high-K Suite Research Articles

Relationships between high-K magmas and Cu–Au porphyry deposits in the Macquarie Arc, Australia

The Macquarie Arc is a long-lived volcanic arc terrane which began in the latest Cambrian and continued until the early Silurian. Early Silurian high-K suites associated with Cu–Au porphyry mineralization are typically considered shoshonitic; however, comparisons with well-studied examples of shoshonites in continental arcs and associated settings suggest few Macquarie Arc rocks would classify as shoshonitic. Notably, the Au-rich Cadia Intrusive Complex, which is marginally shoshonitic, and most high-K to ‘shoshonitic’ Macquarie Arc rocks are essentially high-K calc-alkaline, analogous to ‘primitive’ shoshonites generated within intraoceanic arc settings. Arc magmatism in the Macquarie Arc is consistently primitive and mantle derived prior to an increase in siliciclastic input and crustal thickening during the Silurian Benambran Orogeny, consistent with prior isotopic studies. Magma source regions can thus be interpreted as varying spatially and temporally due to variable enrichment from slab and mantle derived components, reflecting a maturing and potentially migrating arc system.

Chromium isotope fractionation during magmatic processes: Evidence from mid-ocean ridge basalts

Chromium (Cr) isotopes represent a powerful tool for tracing the redox conditions during planetary magmatic evolution. However, so far, the systematic investigation of Cr isotope variation has been only performed on ocean island basalts (OIBs) in terrestrial magmatic rocks. Therefore, the Cr stable isotope compositions (expressed as δ53Cr relative to NIST SRM 979) of other magmatic rocks, formed under different oxygen fugacity, have remained unconstrained. In this paper, we present the first Cr stable isotopic data of mid-ocean ridge basalts (MORBs) from the Eastern Pacific Ocean ridge, the Indian ridge, and the Atlantic Ocean ridge. The oxygen fugacity of such basalts is different from that of the above-mentioned OIBs. The Rhyolite-MELTS model shows that the chemical composition variations in the studied basalts are induced by varying extents of fractional crystallization of olivine, clinopyroxene, plagioclase, and spinel. The δ53Cr values of the MORBs range from −0.27 ± 0.03‰ to −0.07 ± 0.02‰ (n = 28), and two distinct groups of basalts are identified based on the correlation between δ53Cr and MgO. On the one hand, the δ53Cr of group I basalts (−0.27 ± 0.03‰ to −0.14 ± 0.03‰; n = 24) are systematically lower than the established average value of the Bulk Silicate Earth (BSE) (δ53Cr = −0.12 ± 0.04‰; 2SD), which are positively correlated with their MgO and Cr concentrations, indicating that Cr isotopes are fractionated during magmatic differentiation. Moreover, Rayleigh fractionation modelling suggests that the crystallization of olivine, clinopyroxene, and spinel gives rise to the Cr depletion and thereby decreases δ53Cr values. On the other hand, group II basalts (−0.10 ± 0.03‰ to −0.07 ± 0.02‰; n = 4) exhibit higher δ53Cr values than group I with identical MgO and Cr concentrations. This is possibly associated with the crystallization of clinopyroxene under low pressure.The average Cr isotope composition (δ53Cr = −0.16 ± 0.02‰, n = 3) of the primitive basalts (MgO > 9%) represents that of the primary MORB melt, which is lighter than the average value of BSE. Using the non-modal melting equations, the δ53Cr of the MORB mantle source is estimated to be −0.12 ± 0.02‰ (2σ), which is consistent with that of BSE. Compared with the Cr isotopic data of OIBs from Fangataufa island, we find that the equilibrium fractionation factors (Δ53Crcrystal-melt = +0.04‰ to +0.13‰) of MORBs during fractional crystallization are larger than that of Fangataufa island lavas (Δ53Crcrystal-melt = 0.010 ± 0.005‰ for low-K suite, and 0.020 ± 0.010‰ for high-K suite; Bonnand et al., 2020a), indicating that the basalts from Fangataufa island have higher oxygen fugacity than those of MORBs analyzed in this study, which is strongly supported by their higher V/Sc ratios.

Geology and geochronology of the Archean plutonic rocks in the northeast Democratic Republic of Congo

Here we report the first regional-scale study of Archean plutonic rocks from 50,000 km2 of the northeast Democratic Republic of Congo (DRC). We include 50 new U-Pb zircon Sensitive High-Resolution Ion MicroProbe (SHRIMP) ages supported by petrographic and whole rock geochemical data from a further ~400 samples, and 39 additional U-Pb zircon ages reported by Kabete et al. (submitted) and Bird (2016).Felsic-intermediate magmatism across the northeast DRC occurred from ~ 3200 to 2530 Ma. Plutonism before ~ 2670 Ma is restricted to the West Nile Gneiss in eastern and northern-most DRC, whereas plutonism across the remainder of the northeast DRC initiated between 2670 and 2640 Ma. Two Neoproterozoic granite plutons ~ 1000–950 Ma are also present within the West Nile Gneiss. Magmatism between ~ 2670 and 2530 Ma marks the principal period of crustal growth in the northeast DRC. Felsic-intermediate plutonism between ~ 2670 and 2600 Ma included two suites of magnesian, calc-alkaline rocks, (1) a sodic-calcic suite with high Sr/Y ratios (>40) comparable with tonalite-trondhjemite-granodiorite (TTG) suites in other Archean terranes, and (2) a calc-alkaline to high-K suite of granitoids with low Sr/Y ratios (<40) and lower Na and Al contents, comparable with “low-Ca” granites in the Yilgarn Craton. In contrast, most granitoids emplaced between ~ 2600 and 2530 Ma have ferroan, high-K calc-alkaline compositions, and are enriched in high field strength (HFSE) and rare Earth elements (REE). Although the HFSE-elevated granitoids are comparable with Closepet-type, and “mafic granites” in other Neoarchean terranes, their volume within the northeast DRC appears unprecedented.Before ~ 2600 Ma partial melting of hydrous metabasalts at depths > 40 km is inferred to have produced the high Sr/Y magnesian granitoids, whereas the low Sr/Y magnesian granitoids were derived from melting-assimilation-storage-hybridization (MASH) processes in the overlying low-mid crust during the same period. Trans-lithospheric extension after ~ 2600 Ma, enabled asthenospheric mantle to interact with the base of the crust, resulting in high-temperature melting that produced the HFSE-elevated granitoids. Inherited Meso- and Paleoarchean zircons are almost completely restricted to the area within 30–40 km of the West Nile Gneiss, implying most of the northeast DRC is underlain by juvenile Neoarchean crust.

Genesis and evolution of a Paleoproterozoic basement inlier within West Gondwana addressed by Sm-Nd isotopic geochemistry and Zr saturation thermometry

This paper presents new isotopic Sm-Nd ID-TIMS data and applies the Zr saturation thermometry on orthogneisses of the Quirino Complex to address the mechanisms of magma genesis, evolution and tectonic setting. Quirino Complex is considered a Paleoproterozoic basement inlier within a Neoproterozoic belt, comprised of two distinct calcalkaline magmatic suites: a Siderian to Rhyacian high-K suite (2.30–2.19 Ga); and a Rhyacian medium-K suite (2.17–2.16 Ga). High zircon solubility and εNd(t) values (+2.7 to −5.8) indicate that the protoliths of the high-K suite gneisses can be related to high-temperature (TZr = ca. 810 °C) I-type granites. The gneisses of the medium-K suite display isotopic signature for crustal residence (εNd(t) = −17.9 and −8.1). Their protoliths, rich in inherited zircons (ca. 3.0 Ga) and other restitic/refractory components (hornblende clots and mafic enclaves), were derived from partial melting of Archean crust at relatively low temperature (TZr = ca. 750 °C) within a collisional tectonic setting.

Multiple fluid sources/pathways and severe thermal gradients during formation of the Jílové orogenic gold deposit, Bohemian Massif, Czech Republic

The Jílové deposit in the central part of the Bohemian Massif represents a vein to stockwork type of orogenic-type gold deposit. It is hosted by Neoproterozoic rocks of the Jílové Belt and by various magmatic dikes related to the ~355 to ~335Ma Central Bohemian Plutonic Complex. The deposit is situated along the terrane boundary of the Teplá Barrandian and Moldanubian units.The deposit offered an exceptional opportunity to trace O, C, S and Sr stable isotope evolution of parent fluids based on combined mineralogical and geochemical study of carbonate, quartz, scheelite, and sulfide minerals, which represent six stages of mineralization, including the gold-bearing event.Stable isotope data and mineral and isotope thermometry indicate gangue and ore mineral formation between ~350°C and <100°C, which can be divided into 6 stages. Scheelite-bearing assemblages (stages 2–3) precipitated at 292±8°C from a fluid with calculated values: δ18OSMOW=+4.2±0.5‰ and δ13CPDB=−11±1‰. Gold precipitation (stage 5) probably started at about 300°C, but the major event probably occurred at 230±30°C from a fluid with more variable isotope values (δ18OSMOW=+2.5 to +5‰ and δ13CPDB=−9 to −13.5‰). The carbon speciation was characterized by predomination of dissolved CO2 (H2CO3ap.) in the fluids. Some gold, however, undoubtedly precipitated from bicarbonate dominated fluids even at <120°C.Extreme variations in the δ18O values of carbonate minerals, obtained from sampling profiles across individual veins with macroscopic gold, revealed severe thermal gradients during vein formation (~50 to ~100°C difference of crystallization temperatures between the vein margin and core).The sulfur stable isotope composition of sulfide minerals indicates the dominant role of sulfur remobilization from Neoproterozoic rocks and stratiform mineralizations of the Jílové Belt by Variscan hydrothermal fluids. Similarly, the Sr-isotope composition of carbonates indicates both relatively primitive (87Sr/86Sr=0.7055) and more evolved (87Sr/86Sr~0.7090) fluid compositions, probably indicating fluid exchange with the Jílové Belt and the Central Bohemian Plutonic Complex rocks, respectively.Age determination of hydrothermal muscovite (related to stage 2) via 40Ar/39Ar indicated an age of 339.0±1.5Ma for the quartz veins. The mineralization is essentially coeval with the late intrusive phases of the Central Bohemian Plutonic Complex (i.e. the ultrapotassic suite) and with late-orogenic large-scale tectonic movements at the boundary between the two crustal terranes (Teplá-Barrandian and Moldanubian).Based on evaluation of the available age data on the hydrothermal and magmatic activity within the broader area of the Central Bohemian Plutonic Complex, we suggest two intervals of gold mineralization: 347 to 341Ma and 340 to 337Ma. The former interval overlaps with the intrusive activity of the Blatná high-K suite (granodiorite). The associated gold deposits (Mokrsko and Petráčkova hora) exhibit strong affiliation to the intrusion-related-gold-type deposit. The later interval overlaps with the ultrapotassic magmatism and is associated with more or less “classical” orogenic-gold-type deposits (Jílové, Bělčice, Libčice deposits).

U–Pb zircon geochronology of the eastern part of the Southern Ethiopian Shield

The Southern Ethiopian Shield (SES) in the central East African Orogen lies at the junction of Neoproterozoic (880–550Ma) largely greenschist-facies juvenile crust of the Arabian-Nubian Shield in the north and more metamorphosed and remobilized older crust of the Mozambique Belt to the south. The SES exposes a polycyclic sialic gneissic basement (represented by the Alghe Terrane in this study) with interleaved ophiolitic–volcano–sedimentary (Kenticha, Megado, and Bulbul) terranes. U–Pb zircon SHRIMP and LA-ICP-MS dating of 8 igneous and metamorphic intermediate and felsic bodies in the eastern SES yield Neoproterozoic crystallization ages: 847±11, 855±14, 732±5, 665±8, 657±6, 560±8, and 548±8Ma. From these and previously published ages, we infer 4 principal magmatic episodes: 840–890Ma (Late–Tonian–Early Cryogenian), 790–700Ma (Cryogenian), ∼660Ma (Moyale Event), and Pan-African (630–500Ma; Ediacaro-Cambrian). Neoproterozoic zircon xenocrysts (941, 884, 880, 863, 762, 716 and 712Ma) confirm the dominance of Neoproterozoic crust in the study area. One sample of undeformed granite from the Alghae Terrane has abundant Archean zircons and may be a ∼550Ma melt of ∼2.5Ga crust, demonstrating for the first time that Archean crust or sediments with abundant Archean zircons exists in the SES. In spite of ∼300 million years of Neoproterozoic igneous activity, we see no evidence of systematic compositional evolution in SES igneous rocks from early low-K suites to late high-K suites. Ediacaran deformation and magmatism of the SES reflects Late Tonian and Cryogenian formation of mostly juvenile crust that was subsequently deformed and chemically modified as a result of collision between large fragments of East and West Gondwana. Terminal collision began at ∼630Ma and caused crustal thickening, melting, uplift, erosion, orogenic collapse, and tectonic escape over a broad region of the East African Orogen, including up to ∼25km of erosion of SES crust. Plate convergence was likely continuous from ∼630Ma, forming major N-S structures. Deformation stopped at ∼550Ma and was followed by exhumation between ∼ 530 and 500Ma.

Lithospheric Removal as a Trigger for Flood Basalt Magmatism in the Trans-Mexican Volcanic Belt

The voluminous succession of tholeiitic basalts, calc-alkaline andesites and minor high-K basalts that form the Late Miocene Altos de Jalisco mafic province of the western Trans-Mexican Volcanic Belt is interpreted as the magmatic manifestation of a lithospheric dripping event, which removed mantle lithosphere and lower crustal lithologies beneath the study area. During this process, the release of fluids from the foundering materials, coupled with mantle upwelling around the sinking mass, promoted abundant melting of a spinel peridotite and the production of large volumes of tholeiitic magma with low La/Yb and Gd/Yb ratios. Negative correlations of these ratios with MgO contents, Nd isotopes and Rb/Nd ratios indicate that the parental basalts subsequently experienced high-pressure fractional crystallization and contamination with a newly exposed felsic continental crust, thus producing the more evolved calc-alkaline compositions. Stronger garnet signatures and marked enrichments in highly incompatible elements in the high-K suite support derivation from a garnet- and phlogopite-bearing pyroxenitic source, presumably formed by reaction of mantle peridotites with hydrous silicic melts derived from the foundering lithologies.This new petrogenetic model for the Altos de Jalisco volcanic district suggests that the loss of mafic lower crust during lithospheric dripping might be balanced by production of abundant flood basalts within continents, and thus indicates that additional mechanisms may be required for the stabilization of andesitic crust on Earth.

Petrogenesis of Carboniferous adakites and Nb-enriched arc basalts in the Alataw area, northern Tianshan Range (western China): Implications for Phanerozoic crustal growth in the Central Asia orogenic belt

Carboniferous volcanic rocks in the Alataw area, Northern Tianshan Range (Xinjiang), consist of early Carboniferous (ca. 320 Ma) adakites and Nb-enriched arc basalts and basaltic andesites (NEBs), and late Carboniferous (ca. 306–310 Ma) mainly high-K calc-alkaline andesites, dacites and rhyolites. The adakites are calc-alkaline, and characterized by high Na 2O/K 2O (1.52–3.32) ratios, negligible to positive Eu anomalies, strong depletion of heavy rare earth elements (e.g., Yb = 0.74–1.47 ppm) and Y (6.7–14.9 ppm), positive Sr and Ba but negative Nb and Ti anomalies, and relatively constant ε Nd( T) values (+ 3.4–+ 6.6) and ( 87Sr/ 86Sr) i ratios (0.7035–0.7042). Some andesitic and dacitic adakite samples exhibit high MgO contents similar to magnesian andesites. The NEBs are sodium-rich (Na 2O/K 2O = 2.03–8.06), and differ from the vast majority of arc basalts in their higher Nb, Zr, TiO 2 and P 2O 5 contents and Nb/Th, Nb/La and Nb/U ratios, and minor negative to positive anomalies in Ba, Nb, Sr, Zr and Ti. They have the highest ε Nd( T) values (+ 6.4–+ 11.6) but varying ( 87Sr/ 86Sr) i ratios (0.7007–0.7063). The high-K calc-alkaline suite is similar to typical ‘normal’ arc volcanic rocks in terms of moderately fractionated rare earth abundance and distinctly negative Eu, Nb, Sr and Ti anomalies. They have ε Nd( T) values (+ 1.2–+ 6.4) and ( 87Sr/ 86Sr) i ratios (0.7018–0.7059). Geochemically, they are similar to coeval I-type granitoids in the Alataw area. Given the presence of early Carboniferous ophiolites in the Northern Tianshan Range, and the isotopically inappropriate compositions of Proterozoic metamorphic basement in the Alataw area, we argue that the Alataw adakites were most probably related to the melting of young subducted crust of the Northern Tianshan Ocean. The NEBs likely originated from mantle wedge peridotites metasomatized by adakites and minor slab-derived fluids. The later high-K calc alkaline suite was generated by AFC processes that acted on melts derived from a mantle wedge metasomatized by hydrous fluids. The larger range of isotopic compositions exhibited by both the NEB and high-K suite, relative to the adakites, suggests that the mantle wedge was heterogeneous prior to slab- or fluid-mediated metasomatism. Continental crustal growth of the Central Asian orogenic belt was dominated by contributions of the juvenile materials from the depleted mantle prior to 270 Ma and possibly afterwards. The results of this study suggest that other Carboniferous Nb-enriched basalts in the Tianshan Range were generated by subduction processes rather than by intraplate tectonics as previously proposed.

Petrology and Geochemistry of the Lamongan Volcanic Field, East Java, Indonesia: Primitive Sunda Arc Magmas in an Extensional Tectonic Setting?

New geochemical data are presented from prehistoric and historical multifarious process, involving possible inputs from the subducted oceanic lithosphere, subducted oceanic sedieruptive products of the Lamongan volcanic field (LVF), East Java; a region of the Sunda arc covering >260 km and containing ments and fluids, the asthenospheric and lithospheric portions of the mantle wedge above the subduction zone, >90 eruptive vents plus the historically active Lamongan volcano. LVF lavas include medium-K basalts and basaltic andesites from and the arc crust (e.g. Tatsumi & Eggins, 1995). Following melt generation, processes such as crystal fractionation, historical eruptions of Lamongan and prehistoric eruptions in the eastern LVF, along with a high-K suite represented by prehistoric accumulation and crustal assimilation during transit to the surface may obscure the true nature of the magma deposits in the western LVF. Although lacking some of the characteristics of truly primary basalts, the least evolved lavas identified source region. The effects of these processes on the compositions of erupted lavas must therefore be removed in the LVF have some of the lowest SiO2 contents (>43 wt % SiO2 ) yet reported in Sunda arc volcanic rocks. Mass balance before models of arc magma genesis can be tested. For this reason it is desirable to sample magmas that considerations indicate that two chemically distinct LVF magmas may be parental to suites currently being erupted from the neighbouring have suffered minimal modification since segregation from their source region. The compositions of the primary volcanoes, Semeru and Bromo. Lamongan’s historical lavas can be related to the medium-K andesitic products of Semeru by fractional or primitive magmas from different arcs provide a window on the diverse mechanisms of, and the components crystallization, despite the former’s location at the same distance from the trench as Bromo, a high-K volcano. Extensional tectonics, involved in, melt production at convergent margins, and as such are essential to our understanding of the possibly related to arc segmentation in the region of the LVF, creating subduction zone environment. conditions that promote the rapid ascent of parental magmas, is Many studies have highlighted the correlation between probably responsible for this and several other features of the complex. extensional tectonics in volcanic arcs and the eruption of lavas with primitive chemical characteristics (e.g. Luhr & Carmichael, 1981; Knittel & Oles, 1995; Bacon et al.,

The Merzbacher &amp; Eggler (1984) Geohygrometer: a Cautionary Note on its Suitability for High-K Suites

The Merzbacher &amp; Eggler (1984) Geohygrometer: a Cautionary Note on its Suitability for High-K Suites

The Plio–Quaternary Ambon arc, Eastern Indonesia

Plio–Quaternary lavas and granites have been collected from Ambon, Seram, Kelang, Haruku, Saparua, Ambelau and Banda Api islands, Eastern Indonesia. They include low-K calc-alkaline basalts, andesites, dacites and rhyolites and high-K calc-alkaline andesites, dacites, rhyolites and granites. All these rocks present the usual chemical characteristics of island-arc magmas. The high-K suite of Ambon is mostly represented by cordierite-bearing dacites (known as ambonites) and granites. Low-K and high-K magmas were emplaced in neighbouring islands or even in the same island (Ambon), often concomitantly, during two magmatic pulses at 5–3.2 Ma and 2.3–1 Ma, respectively. We propose that the low-K suite results from the evolution of basaltic magmas derived from mantle melting above the Western Irian Jaya plate which subducts along the Seram trough. Intermediate and acidic rocks of the high-K suite (e.g. ambonites) are thought to derive from low-K mafic magmas through massive assimilation of the Seram–Ambon continental crust, as originally proposed by Van Bemmelen in 1949. The timing of magmatic events and the geochemical features of the studied lavas are clearly different from those of the southern part of the Banda arc, in which the low-K suite is lacking. In agreement with earlier seismic evidence for two different slabs subducting beneath the Seram–Ambon continental block and beneath the southern Banda arc (from Wetar to Manuk), respectively, we propose to recognise a new Plio–Quaternary island arc, i.e. the Ambon arc, extending west–east from Ambelau to the Banda Archipelago active low-K volcanoes through Kelang, southwestern Seram, Ambon, Haruku and Saparua.

Massive Assimilation of Continental Crust by Low-K Subductionrelated Magmas: The Plio-Quaternary Ambon Arc (Eastern Indonesia)

The Ambon Plio-Quaternary arc includes the islands of Ambelau, Kelang, Seram, Ambon, Haruku, Saparua, and Banda Api, Eastern Indonesia. It results from the subduction of the Western PapuaNew Guinea continental shelf along the Serum trough. Its magmatic rocks present the common petrologic and chemical characteristics of island-arc magmas. They include low-K calc-alkaline basalts, andesites, dacites and rhyolites together with high-K calc-alkaline andesites, dacites, rhyolites and granites. Low-K and high-K magmas were emplaced, often concomitantly, during two events which occurred at c. 5-3.2 Ma and 2.3-1 Ma. The low-K suite parent magmas likely derived from the melting of a depleted mantle wedge having incorporated a continental component as described by Vroon et al. (1993) for the Banda arc, possibly through contribution of subducted continent-derived sediments. The high-K suite is represented in Ambon by cordierite-bearing dacites (ambonites) and granites. Both of them contain numerous Al-rich sanidinite-facies xenoliths which display various stages of thermal metamorphism, melting and assimilation of cordierite-sillimanite-garnet-bearing migmatites from the substratum of the arc. The highK suite is characterised by a strong continental crust

Temporal chemical variations in late Cenozoic volcanic rocks around The Bandung Basin, West Java, Indonesia.

The Bandung Basin is located on axis of the Sunda arc in the West Java, Indonesia. This basin is underlain by dominant late Cenozoic volcanics. Lava samples were corrected from Quaternary Sunda and Tangkuban Perahu volcanoes and volcanics underlying these. Age of the lavas have been dated by K-Ar method to be ca. 4.1 Ma-ca. 0.04 Ma. Lavas are basalt to dacite in composition, and SiO2 contents range 51-63 wt.%. Both tholeiitic and calc-alkaline suites exist, and K2O 1evel covers medium-K to high-K suites. We used trace element for the estimation of primary basalt compositions. Model calculations using primary magma incompatible element contents examined the degree of partial melting of the source mantle occurred over the life of the suites. Continuous cooling of the mantle wedge was estimated, and a resurgence of degree of melting seems to have occurred between ca. 1.1 and ca. 0.6 Ma. Axial uplift took place in the Sunda arc during the period. This tectonic event may have been induced by a kinematic change in the subduction of the Indian-Australian Plate beneath the Sunda arc, which also affected the melting state of the upper mantle beneath the area.

The Island Arc Setting of a New Zealand Cambrian Volcano-Sedimentary Sequence: Implications for the Evolution of the SW Pacific Gondwana Fragments

The Waingaro Fault Slice, one of several structural units of the early Paleozoic Takaka Terrane in Northwest Nelson, South Island, New Zealand, consists of six fault-bounded subslices that represent different sections of a late Middle to early Late Cambrian volcano-sedimentary succession. Three basaltic to basaltic-andesitic suites and their volcaniclastic equivalents (150-500 m thick; previously mapped as Devil River Volcanics) are intercalated in a nonvolcanic sequence of turbiditic sandstones and siltstones (approximately 1800 m thick). Compositional systematics of the relatively immobile high field strength elements (HFSE) and rare earth elements (REE) as well as the depositional facies provide strong evidence for an island arc setting for the three basaltic to basaltic-andesitic suites. The oldest (Benson volcanic suite) has incompatible trace elements similar to arc-tholeiites, whereas the two younger suites (Cobb Flat and Peel volcanic suites) show affinities with calc-alkaline and high-K suites. E...

The geochemistry and isotopic composition of the mafic and intermediate igneous components of the Cape Granite Suite, South Africa

A number of small outcrops of igneous rocks, gabbroic to granodioritic in composition, occur scattered throughout the western Cape in South Africa. These outcrops, which are mostly intrusive into the Malmesbury metasedimentary sequence, define a small number of medium-sized plutons. A characteristic feature of the larger plutons is their composite nature. Each pluton consists of multiple intrusive phases; a few show extensive hydrothermal alteration. Plutons from Yzerfontein and Mud River, which occur in the Tygerberg terrane, are high-K calcalkaline (K-trans-alkaline) while all plutons from Malmesbury, which occurs in the Swartland terrane, are tholeiitic. The high-K suite correspond to other I-type Cape granites, whereas the tholeiitic Malmesbury plutons may be a precursor of A-type Cape granites in both the Tygerberg and Swartland terranes. Monzonite from the Yzerfontein pluton yielded a UPb age of 519±7 Ma on zircon mineral separates. Rb-Sr isotope data did not yield reliable ages, possibly due to alteration, but yielded distinct initial isotope ratios for the high-K calc-alkaline and the tholeiitic plutons. The δ 34S isotope ratios of pyrite range from 1 to 2.7% for Yzerfontein and from 3.6 to 4.6% for Mud River, suggesting a magmatic source for the pyrite. PbPb data on whole rock and pyrite samples imply crustal input into all the original magmas.

Pliocene Intrusions in the Gunung Bijih (Ertsberg) Mining District, Irian Jaya, Indonesia: Major- and Trace-Element Chemistry

A group of small, hypabyssal Pliocene intrusions are associated with Cu-Au ore deposits in the Gunung Bijih (Ertsberg) Mining District of Irian Jaya (western New Guinea), Indonesia. Several skarn orebodies (GB, GBT, Dom, and Big Gossan) are located around the margins of the Ertsberg Intrusion. All but the Big Gossan deposit are related genetically to the Ertsberg Intrusion, at least in terms of calc-silicate formation. The supergiant Grasberg porphyry copper deposit is hosted in and related to the first two stages of intrusions in the Grasberg Complex. The intrusions are intermediate in composition, and can be divided into a high-K suite of latite to trachydacite and minor trachyte, and a low-K suite of andesite and dacite. All intrusions are enriched in large-ion lithophile elements (LILE) and have Nb and Ti depletions. The low-K suite tends to have higher La/Th and Ba/Th ratios than the high-K suite. Chemical variation in the Ertsberg Intrusion is largely the result of in situ fractional crystallization...