Metasedimentary Inclusions Research Articles

Two-stage hybrid origin of Lachlan S-type magmas: A re-appraisal using isotopic microanalysis of lithic inclusion minerals

Microanalysis of accessory minerals was carried out to elucidate the controversial origin of the classical, ~430 Ma (Silurian) S-type granites and related lithic inclusions from the eastern Lachlan Orogen, southeastern Australia. Inclusions were separated into igneous-derived, mafic microgranular enclaves (MME) and metasedimentary inclusions. Apatite ɛNd(t), and monazite ɛNd(t) isotopic values for granite host and inclusions cover a similar range (~ −4 to −13), but their ɛNd(t) peaks are distinct. Whereas similar unimodal peaks exist for both minerals in the granites, suggesting widespread equilibration during hybridisation of two magmatic components in the host S-type magma, skewed or bimodal peaks characterize the MME and metasedimentary inclusions, indicating their different petrogenetic histories. The isotopic data suggests two endmembers: a mature metasedimentary source with ɛNd(t) of −12, represented by ubiquitous Ordovician quartzose turbidites, and a more radiogenic component represented by a − 5 ɛNd(t) peak in the MME. Zircon ɛ(Hf) values from −3 to −10, and δ18O values from 7.5‰ to 10.5‰, reflect variable incorporation (40–80%) of metasedimentary material with a mantle-like endmember. Temperature estimates for magmatic equilibration are 750-800 °C, though evidence exists for transient higher temperatures in some inclusions. Phase equilibria modelling using the average composition of the Ordovician turbidites as a potential source-rock suggests that melt compositions were uniform, silicic (75–76 wt% SiO2) and peraluminous (ASI = 1.13–1.22), for a wide range of temperatures and water contents. However, neither the modelled residual rock (restite) compositions, nor the analysed metasedimentary inclusions, lie along the Lachlan S-type compositional array. Rather, the compositional array projects to a mafic endmember defined by the MME, suggesting widespread, efficient hybridization with weakly peraluminous, andesitic magmas. Source mixing models (without significant mantle-derived magma input) are rejected on petrological and geological criteria. The andesitic MME-type magmas were hybridized in the lower crust, probably during mafic magma underplating, before incorporation into large, cool, midcrustal S-type magma reservoirs, and the observed MME represent the final mingled stage of that interaction during emplacement at shallow crustal levels. Using a modern example from the Andes, a model is presented of 2-stage hybridization, initially near the Moho, then in the midcrust as hot, hybrid andesitic magmas repeatedly infused a resident, giant, low-T, S-type magma body. Late-stage injection of the MME generated localized inclusion swarms, as at Cowra and Deddick, and promoted eruption of vast S-type ignimbrite sheets during a major magmatic flare-up in the Lachlan orogen.

Sulfur isotope behavior during metamorphism and anatexis of Archean sedimentary rocks: A case study from the Ghost Lake batholith, Ontario, Canada

Recycling of surface-derived sulfur into the deep earth can impart distinct sulfur isotope signatures to magmas. The details of sulfur transfer from sedimentary rocks to magmas (and ultimately igneous rocks) through metamorphism and devolatilization and/or partial melting, however, is difficult to trace. To understand this process in detail we studied multiple-sulfur isotope compositions of sulfides in the Archean (c. 2685 Ma) Ghost Lake batholith (GLB) and its surrounding host metasedimentary rocks of the Superior Craton (Ontario, Canada) by high spatial resolution secondary ion mass spectrometry, complemented by high-precision gas source isotope ratio mass spectrometry measurements. The GLB comprises strongly peraluminous biotite+cordierite, biotite+muscovite, and muscovite+garnet+tourmaline granites to leucogranites, which are thought to represent the partial melts of surrounding metagreywackes and metapelites. The metasedimentary rocks display a range of metamorphic grades increasing from biotite-chlorite (280-380°C) at ∼5 km away from the GLB to sillimanite-K-feldspar grade (∼660°C) immediately adjacent to the batholith, thus providing a natural experiment to understand sulfur isotope variations from low- to high-grade Archean sedimentary rocks, as well as granites representative of their partial melts.We find that metasedimentary sulfide δ34S values increase with progressive metamorphism at most 2-3‰ (from −1‰ up to +1 to +2‰). An increase in δ34S values in pyrrhotite during prograde metamorphism can be explained through Rayleigh fractionation during pyrite desulfidation reactions. Pyrite from all but one of the granite samples preserve δ34S values similar to that of the high-grade metasedimentary rocks, indicating that partial melting did not result in significant fractionation of δ34S. The exception to this is one granite sample from a part of the batholith characterized by abundant metasedimentary inclusions. This sample contains pyrite with heterogeneous and low δ34S values (down to −16‰) which likely resulted from incomplete homogenization of sulfur between the granitic melt and metasedimentary inclusions. Small (several tenths of a permil), mostly positive Δ33S are observed in both the metasedimentary rocks and granites.Our results suggest that Archean strongly peraluminous granites could be a high-fidelity archive to quantify the bulk sulfur isotope composition of the Archean siliciclastic sediments. Further, our findings indicate that subduction of reduced sulfur-bearing sediments in the Archean with δ34S at or near 0‰ should result in release of sulfur-bearing fluids in the mantle wedge with similar values (within a few permil). S-MIF (if initially present in Archean surface material) may be preserved during this process. However, the absence of S-MIF in igneous rocks does not preclude assimilation of Archean sedimentary material as either S-MIF may not be originally present in the Archean sedimentary sulfur and/or homogenization or dilution could obscure any S-MIF originally present in assimilated Archean sediments.

Magmatic differentiation processes at Merapi Volcano: inclusion petrology and oxygen isotopes

Indonesian volcano Merapi is one of the most hazardous volcanoes on the planet and is characterised by periods of active dome growth and intermittent explosive events. Merapi currently degasses continuously through high temperature fumaroles and erupts basaltic-andesite dome lavas and associated block-and-ash-flows that carry a large range of magmatic, coarsely crystalline plutonic, and meta-sedimentary inclusions. These inclusions are useful in order to evaluate magmatic processes that act within Merapi's plumbing system, and to help an assessment of which phenomena could trigger explosive eruptions. With the aid of petrological, textural, and oxygen isotope analysis we record a range of processes during crustal magma storage and transport, including mafic recharge, magma mixing, crystal fractionation, and country rock assimilation. Notably, abundant calc-silicate inclusions (true xenoliths) and elevated δ18O values in feldspar phenocrysts from 1994, 1998, 2006, and 2010 Merapi lavas suggest addition of limestone and calc-silicate materials to the Merapi magmas. Together with high δ13C values in fumarole gas, crustal additions to mantle and slab-derived magma and volatile sources are likely a steady state process at Merapi. This late crustal input could well represent an eruption trigger due to sudden over-pressurisation of the shallowest parts of the magma storage system independently of magmatic recharge and crystal fractionation. Limited seismic precursors may be associated with this type of eruption trigger, offering a potential explanation for the sometimes erratic behaviour of Merapi during volcanic crises.

Geology, Geochemistry, and Mineralogy of the Worthington Offset Dike: A Genetic Model for Offset Dike Mineralization in the Sudbury Igneous Complex

The Worthington offset dike extends for approximately 15 km away from the southwestern margin of the 1.85 Ga Igneous Complex. The dike is zoned with respect to inclusion and sulfide contents. Marginal chilled quartz diorite is transitional into medium-grained quartz diorite. These rocks are sulfide undersaturated, contain small inclusions from the wall rocks, and are preserved along much of the dike. Locally, the dike contains a core of inclusion-rich quartz diorite, which can be choked with inclusions surrounded by semimassive to massive sulfide. The more heavily mineralized inclusion-rich quartz diorite contains 10 to 75 percent amphibolite inclusions, which are petrologically and geochemically similar to the immediately adjacent country-rock amphibolites, locally termed Sudbury gabbros. The semimassive to massive sulfide zones form subvertical pipes, much like the deposits of the Copper Cliff offset dike, and these are associated with locations where the Worthington dike widens from 20 to 30 m to 50 to 80 m. The average metal tenors of the sulfide with > = 5% sulfur are calculated to be 7 percent Ni and 13 percent Cu. Thus the dike ores have a much higher Cu/Ni ratio than orebodies within the contact sublayer (Cu/Ni ~ 1). The medium-grained inclusion-poor quartz diorite and inclusion-rich quartz diorite differ in Ni, Cu, Pt, and Pd abundances, but they have similar major and lithophile trace element abundance levels despite having different inclusion and sulfide contents. Assimilation of inclusions has therefore not significantly changed the composition of the silicate matrix of the inclusion-rich quartz diorite. The composition of the inclusion-poor quartz diorite is a close approximation to average crust and also to the bulk composition of the Igneous Complex rocks. Regional differences in offset geochemistry exist between the North and South Range offset dikes, but we believe that the formation of the inclusion-poor quartz diorite and inclusion-rich quartz diorite silicate magmas predated significant silicate differentiation or silicate gravitational segregation of the melt sheet and that the differences may record primary silicate heterogeneity of the melt sheet. These differences may have developed in response to the different proportions of Archean granitoids relative to Proterozoic sediments and volcanics that contributed to the melt sheet on the North and South Ranges of the Igneous Complex. The first quartz diorite melt was sulfide undersaturated and devoid of amphibolite inclusions. The introduction of this melt took place during or shortly after the generation of the melt sheet and represented a very rapid introduction of quartz diorite magma into radial and concentric dikes with local incorporation of metasedimentary inclusions. The second phase of activity involved the introduction of a sulfide-bearing melt into portions of the quartz diorite melt that were not completely crystallized at the center of the offset dike; this produced the inclusion-rich quartz diorite. The main control on the location of injection of the sulfide-bearing melt appears to be widened domains of the partially crystallized quartz diorite dike that often correspond to contacts between different country-rock types and sulfides, especially where there are gabbro country rocks and/or local development of breccia. The injection of sulfide-enriched melt is interpreted to have taken place along steeply oriented pipes through these heavily brecciated country rocks; this appears to explain why there is a contact relationship between inclusion-rich quartz diorite and inclusion-poor quartz diorite, and also explains the presence of fragments of inclusion-poor quartz diorite and country-rock amphibolites in the inclusion-rich quartz diorite. The introduction of sulfides into these pipes is interpreted to have taken place from the overlying melt sheet and it is suggested that the high Cu/Ni ratios and platinum-group element tenors of these sulfides, compared to contact-style sulfides, indicate that these were some of the earliest sulfides to segregate from the overlying melt sheet. The type and style of brecciation developed in the country rocks along different offset dikes controlled the continuity of the quartz diorite and the location of the orebodies. Intersections of the quartz diorite with domains of breccia appear to have been important in the Copper Cliff offset dike and the development of mineralization may be a response to the presence of favorable channels through which magmatic sulfides were introduced into the dike from the overlying melt sheet. We propose that the quantity of sulfide in a dike is related to the thickness of the overlying melt sheet. The melt sheet thickness was presumably greatest at the Copper Cliff and Frood-Stobie offset dikes, the Worthington offset dike is located below a thinner melt sheet, and the Manchester and Foy offset dikes developed beneath very thin portions of the melt sheet.

Planar deformation features and impact glass in inclusions from the Vredefort Granophyre, South Africa

Abstract— The Vredefort Granophyre represents impact melt that was injected downward into fractures in the floor of the Vredefort impact structure, South Africa. This unit contains inclusions of country rock that were derived from different locations within the impact structure and are predominantly composed of quartzite, feldspathic quartzite, arkose, and granitic material with minor proportions of shale and epidiorite. Two of the least recrystallized inclusions contain quartz with single or multiple sets of planar deformation features. Quartz grains in other inclusions display a vermicular texture, which is reminiscent of checkerboard feldspar. Feldspars range from large, twinned crystals in some inclusions to fine‐grained aggregates that apparently are the product of decomposition of larger primary crystals. In rare inclusions, a mafic mineral, probably biotite or amphibole, has been transformed to very fine‐grained aggregates of secondary phases that include small euhedral crystals of Fe‐rich spinel. These data indicate that inclusions within the Vredefort Granophyre were exposed to shock pressures ranging from <5 to 8–30 GPa.Many of these inclusions contain small, rounded melt pockets composed of a groundmass of devitrified or metamorphosed glass containing microlites of a variety of minerals, including K‐feldspar, quartz, augite, low‐Ca pyroxene, and magnetite. The composition of this devitrified glass varies from inclusion to inclusion, but is generally consistent with a mixture of quartz and feldspar with minor proportions of mafic minerals. In the case of granitoid inclusions, melt pockets commonly occur at the boundaries between feldspar and quartz grains. In metasedimentary inclusions, some of these melt pockets contain remnants of partially melted feldspar grains. These melt pockets may have formed by eutectic melting caused by inclusion of these fragments in the hot (650 to 1610 °C) impact melt that crystallized to form the Vredefort Granophyre.

Metavolcanic and Metasedimentary Inclusions in the Bundelkhand Granitic Complex in Tikamgarh District, Madhya Pradesh

Abstract The Bundelkhand Granitic Complex of central India contains inclusions of Archaean volcanic and sedimentary rocks and orthogneisses, some of which are 15 km by 6 km in size, Earlier workers considered that these inclusions belong to two distinct lithostratigraphic formations separated in time by a migmatization event. Our new data suggest that the migmatization event post-dated all of the volcanic and sedimentary rocks and hence the two-fold stratigraphic subdivision is not viable. Further, intrusion of orthogneisses resulted in an increase in grade of metamorphism in some volcanic and sedimentary rocks specifically near the contact.

Genesis of the Kinabalu (Sabah) granitoid at a subduction-collision junction

The Kinabalu batholith is a late Neogene granitoid in northwestern Sabah (East Malaysia) apparently marking the locus at which subducted South China Basin lithosphere interacted with roots of the northern Sabah collision suture. The exposed batholith comprises a relatively small core of biotite quartz monzodiorite (BQM) grading out to dominant hornblende quartz monzonite (HQM). Both lithologies contain mafic igneous and metasedimentary inclusions and are cut by late-stage aplite dikes. Major element data indicate the BQM (K2O/Na2O ratios ranging 0.72–1.03) represents a low-K type while HQM (K2O/Na2O ratios ranging 1.35–5.58) is a distinct high-K type. Magmaphile element distributions support this distinction, HQM showing higher K, Rb, LREE and lower Ta and Nb contents than BQM, indicating the more extensive interaction of HQM with sial. Least-squares mass balance models suggest that HQM evolved through the combined effects of fractional crystallization and crustal assimilation while BQM was dominated by fractional crystallization. However, similar plagioclase zoning patterns in both lithologies suggest they are comagmatic rather than generated by melting of separate sources. It is concluded that low-K type melts, formed by subduction-induced remelting of underplated lower crust, underwent high pressure sialic contamination with development of high-K character. These provided access to later-formed low-K melts which were less contaminated and consolidated to form the pluton core. The unusual zonation from inner low-K to outer high-K type compositions may indicate that the cessation of subduction prevented upward migration of the melting anomaly and thermal maturation of the pluton.

Chemical and Isotopic Evidence for the Petrogenesis of the Northeastern Idaho Batholith

Sr, Pb, and REE data from samples from igneous rocks, metasedimentary inclusions, and intruded metasedimentary country rocks of the northeastern portion of the Idaho batholith indicate a significant contribution from a lower crustal source region to the intrusive rocks. Rb-Sr and U-Pb data indicate that the source region was depleted in Rb and U and that it was heterogeneous. Zircons separated from a sample collected from the interior of the batholith were found to have rounded cores and euhedral overgrowths. U-Pb isotopic analysis of these zircons and monazite yields a lower intercept age of 60 Ma and an upper intercept age of ~1710 Ma. This is interpreted to be a mixing line between the crystallization age of the granitic rocks (60 Ma old overgrowths) and the average age of the source (1710 Ma old cores). The zircon cores are most likely inherited from the source region of the granite, and not assimilated from the intruded country rocks.

Polyphase metamorphic sedimentary xenoliths from Mt. Amiata volcanics (Central Italy); evidence for a partially disrupted contact aureole

Metasedimentary inclusions in the crustal anatectic rhyodacites of Mt. Amiata volcanic complex can be subdivided into a silica-poor group, rich in aluminous minerals, and a small silica-rich group containing abundant quartz. Textural observations, supported by mineral chemistry provide evidence for three metamorphic events. The regional metamorphic M0 is often associated with deformation, and two subsequent progressive thermometamorphic events, M1 and M2, are caused by the magmatic heat. Mineral assemblages of M1 are indicative for the pyroxene-hornfels facies, and assemblages of M2, in combination with evidence for partial melting and sanidinization, suggest sanidinite facies conditions. The inclusions are interpreted mainly as xenolithic fragments of a contact aureole, which were remetamorphosed after incorporation by the magma. The aureole formed in pre-mesozoic formations during accumulation of the melt in a magmachamber. Constraints on pressure conditions for M1 indicate the possibility of a fairly large depth of the heat-source giving rise to Mt. Amiata geothermal field.

Geochemistry of a peraluminous granitoid suite from North-eastern Victoria, South-eastern Australia

The Koetong Suite of Silurian, 2-mica granitoids was derived from a metasedimentary source and emplaced into Ordovician sediments and metasediments along the eastern margin of the Western Metamorphic Belt of South-eastern Australia. Whole-rock geochemical considerations preclude derivation of the magmas represented by the granitoids from exposed Ordovician metasediments. The magmas were generated by partial melting of material similar in composition to garnet-cordierite gneisses exposed in the adjacent metamorphic belt. Melting at pressures in excess of 5 Kb and temperatures about 750°C produced peraluminous magmas and, when the degree of partial melting approached 25–30%, these magmas became mobile and moved vertically into the overlying Ordovician sediments. During movement from the source region to the zone of emplacement, separation of the melt and refractory residue components of the magma resulted in a range of compositions so that whole-rock analyses of the granitoids are linearly related on major and trace element variation diagrams. Processes such as crystal fractionation and crystal accumulation may have operated locally. The magmas were largely composed of solid material throughout their emplacement histories and the amount of melt may not have exceeded 30–45% at any stage. Metasedimentary inclusions are a reflection of source heterogeneity. After emplacement of the magmas, in situ crystallization of a relatively anhydrous assemblage of minerals led to water contents in residual, intercrystalline, melts sufficiently high for muscovite to begin crystallization at pressures around 4 Kb. Subsequent saturation of intercrystalline residual melt and loss of the resultant volatile phase caused the development of eutectoid intergrowths involving muscovitebiotite-quartz and alkali feldspar.

U-Pb Studies of Zircon Cores and Overgrowths, and Monazite: Implications for Age and Petrogenesis of the Northeastern Idaho Batholith

U-Pb isotopic studies of zircons, many containing xenocrystic cores with euhedral overgrowths, and monazite from igneous rocks and metasedimentary inclusions of the northeastern Idaho batholith yield linear arrays on concordia diagrams. We interpret these as mixing lines between an old component (cores) and a young component (overgrowths and zircons without cores). The lower intercept of such arrays with concordia may yield the minimum age of the rocks if the overgrowths and zircons without cores are discordant, or the crystallization age if they are concordant. Monazites yield apparently concordant ages either equal or less than the lower intercept zircon ages. The samples studied yield lower intercept ages ranging from $73.5 \pm 6 m.y.$ (foliated quartz diorite) to $46.5 \pm 1 m.y.$ (feldspar megacryst granite); ages obtained are consistent with crosscutting relations observed in the field. Upper intercepts yield ages of 1700 to 2349 m.y. These are interpreted to indicate the mean age of xenocrystic zir...