti, resembling low-MgO high-alumina basalts or basaltic andesites. These plutonic rocks exhibit depleted whole-rock εNd(t) values of ∼+5 and zircon εHf(t) values peaking at ∼+14. These features resemble those of rocks in a subduction-related arc setting.We also completed detrital zircon U-Pb dating and microstructure analysis for the sandstones of the Langjiexue Group in the Tethyan Himalaya. Zircon grains with ages >300 Ma are dominated by preweathered and weathered surfaces as well as fairly rounded to completely rounded scales, indicating a high degree of polycyclicity. In contrast, 300–200 Ma ones are characterized by fresh surfaces and completely unrounded to poorly rounded scales, indicating nearby sources. Collectively, our data, combined with published results, support that the subduction initiation of the Neotethys began no later than the Middle Triassic. Arc-affinity magmatic rocks supplied some materials to the Langjiexue Group. This scenario sheds new light on the provenance of the Langjiexue Group and the Triassic paleogeography of the Neotethyan realm.

Amphibole and apatite insights into the evolution and mass balance of Cl and S in magmas associated with porphyry copper deposits

Chlorine and sulfur are of paramount importance for supporting the transport and deposition of ore metals at magmatic–hydrothermal systems such as the Coroccohuayco Fe–Cu–Au porphyry–skarn deposit, Peru. Here, we used recent partitioning models to determine the Cl and S concentration of the melts from the Coroccohuayco magmatic suite using apatite and amphibole chemical analyses. The pre-mineralization gabbrodiorite complex hosts S-poor apatite, while the syn- and post-ore dacitic porphyries host S-rich apatite. Our apatite data on the Coroccohuayco magmatic suite are consistent with an increasing oxygen fugacity (from the gabbrodiorite complex to the porphyries) causing the dominant sulfur species to shift from S2− to S6+ at upper crustal pressure where the magmas were emplaced. We suggest that this change in sulfur speciation could have favored S degassing, rather than its sequestration in magmatic sulfides. Using available partitioning models for apatite from the porphyries, pre-degassing S melt concentration was 20–200 ppm. Estimates of absolute magmatic Cl concentrations using amphibole and apatite gave highly contrasting results. Cl melt concentrations obtained from apatite (0.60 wt% for the gabbrodiorite complex; 0.2–0.3 wt% for the porphyries) seems much more reasonable than those obtained from amphibole which are very low (0.37 wt% for the gabbrodiorite complex; 0.10 wt% for the porphyries). In turn, relative variations of the Cl melt concentrations obtained from amphibole during magma cooling are compatible with previous petrological constraints on the Coroccohuayco magmatic suite. This confirms that the gabbrodioritic magma was initially fluid undersaturated upon emplacement, and that magmatic fluid exsolution of the gabbrodiorite and the pluton rooting the porphyry stocks and dikes were emplaced and degassed at 100–200 MPa. Finally, mass balance constraints on S, Cu and Cl were used to estimate the minimum volume of magma required to form the Coroccohuayco deposit. These three estimates are remarkably consistent among each other (ca. 100 km3) and suggest that the Cl melt concentration is at least as critical as that of Cu and S to form an economic mineralization.

GEOLOGY, MINERALOGY AND RADIOACTIVITY STUDIES OF WADI KHUDA STREAM SEDIMENTS, SOUTH EASTERN DESERT, EGYPT

This study concerned with the geologic features, mineralogical composition and radioactivity of Wadi Khuda stream sediments south Eastern Desert of Egypt.Geologically, the basement rocks cropping out at Wadi Khuda area are gneisses, gabbro-diorite complex, tonalite-granodiorite, monzogranite and albite granite. The area is drained by several Wadis as Wadi Khuda, Wadi Hindia, Wadi Shut and Wadi Salib el Abiad. The stream sediments samples indicate significant enriched in some HFSE (high field strength elements) such as Zr, Y, Nb compared to the probable source rocks. This indicate that these trace elements in the stream sediments derived from the muscovite granite and granitic pegmatities.Mineralogical studies on the heavy minerals of Wadi Khuda recorded by magnetite, ilmenite, zircon, thorite, titanite, fluorite, and garnet. The radiometric measurements of the studied stream sediments show that eU varies from 3 to 16 ppm with an average of 7.6 ppm, while eTh ranges from 6 to 40 ppm with an average of 21.5 ppm. The average concentration of Ra and K% are 4.2 ppm and 1.8% respectively.

Petrological Evolution of the Magmatic Suite Associated with the Coroccohuayco Cu(–Au–Fe) Porphyry–Skarn Deposit, Peru

The petrological evolution of magmatic rocks associated with porphyry-related Cu deposits is thought to exert a first-order control on ore genesis. It is therefore critical to understand and recognize petrological processes favourable to the genesis of porphyry systems. In this study we present new petrographic, geochemical (whole-rock and mineral), and isotopic (Pb, Sr, Nd) data for rocks from the magmatic suite associated with the Eocene Coroccohuayco porphyry–skarn deposit, southern Peru. Previously determined radiometric ages on these rocks provide the temporal framework for interpretation of the data. Arc-style magmatic activity started at Coroccohuayco with the emplacement of a composite precursor gabbrodiorite complex at c. 40·4 Ma. After a nearly 5 Myr lull, magmatic activity resumed at c. 35·6 Ma with the rapid emplacement of three dacitic porphyries associated with mineralization. However, zircon antecrysts in the porphyries show that intra-crustal magmatic activity started c. 2 Myr before porphyry emplacement and probably built a large intra-crustal magmatic body with an associated large thermal anomaly. Our data suggest that all magmas underwent a period of evolution in the deep crust before transfer and further evolution in the upper crust. The gabbrodiorite complex was sourced from a heterogeneous deep crustal reservoir and was emplaced at a pressure of 100–250 MPa where it underwent a limited amount of fractionation and formed a chemically zoned pluton. Its initial water content and oxygen fugacity were estimated to be around 3 wt % H2O and NNO ± 1 (where NNO is the nickel–nickel oxide buffer), respectively. The deep crustal source of the porphyries appears to have been more homogeneous. The porphyries are interpreted to be the product of advanced differentiation of a parental magma similar to the gabbrodiorite. Most of this evolution occurred at deep crustal levels (around 800 MPa) through fractionation of amphibole + pyroxene + plagioclase ± garnet, leading to the development of a high Sr/Y signature characteristic of porphyry-related magmatism worldwide. Subsequent upper crustal evolution (100–250 MPa) was dominated by crustal assimilation, cannibalism of previously emplaced magma batches (proto-plutons) and magma recharge. Water content and oxygen fugacity were estimated to be around 5 wt % H2O and NNO + 1 to NNO + 2, respectively, at the end of the period of upper crustal evolution. This high oxygen fugacity is inferred to have favoured sulphur and metal enrichment in the melt. The high thermal regime generated through 2 Myr of sustained magmatism in the upper crust favoured crustal assimilation, proto-pluton cannibalism, and efficient metal extraction upon fluid exsolution. The Coroccohuayco magmatic suite appears to have acquired its metallogenic potential (high fO2, high Sr/Y) through several million years of deep crustal evolution.

LiDAR offshore structural mapping and U/Pb zircon/monazite dating of Variscan strain in the Leon metamorphic domain, NW Brittany

An exceptional structural picture of the immerged Variscan basement, offshore the Leon metamorphic domain, is supplied by high-resolution LiDAR and echosounder data recorded in the Molène archipelago, western Brittany (France). Various types of fabrics are identified and, from in situ rock sample analyses further combined with field structural data, are interpreted on a lineament trajectory map as the trace of magmatic and tectonic structures. Our onshore/offshore study leads us to propose a two-phase kinematic model that emphasizes the role of a strike-slip duplex in an EW-trending relay zone linking the North Armorican and Pierres Noires ductile shear zones (NASZ, PNSZ). Dextral shearing occurred within a transtensional setting, synchronously with magmatic intrusions (St-Renan granite and an offshore gabbro-diorite complex) dated at 314–320Ma by new U–Th/Pb ages. It post-dated an early regional foliation related to top-to-the-NE ductile transpressional shearing. Our study emphasizes the key role of strike-slip tectonics in the NW part of the Armorica Variscan belt.

Mantle upwelling during Permian to Triassic in the northern margin of the North China Craton: Constraints from southern Inner Mongolia

The nothern margin of the North China Craton (NCC) preseves the tectonic imprints of early Paleozoic to Triassic tectonic processes. Here we investigate the geochronology, geochemistry, zircon hafnium and whole rock Nd isotope data on a suite of magmatic rocks from the Ningcheng pyroxenite–gabbro–diorite complex and the Jiwangyingzi Formation in the Inner Mongolia region of the northern NCC. The zircon U–Pb dating identifies multiple stages of mafic to intermediate magmatism from Early Permian (298±4Ma) to Middle Triassic (219±1Ma) in this area and their hafnium isotopes display diverse mantle sources. An integration of field observations, geochronology and elemental and isotopic geochemistry indicates a complex petrogenetic history for the magmatic suite. The Ningcheng pyroxenite–gabbro–diorite complex is a product of multiple pulses of magmas with crystal fractionation, hybridization and assimilation, with the involvement of melts derived from lithospheric mantle and even the asthenosphere. Our data reveal an increasing involvement of mantle component from Late Permian to Triassic. The features of progressive mantle upwelling correlate with tectonic processes associated with the closure of Paleo-Asian Ocean and subsequent post collisional extension, resulting in active magmatism from Carboniferous to Triassic in this region.

Microchemistry and stable isotope systematics of gold mineralization in a gabbro–diorite complex, SE Egypt

Electron probe micro-analysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data of ore minerals from the Um Eleiga deposit (SE Egypt) suggest that primary Au was sequestered within Ni–pyrrhotite and Co–Ni–Fe sulfarsenide, while refined, free-milling gold grains (~93wt.% Au) are associated with late pyrite, or occur along micro-fractures in quartz veins. A magmatic source of gold is most likely, yet endowment by circulating metasomatic fluid was crucial for Au concentration.Sulfur stable isotope values (δ34S) of sulfide minerals disseminated in the quartz veins (pyrite: −0.5 to 1.5‰, chalcopyrite: −0.3 to 0.6‰, and sphalerite: −0.1 to −0.4‰), indicate a generally light sulfur source. Similarly, measured δ13C of vein calcite (−3.1 to −1.8, n=9) and calculated fluid δ13CCO2 values (−0.6 to 0.6‰, for the temperature range 350–375°C of associated hydrothermal chlorite) cluster around 0‰, consistent with a magmatic source for carbon. Oxygen isotope data for vein calcite (10.3–13‰, n=9) and quartz (11.7–14.3‰, n=10) indicate average fluid δ18OH2O values of 7.4 and 7.8‰, respectively, which implicate fluids similar to those in intrusion-related systems.

Geochemistry and magmatic setting of Wadi El-Markh island-arc gabbro–diorite suite, central Eastern Desert, Egypt

Wadi El-Markh gabbro–diorite complex is composed of pyroxene hornblende gabbros, hornblende gabbros, diorites and quartz diorites. According to their bulk rock geochemistry and mineral chemistry, the gabbroic and dioritic rocks represent fractionates along a single line of descent and crystallized from a calc-alkaline mafic magma. When compared to the primitive mantle, all members of the gabbroic–dioritic rock suite are enriched in the large ion lithophile elements relative to the high field strength elements and display distinctive negative Nb and P 2O 5 anomalies. This signals an arc setting. Fractionation modeling involving the major elements reveals that the hornblende gabbros were generated from the parent pyroxene hornblende gabbros by 61.86% fractional crystallization. The diorites were produced from the hornblende gabbros by fractional crystallization with a 58.97% residual liquid, whereas the quartz diorites were formed from the diorites by 26.58% fractional crystallization. According to geothermobarometry based on amphibole mineral chemistry, the most primitive pyroxene hornblende gabbros crystallized at ∼830 °C/∼5 kbar. The crystallization conditions of the quartz diorites were estimated at ∼570 °C/∼2 kbar. In consequence the Wadi El-Markh gabbro–diorite complex represents a single magmatic suite of which fractionates crystallized in progressively shallower levels of an arc crust.

Rare-earth and trace element imprints on the origin and tectonic setting of gabbro-diorite complex in the Pan-African belt of Southeast Obudu Plateau, Nigeria

A gabbro-diorite plutonic complex from the Southeast Obudu Plateau, representing limited volumes of magma, was studied for its trace and rare-earth element characteristics, in an attempt to document its genetic and geodynamic history. Geochemical studies indicate that the gabbro samples are characterized by variable concentrations and low averages of such index elements as Cr (40×10−6−200×10−6; av. 80×10−6), Ni (40×10−6−170×10−6; 53.33×10−6) and Zr (110×10−6−240×10−6; 116.67×10−6); variable and high averages of Rb (3×10−6−270×10−6; 80.67×10−6), Sr (181×10−6−1610×10−6; 628.17×10−6) and U (0.14×10−6−3.46×10−6; 1.51×10−6), and fairly uniform Co (34×10−6−49×10−6; 36.33×10−6) and Sc (23×10−6−39×10−6; 34.5×10−6), while the diorite samples exhibit higher trace element compositions. The range of REE contents and distinctive chondrite-normalized patterns indicate moderate fractionation with slight positive Eu anomaly in the diorites to very low fractionation with flat patterns and slight positive Eu anomaly in the gabbros. However, the general element systematics of the samples, especially LILE (Ba, Rb, Sr, Cs and Pb), HFSE (Zr, Th, U, Hf, Mo, W, Nb and Sn), relatively immobile elements (Zr, Ni, Cr) and REE, suggests a differentiation model, involving fractional crystallization of olivine and clinopyroxene from a partial melt generated beneath an island arc complex. A possible model for the complex is therefore an island arc setting, the development of which was dominated by calc-alkaline magmatism across the Obudu Plateau.

The Neoproterozoic Dubr intrusives, Central Eastern Desert, Egypt: petrological and geochemical constraints on the evolution of a mafic-felsic suite

The Neoproterozoic magmatism in the Dubr area took place in two suites namely: (1) a mafic suite of a gabbro-diorite complex and (2) a felsic suite of a granodiorite and monzogranite composition. Pressures estimated for the gabbro and diorite rocks show that they have originated over a wide range of pressures (1.5-6.5 kbar). The gabbro and diorite rocks predict crystallization temperatures ranging from 820 to 940 °C and 660 to 710 °C, respectively. The rare-earth element abundances of the Fe--tholeiite gabbro-diorite complex display fractionated {(La/Lu) n = 1.53-2.96} and LREE-enriched {(La/Sm) n = 1.12-2.62} patterns. The REE patterns of the gabbro-diorite complex suggest that it can be generated from a spinel-bearing mantle source. Transition metal contents of the gabbro-diorite complex are variable and low (Ni, 9-133 ppm; Cr, 20-325 ppm; Sc, 17.7-34 ppm), indicating that none of the samples are primitive. The gabbro-diorite complex may have formed via relatively high degree partial melting (25-30 %) of an enriched mantle source followed by fractional crystallization of olivine, Cr-spinel with minor clinopyroxene and plagioclase. Chemical evidences imply that crustal contamination is likely to be a minor factor in trace element variation of the gabbro-diorite complex. The granodiorite and monzogranite are calc-alkaline showing metaluminous affinity for the granodiorite and a slightly peraluminous nature for the monzogranite. They display trace element and REE characteristics of I-type granites formed in a subduction-related arc environment. The hornblende of the granodiorite and monzogranite yielded pressures of an average 3 kbar and 2.5 kbar, respectively. Amphibole-plagioclase geothermometer yielded crystallization temperatures ranging from 680 to 700 °C and 650 to 675 °C for the granodiorite and monzogranite, respectively. The co-linear trends on variation diagrams for the granodiorite and monzogranite indicate a genetic link between them through fractional crystallization. It appears likely that the restite-unmixing model is responsible for the petrological and chemical features of the granodiorite. The most probable source for the granodiorite is a dioritic rock. The monzogranite was derived as a result of crystallization of plagioclase, biotite, hornblende, quartz, ilmenite and magnetite phases from a granodioritic melt.

Isotopic responses to basaltic injections into silicic magma chambers: a whole-rock and microsampling study of macrorhythmic units in the Pleasant Bay layered gabbro-diorite complex, Maine, USA

The Pleasant Bay layered gabbro-diorite complex (420 Ma) formed via repeated injections of mafic magma into a felsic magma chamber. It is dominated by repeating sequences (macrorhythmic units) with chilled gabbroic bases which may grade upward into medium-grained gabbro, diorite and granite. Each unit represents an injection of mafic magma into the chamber followed by differentiation. Increases in Sri and decreases in eNdi with stratigraphic height indicate open-system isotopic behaviour and exchange between the mafic and felsic magmas. Isotopic variations of whole-rock samples in individual macrorhythmic units do not conform to bulk mixing or AFC models between potential parental magmas. Sr isotopic studies of single feldspar crystals from one macrorhythmic unit indicate that exchange of crystals between the resident felsic magma and mafic influxes was important, that some of the rocks contain feldspar xenocrysts, and that the rocks are isotopically heterogeneous on an intercrystal scale. Xenocryst abundance increases with stratigraphic height, suggesting that crystal exchange occurred in situ. The lack of disequilibrium textures in the xenocrystic feldspar indicates the evolved macrorhythmic magma and resident silicic magma were of a similar composition and likely in thermal equilibrium at the time of crystal transfer. Mafic chilled margins are enriched in alkalis and isotopically evolved compared with mafic dikes (representing the parental melts) and suggest rapid in-situ diffusional exchange following emplacement of individual mafic replenishments.

Geochemical and petrological evidence of calc-alkaline and A-type magmatism in the Homrit Waggat and El-Yatima areas of eastern Egypt

The Neoproterozoic plutonic complex in the Homrit Waggat and El-Yatima areas, central Eastern Desert of Egypt, comprises a deformed calc-alkaline I-type metagabbro-diorite complex and tonalite-granodiorite suite invaded by felsic high level intrusions of A-type characteristics. The metagabbro-dionte complex exhibits petrological and geochemical characteristics of mantle-derived island-arc basalts, and its magma was derived possibly from partial melting of a mantle wedge above an early Pan-African subduction zone. The rocks of the ton alite-granodiorite suite have a wide range of SiO 2 (62–71%), and K, Rb and Ba enrichment relative to Nb and Y. Their chemical variations suggest that they are not related to the gabbro-diorite complex, but most probably derived by partial melting of the amphibolitic lower crust in a subduction zone. The A-type granites are mainly syeno- and alkali-feldspar granites characterised by sub- and hyper-solvus textures, late magmatic interstitial biotite and interstitial or vein-fluorite. They are geochemically evolved (SiO 2=74–78%), metaluminous to mildly peraluminous, enriched in Fe, Y, Nb, Rb, Zr and F, and depleted in CaO, MgO, Ba and Sr. Although they can be classified as A-type and within-plate granites, the least differentiated samples have F, Nb, Y and Rb contents similar to those in the surrounding I-type ton alite-granodiorite suite. This similarity suggests that high concentrations of these elements in the A-type granite are mostly related to unusual fractionation processes rather than to source rock (A-type source). This simply indicates that these granites are I-type and their classification as A-type reflects the process of evolution. A petrogenetic model of dehydration partial melting of an early Pan-African lower crust along major shear zones in a post-collisional environment to produce granodioritic melt seems likely. Fractional crystallisation of this granodioritic melt gave silicic granites, during which a F-rich fluid phase was evolved. Late magmatic F-rich fluid-rock interaction and F complexing played an important role in the evolution and chemical characterisation of the A-type granites.

Geochemical and Sr–Nd–Pb isotopic data bearing on the origin of Pan-Africangranitoids in the Kid area, southeast Sinai, Egypt

The Kid plutonic suite in southeast Sinai, Egypt is composed of: (1) a gabbro–diorite complex; (2) quartz monzodiorite; (3) granodiorite–monzogranite association; (4) syenogranite. Major- and traceelement analyses and radiogenic isotope data (Rb/Sr, Sm/Nd and Pb/Pb) are presented for these rocks. Rb–Sr isochrons indicate ages of 581±11 Ma, 576±11 Ma and 570±4 Ma for the quartz monzodiorite–granodiorite–monzogranite association and syenogranite, respectively. The gabbro–diorite complex is a layered intrusion varying in SiO2 from 42 to 59 wt%. It shows a calc-alkaline trend, and the trace element characteristics of arc lavas. Subsequently, this magma fractionated to give quartz monzodiorite andsyenogranite magmas. The quartz monzodiorite has low initial 87 Sr/ 86 Sr ratios (0.703) and high ɛ Nd values (+4.4±0.3), with trace element characteristics of volcanic-arc granites. The granodiorite–monzograniteassociation has higher initial Sr ratio (0.7044), lower ɛ Nd (+1.5–+2.7) and relatively high 207 Pb/ 204 Pb and 208 Pb/ 204 Pb, significantly different from those of the older rocks. Isotopic data and REE modelling suggestthat it formed by partial melting of an early Pan-African amphibolitic lower crust. The syenogranite isa typical highly fractionated post-orogenic granite, with a deep negative Eu anomaly, HFSE and REEenrichment, a 87 Sr/ 86 Sr initial ratio of 0.7033, ɛ Nd of 4.5 & 0.5 and distinct variations in the 206 Pb/ 204 Pbratio. The isotopic compositions of Sr, Nd and Pb indicate that the syenogranite and the quartzmonzodiorite are comagmatic. Increasing fluorine concentration from the quartz monzodiorite to thesyenogranite suggests that fluid fractionation involving fluorine complexing played an important role inthe genesis of the syenogranite. This process led to the enrichment of HFSE and HREE, to extensivefractional crystallization, to changes in the U/Pb ratios and a distinct variation in 206 Pb/ 204 Pb.

Combined oxygen isotope – compositional studies of some granitoids from the Grenville Province of Ontario, Canada: implications for source regions

Oxygen isotopic compositions of whole rocks and coexisting quartz–feldspar pairs have been determined for nine pre-, and syn- to late-kinematic granitoid plutons in the Grenville Province of Ontario. These new data demonstrate that granitoid rocks (Algonquin, Mulock) in migmatite terrain of the Ontario Gneiss Segment possess normal δ18O values (<9.0‰), whereas mesozonal to epizonal plutons (Elphin, Coe Hill, Deloro, Barber's Lake) in the Central Metasedimentary Belt (CMB) are characterized by significantly higher 18O contents (δ18O > 9.0‰), in accord with previous results.In the Algonquin sodic suite, a gross covariance of δ18O with compositional indices is present, from 6.4‰, SiO2 = 50.5 wt. % (gabbro) to 8.7‰, SiO2 = 72 wt. % (trondhjemite), resulting from combined assimilation–fractional crystallization. Mafic members of the sodic suite are 18O enriched overall (5.8–7.9‰) relative to fresh tholeiites (5.7 + 0.3‰), implicating some 18O contamination of the protolith. The dispersion of δ18O values in the Algonquin potassic suite, from 4.3 to 9.3‰, is independent of composition and attributed to isotopic exchange with low-18O thermal waters during emplacement. Biotite–hornblende granite of the Mulock batholith is characterized by a limited oxygen isotope compositional range, where the average δ18O = 8.1 ± 0.5‰; δ18O correlates with SiO2 but not with the zonal distribution of Ba, Rb, and Sr abundances.The Union Lake quartz diorite (δ18O = 8.5 ± 0.1‰) and White Lake trondhjemite (δ18O = 7.3 ± 0.6‰) have oxygen isotope compositions comparable to those of other trondhjemitic suites in the CMB. A systematic enrichment of ~1.2‰ in the Union Lake pluton, together with enhanced Ca, Mg, Fe, and Sr, can be accounted for by assimilation of ~5% marbles and 10% amphibolites from the country rock. Uniformly high δ18O values of 11.5 ± 0.8‰ characterize the Elphin granite–syenite complex. The largest values (11.7–12.7‰) and lowest SiO2 (54–56 wt. %) are in the partially assimilated host gabbro–diorite complex, endorsing the presence of 18O-enriched source regions. The Cheddar biotite–hornblende granite, one of a population of intrusions within the alkalic belt of the western CMB, has a restricted isotopic span, where δ18O = 8.8 ± 0.9‰. An unusual concave rare-earth-element (REE) distribution may result from interaction with a heavy rare-earth -element (HREE) enriched volatile phase. The Coe Hill biotite granite (δ18O = 10.4 ± 0.4‰) is isotopically in compliance with other granites and syenites of the CMB. Covariance of δ18O and SiO2, in conjunction with smooth and continuous geochemical trends, is interpreted in terms of assimilation–fractional crystallization.Peralkaline granite of the Deloro pluton includes a hypersolvus phase with high, scattered δ18O values (9.1–11.8‰) and a subsolvus counterpart attributed to late influx of water that induced isotopic reequilibration toward a more constrained range (δ18O = 9.2–10.2‰). REE distributions of a calcic syenite phase are compatible with its evolution by fractional crystallization of a low-K tholeiitic magma, and the high-18O character (δ18O = 11.1–12.6‰) requires 18O enrichment of the protolith and (or) 18O contamination of the magma. Peralkaline rhyolitic volcanics, compositionally coherent with the Deloro pluton and possibly representing extrusive equivalents, possess significantly higher and more variable δ18O values, from 11.7 to 14.2‰; this is attributed to 18O enrichment during low-temperature exchange with thermal waters, superimposed on a primary high-18O magma. The Barber's Lake two-mica granite contains enhanced abundances of U (15 ppm) and Th (36 ppm) in conjunction with systematically elevated δ18O values (10.4 ± 0.5‰). Geochemical constraints are compatible with its evolution from a trondhjemitic magma, but the isotopically enriched nature requires extensive 18O contamination of the protolith and (or) magma. These nine granites variously retain "memory" of primary and (or) secondary features, including δ18O of the source region, covariance of isotopic and compositional parameters, and sporadically superimposed disturbance by exchange with thermal waters. During metamorphism, quartz and feldspar were systematically reset to high-temperature fractionations, but the extent of open-system exchange with rock reservoirs was limited.Despite some probable disturbance by metamorphism and the limited data available, O–Sr isotope systematics of the Grenville granitoids indicate that (1) high-18O granites from the Frontenac Axis were derived from in situ anatexis of Grenville Supergroup metasediments, (2) synkinematic granites were derived by mixing of a primary magma generated at a lower crustal (granulite facies) or upper mantle level with the fusion products generated by partial melting of the Archean–Early Proterozoic type metasediments, and (3) the tonalite–trondhjemite suite in this part of the Grenville Province was derived from a similar lower crustal or upper mantle primary magma by direct fractional crystallization.