Relict Igneous Textures Research Articles

Cadomian metabasites of the Eastern Pyrenees revisited

This study presents a new geochemical, petrological, and geochronological U–Pb dataset from Ediacaran metabasites of the Canigó and Cap de Creus massifs, Eastern Pyrenees. The rocks are composed of calcic amphibole + plagioclase + chlorite + epidote ± quartz plus titanite + apatite + ilmenite ± biotite ± rutile as accessory phases and show relict igneous textures. Peak pressure-temperature determinations share common conditions, ranging 452–482ºC and 5.2–7.7kbar. These intermediate P-T conditions suggest Barrovian-type metamorphism, most likely related to a collisional setting. The metabasites correspond to evolved basaltic rocks (Mg#<0.55) with moderate TiO2 content (up to 2.08wt.%) and relatively low Cr (43–416ppm). The rocks are moderately enriched in light rare earth elements (LREE) relative to heavy rare earth elements (HREE) (average (La/Lu)n of 2.7) and the N-MORB normalized multi-element patterns show negative slopes, with prominent negative Nb anomalies ((Nb/La)NMORB=0.33–0.78). These variations are akin to island arc tholeiites generated in back-arc basins and to other metabasites described in the Eastern Pyrenees with a putative Ediacaran age, and they differ from the Ordovician tholeiitic metabasites from the Canigó massif, which derived from a contaminated E-MORB source. The positive ƐNd(T) values (0.82–3.05) of the studied metabasites preclude a notable contribution from an older continental crust. U-Pb dating (LA-ICP-MS) of one chlorite-rich schist sample in contact with the metabasites from the Canigó massif yielded a main peak at ca. 632Ma. We argue that the Cadomian metabasites from the Pyrenees formed during back-arc extension in the continental margin of Gondwana and were later affected by (probably early Variscan) medium-P metamorphism before the HT-LP metamorphism classically described in the Pyrenees.

Petrochemistry and Sr-Nd isotopes of post-collisional Neoproterozoic (ca. 950 Ma) amphibolite dykes of continental flood basalt affinity from the Simdega area: Implications for the geodynamic evolution of the Chhotanagpur Gneissic Complex, Eastern India

Metamorphosed mafic dykes are significant stratigraphic markers which also represent a shift from extensional to compressional geodynamic processes. We present petrology, bulk-rock geochemistry, and Sr-Nd isotope data on nineteen E-W to ENE-WSW trending mafic dykes metamorphosed under amphibolite facies in the Simdega area in the south western domain of the Chhotanagpur Gneissic Complex (CGC). The CGC is widely regarded as an eastern extension of the Central Indian Tectonic Zone (CITZ) which separates the Northern Indian and the Southern Indian blocks. Calcic amphiboles and plagioclases are the dominant minerals whereas clinopyroxene (diopside), biotite, chlorite, quartz, apatite, titanite, ilmenite and magnetite are present in minor amounts. Relict igneous textures are also present in some of the samples. Conventional thermobarometry and pseudosection modelling gave consistent peak temperatures (619 ± 13 °C) and pressures (5.8 ± 0.1 kbar) corresponding to the amphibolite facies. Major oxide geochemistry of the studied samples show differentiation trends and display arc-tholeiitic, high Fe-tholeiitic, high Mg-tholeiitic and calc-alkaline characters. Based on the LREE and HREE fractionation (La N /Yb N , La N /Sm N and Gd N /Yb N ) the studied samples can be classified into two broad groups: Group-I and Group II. In terms of the 87 Sr/ 86 Sr initial and 143 Nd/ 144 Nd initial contents, both the Group I and II samples are confined to an array of continental flood basalts and EM-1 and EM-2 reservoirs. The T DM (depleted mantle) model age of ca. 1.9 Ga suggest the source enrichment of the dykes during the Paleoproterozoic. Trace element ratios of both groups of the amphibolite dykes reveal that the (i) crustal contamination to be minimum, (ii) subduction-related fluid enrichment influenced their source regions, and (iii) mixed arc-, OIB-, and oceanic plateau- type geochemical signatures to be their characteristic features. Petrogenetic modelling involving REE suggest variable degrees of partial melting ranging from 5 to 10% for most of the samples of G-I and 1–3% for G-II dykes from a mixed source comprising predominantly of spinel lherzolite and minor amounts of garnet lherzolite. A three-stage model is proposed for the evolution of the CGC amphibolite dykes which involves: (i) sub-continental lithopheric mantle (SCLM) source enrichment due to the fluids derived from the subduction zone prior to ca. 1.9 Ga, (ii) emplacement of the mafic dykes in a post-collisional intra-continental extensional tectonic setting due interaction between the mantle plume and SCLM during the Tonian period at ca. 0.95 Ga and (iii) subsequent amphibolite facies thermal event at 0.92–0.88 Ga, associated with the amalgamation of the Rodinia supercontinent, that metamorphosed the mafic dykes as well as the much of the south-western part of CGC. Our findings support models proposing a northward subduction of the southern Indian block beneath the northern Indian block along the CITZ as well as bring out the role of a mantle plume in the generation of the Precambrian mafic dykes in the CGC. • Emplacement in a post-collisional intra-continental extensional tectonic setting • Trace elements show arc- as well as plume-derived components. • Nd isotope model ages highlight ancient (1.9 Ga) subduction event in their genesis. • Amphibolite grade metamorphism linked to Rodinia amalgamation

Tectonothermal imprints in a suite of mafic dykes from the Chotanagpur Granite Gneissic complex (CGGC), Jharkhand, India: Evidence for late Tonian reworking of an early Tonian continental crust

Tectonothermal evolution of a set of mafic dykes in the felsic orthogneiss basement of the Chotanagpur Granite Gneissic Complex (CGGC) is presented. The gneissic basement rocks preserve high pressure granulite facies metamorphism (M2, ~800 °C and ~9 kbar) and develop pervasive planar fabric (S2) during a continent-continent collision event at 1000–950 Ma. Mafic dykes cut the S2 fabric of its basement it intrudes. Together with the S2, the mafic dykes have been deformed by three sets of folds viz. DkF1, DkF2 and DkF3. A conspicuous NS trending foliation defined by metamorphic amphibole is developed parallel to the axial plane of DkF1–2. The mafic dykes preserve relict igneous textures (e.g. intergranular texture) defined by clinopyroxene, orthopyroxene and plagioclase. Numerically computed phase diagram (pseudosection) with a representative bulk composition (with <10 vol% metamorphic minerals namely amphibole+ plagioclase rim + quartz) constrain the depth of emplacement of the mafic dykes (15–18 km). The geological features are consistent with rifting of the early Tonian basement of the CGGC and the then mantle beneath it. This lithospheric extension resulted in outpouring of basaltic magma that punctured the early Tonian basement rocks in the form of mafic dykes. Subsequent metamorphism that affected the mafic dykes and its basement culminated at ~700 °C and ~7.5 kbar (M3 metamorphism).The estimated physical conditions during the M3 metamorphic event suggest burial of the previously rifted basement possibly in a continent-continent collisional tectonic setting. The Th-U-total Pb ages of monazite grains from the host felsic orthogneiss date the M2 (ca. 964 Ma) and M3 (ca. 917 Ma) metamorphism. Based on commonality of geological and geochronological history among the CGGC, the Eastern Ghats Mobile Belt and Rayner complex (and its equivalent parts) in east Antarctica corroborates the view that Indo-Antarctic landmass were united and shared the similar geological histories during the entire Tonian period. During the Tonian period the Indo-Antarctic landmass witnessed two major phases of orogenesis (1000–950 Ma and < 920 Ma) that are punctuated by a phase of lithospheric extension manifested by the emplacement of mafic dykes.

Enigmatic nodule‐bearing rocks in the mafic to ultramafic Diana Mills pluton, Piedmont Province, Virginia

The Silurian Diana Mills pluton is a metamorphosed mafic–ultramafic body in the Piedmont Province of Virginia. Major rock types in the pluton include metadiorite and chlorite‐amphibole rock, with lesser amounts of hornblendite. However, the most visually striking rocks, found at 4 locations, consist of tan‐weathering nodules in a dark green matrix. The purpose of this study was to investigate the origin of these nodular rocks. All rocks show evidence of greenschist‐facies metamorphism, but relict igneous textures are preserved locally. Apart from the nodules, Ca‐amphibole is ubiquitous in all rock types. Nodules are dominated by serpentine or talc (+magnetite ± chromite), and many show a thin (≤5 mm) radially oriented shell of serpentine against adjacent matrix. The nodules are ultramafic (Mg# ~80–90, up to7,563 ppm Cr, up to 2,038 ppm Ni), and their normative mineralogy is dominated by olivine and orthopyroxene. Thus, they represent metamorphosed harzburgites and pyroxenites. Matrix minerals are dominantly amphibole + chlorite, along with variable amounts of talc + magnetite. Some matrix samples are chlorite‐rich, probably reflecting metasomatic reaction with nodules (i.e., they are small‐scale “blackwalls”). Matrix samples are also ultramafic (high Mg#, Cr, and Ni). The matrix of the nodule‐bearing outcrops is essentially the same as other chlorite‐amphibole rocks elsewhere in the pluton. We consider these rocks to represent emplacement of an original hornblende peridotite crystal mush (a mixture of crystals and hydrous melt), which locally carried harzburgitic nodules. The nodules most likely represent the earliest‐formed cumulates from the Diana Mills parent magma.

Tectonic implications of the discovery of a Shawinigan ophiolite (Pyrites Complex) in the Adirondack Lowlands

The Pyrites Complex in the Adirondack Lowlands domain of the Grenville Province forms the core of a large NE-trending, elongate, winged structure ∼15 km long dominated by highly deformed metagabbro, amphibolite, and hornblende schist. A previously unrecognized km-scale boudin of metamorphosed ultramafic rocks associated with the belt is described. It is largely replaced by secondary hydrous minerals, but retains relict igneous textures and some primary minerals such as augite and chromite, and is cut by several 1–2-m-wide lamprophyre dikes. The ultramafic rocks are interpreted as part of an obducted ophiolite complex on the basis of its structural contact with a belt of rocks including marine metasedimentary rocks, pyritic gneisses, metagabbros, and amphibolites with mid-ocean ridge basalt chemistry and were emplaced within a collapsing backarc basin of Shawinigan age. Small (50–200 μ) zircon crystals separated from peridotite and pyroxenite yield minimum ages (1140 ± 7 and 1197 ± 5 Ma) and constrain the timing of metamorphic and thermal events associated with the Shawinigan orogenesis and anorthosite-mangerite-charnockite-granite (AMCG) plutonism. Neodymium T DM ages from the Pyrites Complex range from 1440 to 2650 Ma, are not compatible with derivation from a typical depleted mantle reservoir, and suggest, along with incompatible element concentrations, that these rocks record mantle enrichment, presumably during subduction beneath the leading edge of Laurentia. Rifting and development of oceanic crust between the southern Adirondack Highlands and Lowlands, coincident with a similar backarc rifting in the Central Metasedimentary Belt at ca. 1300 Ma, are proposed. Three mafic suites in the Adirondack Lowlands are distinguished by their field relations, age, geochemistry, and Nd isotopic systematics and reflect the various stages of evolution of the Trans-Adirondack backarc basin. Within the Lowlands interleaved evaporites, metasedimentary and possible metavolcanic rocks, calc-alkaline and transitional plutonic rocks, and ophiolitic rocks of the Pyrites Complex provide constraints on the tectonic processes and sedimentary response to development of a backarc basin, magmatic arc, foredeep sedimentation, and ophiolite obduction during Shawinigan convergence from ca. 1200–1160 Ma, which culminated in slab breakoff and plate delamination resulting in intrusion of the AMCG suite throughout the Frontenac-Adirondack terrane and beyond.

Precambrian mafic magmatism in the Western Dharwar Craton, southern India

Mafic rocks of Western Dharwar Craton (WDC) belong to two greenstone cycles of Sargur Group (3.1-3.3 Ga) and Dharwar Supergroup (2.6-2.8 Ga), belonging to different depositional environments. Proterozoic mafic dyke swarms (2.4, 2.0-2.2 and 1.6 Ga) constitute the third important cycle. Mafic rocks of Sargur Group mainly constitute a komatiitic-tholeiite suite, closely associated with layered basic-ultrabasic complexes. They form linear ultramaficmafic belts, and scattered enclaves associated with orthoquartzite-carbonate-pelite-BIF suite. Since the country rocks of Peninsular Gneiss intrude these rocks and dismember them, stratigraphy of Sargur Group is largely conceptual and its tectonic environment speculative. It is believed that the Sargur tholeiites are not fractionated from komatiites, but might have been generated and evolved from a similar mantle source at shallower depths. The layered basic-ultrabasic complexes are believed to be products of fractionation from tholeiitic parent magma. The Dharwar mafic rocks are essentially a bimodal basalt-rhyolite association that is dominated by Fe-rich and normal tholeiites. Calc-alkaline basalts and andesites are nearly absent, but reference to their presence in literature pertains mainly to carbonated, spilitized and altered tholeiitic suites. Geochemical discrimination diagrams of Dharwar lavas favour island arc settings that include fore-, intra- and back-arcs. The Dharwar mafic rocks are possibly derived by partial melting of a lherzolite mantle source and involved in fractionation of olivine and pyroxene followed by plagioclase. Distinctive differences in the petrography and geochemistry of mafic rocks across regional unconformities between Sargur Group and Dharwar Supergroup provide clinching evidences in favour of distinguishing two greenstone cycles in the craton. This has also negated the earlier preliminary attempts to lump together all mafic volcanics into a single contemporaneous suite, leading to erroneous interpretations. After giving allowances for differences in depositional and tectonic settings, the chemical distinction between Sargur and Dharwar mafic suites throws light on secular variations and crustal evolution. Proterozoic mafic dyke swarms of three major periods (2.4, 2.0-2.2 and 1.6 Ga) occur around Tiptur and Hunsur. The dykes also conform to the regional metamorphic gradient, with greenschist facies in the north and granulite facies in the south, resulting from the tilt of the craton towards north, exposing progressively deeper crustal levels towards the south. The low-grade terrain in the north does not have recognizable swarms, but the Tiptur swarm consists essentially of amphibolites and Hunsur swarm mainly of basic granulites, all of them preserving cross-cutting relations with host rocks, chilled margins and relict igneous textures. There are also younger dolerite dykes scattered throughout the craton that are unaffected by this metamorphic zonation. Large-scale geochemical, geochronological and palaeomagnetic data acquisition through state-of-the-art instrumentation is urgently needed in the Dharwar craton to catch up with contemporary advancements in the classical greenstone terrains of the world.

Petrogenesis and tectonic setting of Cadomian felsic igneous rocks, Sandıklı area of the western Taurides, Turkey

In the Sandikli (Afyon) region, western Taurides, the Late Proterozoic rocks of the Sandikli basement complex are composed of low-grade meta-sedimentary rocks (Guvercinoluk Formation) intruded by felsic rocks (Kestel Cayi Porphyroid Suite, KCPS). The KCPS is a deformed and highly sheared, dome-shaped rhyolitic body with a granitic core. Quartz porphyry dikes intrude both the slightly metamorphic igneous and the sedimentary rocks of the basement complex. Both the quartz porphyries and rhyolites were converted into mylonites with relict igneous textures. Geochemical data show that these felsic igneous rocks are subalkaline and mainly granitic in composition with SiO2 >72 wt% and Al2O3 >11.5 wt%. The chondrite-normalized incompatible trace element patterns are characterized by distinct negative Rb, Nb, Sr, P, Ti, and Eu with enrichment in Th, U, La, Ce, Nd, Sm, and Zr. The REE patterns of the felsic rocks indicate a strong enrichment in LREE but display slightly flat HREE patterns. According to geochemical characteristics and petrogenetic modeling, extrusive and intrusive rocks of the KCPS were probably derived from an upper continental crustal source (partial melting of granites/felsic rocks) by 18–20% fractional melting plus 18–20% Rayleigh fractional crystallization, which seems to be the most effective igneous process during the crystallization of the KCPS. Single zircon age data from the granitoids and fossils from the disconformably overlying sedimentary successions indicate that the metamorphism and the igneous event in the Taurides are related to the Cadomian orogeny. Based on the geological, geochemical and petrogenetic correlation of the post-collisional granitoids it is further suggested that the Tauride belt in western central Turkey was in a similar tectonic setting to the Gondwanan terranes in North Africa (Younger Granitoids) and southern Europe (Spain, France, Bohemia, Brno Massifs) during the Late Cadomian period.

EVIDENCE OF EARLY PALAEOZOIC CONTINENTAL RIFTING FROM MAFIC METAVOLCANICS OF SOUTHERN PELORITANI MOUNTAINS (NORTH-EASTERN SICILY, ITALY)

Pre-Hercynian magmatic rocks are widespread in the Palaeozoic basement of the Peloritani Mountain Belt. The metabasites of the Mongiuffi-Melia area (SE Peloritani) represent the largest magmatic products of Early Palaeozoic. These rocks occur as mafic lava flows and metavolcanoclastites often preserving relict igneous textures. They were weakly metamorphosed during the Hercynian orogenesis and show sub-greenschist metamorphic assemblages overprinting the original igneous parageneses. The geochemical features of these rocks indicate an alkali basaltic composition and a within-plate to P-MORB affinity. Basaltic protoliths of metabasites were produced by partial melting of variably enriched mantle sources and experienced fractionation and little crustal contamination during their ascent. Overall data are consistent with a geodynamic environment related to an early stage of tectonically dominated continental rifting. These metabasites represent the only evidences of a Cambro-Ordovician extensional event in the Peloritani domain of the former Alboran microplate. Their features are consistent with a possible location of this terrane at the south-western termination of the South Armorican Ocean during the Early Palaeozoic.

Geology and Geochemistry of the Pre-early Cambrian Rocks in the Sandikli Area: Implications for the Pan-African Evolution of NW Gondwanaland

The pre-Early Cambrian Sandikli Basement Complex in western Central Anatolia comprises a low-grade meta-sedimentary succession (Güvercinoluk Formation) and meta-rhyolites intruded by meta-quartz porphyry rocks (Kestel Cayi Porphyroid Suite). The Güvercinoluk Formation consists of alternation of meta-siltstones and meta-sandstones with olistostromal conglomerates, rare black chert and cherty meta-dolomite lenses. The Kestel Cayi Porphyroid Suite is a deformed, highly sheared dome-shaped rhyolitic body with quartz porphyry rocks. Quartz porphyry dykes intrude both the volcanic carapace and the meta-sedimentary rocks of the Güvercinoluk Formation. Both the meta-quartz porphyry rocks and meta-rhyolites are typically mylonitic with relict igneous textures. Geochemical data indicate that the felsic rocks of the Kestel Cayi Porphyroid Suite are subalkaline and display characteristic features of post-collisional, I-type granitoids. The basement complex is unconformably overlain by variegated conglomerates, mudstones and arkosic sandstones with andesitic lavas, followed by siliciclastic rocks and carbonates that yielded Early Middle Cambrian fossils. Based on the geochemical characteristics of the felsic rocks of Kestel Cayi Porphyroid Suite and the depositional features of the associated sediments it is suggested that the Sandikli Basement Complex is related to a post-collisional extension event in NW Gondwanaland. Similar occurrences elsewhere have been related to a transition from continental plate convergence to continental plate divergence along the Pan-African Belt.

Structural controls on nickel sulphide ore shoots in Archaean komatiite, Kambalda, WA: the volcanic trough controversy revisited

Structural evaluation of the extensive exploration database for the Kambalda–Tramways area (Archaean Yilgarn Craton, Western Australia) is addressing the long outstanding issue of whether the trough structure embayments that partly confine the nickel sulphide (NiS) ore shoots are volcanic or tectonic in origin. The trough structures are complex linear domains associated with talc–carbonate alteration overprinting thickened komatiite overlying footwall basalt. Two end-member trough structure types are recognised: (1) re-entrant trough structures, and (2) open trough structures. The re-entrant trough structures are narrower at the top than at the base, strongly asymmetric in cross-section, restricted in distribution to Kambalda Dome, and transgress and truncate ore. The open trough structures are wider at the top than at the base, weakly asymmetric in cross-section, prevalent outside Kambalda Dome, and are mineralised along their lateral margins. In addition to trough-associated ore, other NiS ore shoots overlie near-planar komatiite-basalt contacts and are overlain by serpentinite with relict igneous textures. All these features indicate the possibility that the trough structures reflect structural folding, thickening and alteration at rock contacts marking different competency during regional fold–thrusting. Consequently, NiS exploration strategies should consider structural and metamorphic features, as well as volcanologic and stratigraphic features. Recognition of folding and thrusting patterns related to major competency contrasts in favourable komatiite–basalt stratigraphy near the crest of regional anticlines and domes would aid exploration success.

ESTRUTURA E DEFORMAÇÃO BRASILIANA NOS DIQUES MÁFICOS DA SERRA DO ESPINHAÇO MERIDIONAL

Mafic dike swarms are abundant in Southern Espinhaço Mountain Range. They are essential tools for theunderstanding of the geodynamic process, marking the beginning of extension tectonic regimes. This paperdescribes the mafic dike swarm of the Southern Espinhaço, their field relationships, petrography anddeformational history. The dykes are: (1) metagabbros with relict igneous textures, N-S oriented, deformedwith un anastomosed protomylonitic foliation; (2) greenschists (sericite-epidote-chlorite schists) withlepidoblastic textures, fine-grained, east-west trending. These dikes represent significant amounts of maficmelts formed during the first stages of the Brasiliano cycle. They have a foliation parallel to the regionalfoliation of the Espinhaço Supergroup, related with the Brasiliano Orogeny.

Geology of an Archaean metakomatiite succession, Tramways, Kambalda Ni province, Western Australia: Assessing the extent to which volcanic facies architecture and flow emplacement mechanisms can be reconstructed

The Kambalda Ni province, located in the Archaean Norseman‐Wiluna greenstone belt of Western Australia, boasts the largest known concentration of komatiite‐associated magmatic Fe–Ni–Cu sulfide deposits. These are found as long, linear massive to disseminated bodies at the base of a thick komatiite sequence. The sulfide bodies are closely associated with, or contained within, trough structures at the contact with the underlying basaltic unit. In this study, the McComish Prospect, located 40 km south of Kambalda at Tramways, was studied to assess the relationships between volcanic facies, mineralisation and trough structures. The rocks in this region have variably experienced four phases of deformation, upper greenschist ‐ lower amphibolite facies metamorphism, granitoid intrusion, and subsequent alteration. Relict igneous textures are locally preserved at McComish, however, enabling the evaluation of existing geological models and interpretations. The McComish trough is considered to be entirely structural in origin and unrelated to primary volcanic processes (e.g. thermal erosion). The association of volcanic textural facies in individual flow units, and the distribution of flow units across the trough is more complex than predicted by prevailing models, suggesting an alternative komatiite lava emplacement mechanism. Results are consistent with the proposal that komatiites did not flow turbulently as widely accepted, nor did they cool by vigorous convection. Alternatively, the lavas were emplaced as inflated, lobate basalt pahoehoe‐like flows. Although Fe–Ni–Cu sulfide mineralisation at McComish is most likely volcanic in origin, its present distribution appears to be structurally controlled or modified. The zone of weakly to strongly disseminated sulfides at the base of the komatiite sequence is thickened adjacent to a major north‐northwest‐trending fault on the western margin of the trough. This fault is interpreted to have been a fluid conduit, remobilising the ore during metamorphism and deformation.

Meta-igneous (non-gneissic) tonalites and quartz-diorites from an extensive ca. 3800 Ma terrain south of the Isua supracrustal belt, southern West Greenland: constraints on early crust formation

In the Itsaq Gneiss Complex south of the Isua supracrustal belt (West Greenland) some areas of early Archaean tonalite and quartz-diorite are non-gneissic, free of pegmatite veins, and in rarer cases are undeformed with relict igneous textures and hence were little modified by heterogeneous ductile deformation under amphibolite facies conditions in several Archaean events. Such well-preserved early Archaean rocks are extremely rare. Tonalites are high Al, and have bulk compositions close to experimental liquids. Trace element abundances and modelling suggest that they probably originated as melts derived from basaltic compositions at sufficiently high pressures to require residual garnet + amphibolites ± clinopyroxene in the source. The major element characteristics of the quartz-diorites suggest these were derived from more mafic magmas than the tonalites, and underwent either igneous differentiation or mixing with crustal material. As in modern arc magmas, high relative abundances of Sr, Ba, Pb, and alkali elements cannot be generated simply from a basaltic source formed by large degrees of melting of a depleted mantle. This may indicate an important role for fluids interacting with mafic rocks in generating the earliest preserved continental crust. The high Ba/Th, Ba/Nb, La/Nb and low Nb/Th, Ce/Pb, and Rb/Cs ratios of these tonalites are also observed in modern arc magmas. SHRIMP U-Pb zircon geochronology was undertaken on seven tonalites, one quartz-diorite, a thin pegmatitic vein and a thin diorite dyke. Cathodoluminescence images show the zircon populations of the quartz-diorite and tonalites are dominated by single-component oscillatory-zoned prismatic grains, which gave ages of 3806 ± 5 to 3818 ± 8 Ma (2σ) (quartz-diorite and 5 tonalites) and 3795 ± 3 Ma (1 tonalite). Dating of recrystallised domains cutting oscillatory-zoned zircon indicates disturbance as early as 3800–3780 Ma. There are rare ca. 3600 Ma and 3800–3780 Ma (very high U and low Th/U) ≤ 20 μm wide partial overgrowths on the prismatic grains. Given likely Zr-undersaturation of precursor melts and evidence of zircon recrystallisation and metamorphic regrowth as early as 3800–3780 Ma, the age determinations on the prismatic oscillatory-zoned zircon populations give the igneous crystallisation age of the tonalite and quartz-diorite protoliths. When the coherency of the geochemistry is considered, these samples represent the best preserved suites of ca. 3800 Ma felsic igneous rocks yet documented.

KREEP cumulates in the western lunar highlands; ion and electron microprobe study of alkali-suite anorthosites and norites from Apollo 12 and 14

Alkali suite anorthosites and norites are the second most common plutonic rock association in the western lunar highlands (after the magnesian suite), but their origin poses an enigma for most petrologic models of lunar crustal evolution. Some models suggest that the alkali and magnesian suites formed from distinct, unrelated parental magmas, whereas other models propose that both suites formed from the same parental magma. The contrast in major element chemistry of the cumulus phases in each suite is difficult to reconcile with their similar incompatible element chemistry. We present herein a detailed ion microprobe (SIMS) and electron microprobe study of seven alkali suite rocks. Our data show that most alkali suite anorthosites preserve major and trace element characteristics acquired during their formation as igneous cumulate rocks, and that these characteristics can be used to reconstruct the parental-magma composition. The data indicate that cumulates of the alkali suite crystallized from magmas with rare-earth element (REE) contents approximately 0.6-2.0Xhigh-K KREEP, and small but consistently positive Eu anomalies (Eu/Eu (super *) approximately 2) relative to KREEP. Snyder and others (1995a) have proposed that the alkali suite parental magma is similar to Apollo 15 pristine KREEP basalt. Our model suggests that the major element composition of cumulus plagioclase in most alkali suite rocks is too sodic for the calculated crystal line-of-descent of pristine KREEP basalt, and that assimilation of pre-existing calcic anorthosite is required. This conclusion is supported by the REE patterns of the alkali-suite parental magma determined here. We propose that alkali suite anorthosites formed as flotation cumulates in KREEPy plutons that may have formed norites as complementary bottom cumulates. The alkali flotation cumulates reflect fractional crystallization of their parental pluton, local equilibrium crystallization, assimilation of plagioclase-rich roof rock, and episodic magma-mixing during convective overturn of the crystallizing magma bodies. Texturally pristine alkali anorthosites exhibit petrographic characteristics that are consistent with their origin as cumulates in a KREEPy pluton, including abundant modal plagioclase, post-cumulus pyroxenes (both augite and pigeonite) that generally lack exsolution lamellae and that have equilibration temperatures of 950-1100 degrees C, relict igneous textures and, in some cases, igneous lamination. The lack of cumulus mafic phases in rocks that should be pyroxene-saturated suggests separation of the plagioclase by flotation, not sinking. Assimilation of plagioclase from older, anorthositic highlands crust is indicated by the high Eu contents of the cumulates and by the positive Eu anomalies in their calculated parental melts relative to high-K KREEP. Mixing of the evolved alkali-suite parental magma with primitive melt occurred episodically, as shown by reverse zoning profiles in some cumulus plagioclase. Injection of this primitive, hot magma into the crystallizing pluton may have induced convective overturn of the magma chamber.

Coronitic metagabbro and eclogite from the Grenville Province of western Quebec: interpretation of U–Pb geochronology and metamorphism

We present new U–Pb and metamorphic data on high-pressure coronitic metagabbros from three distinct structural settings in the Parautochthonous Belt of the Grenville Province in western Quebec. Intrusive ages are (i) [Formula: see text], for metagabbro close to the Grenville Front, correlative with the Sudbury dykes, defined in Ontario; (ii) [Formula: see text] for an eclogitized lens at the base of the highest structural level (SL4), a new age for mafic magmatism in the western Grenville; and (iii) [Formula: see text] for metagabbro from SL4, interpreted as correlative with metagabbros from the Algonquin and Shawanaga domains in Ontario. Metamorphism in all cases is Grenvillian, with the best constrained age of 1069 ± 3 Ma for the metagabbro of SL4. Metamorphic grade increases from the Grenville Front to the south. The mafic rocks preserve relict igneous textures overprinted by garnet + clinopyroxene that developed as coronas and (or) pseudomorphs after igneous phases. The highest grade metagabbros contain omphacite and some lack primary plagioclase, therefore being eclogites. However, interpretation of textures and mineral chemistry indicates that they were equilibrated during decompression (at 1350 MPa and 720 °C, sample 51; and at 1200 MPa and 740 °C, sample 29), so maximum depths of burial remain unconstrained. Their evolution is interpreted as follows: (i) high-pressure metamorphism by burial of the Laurentian margin under accreted terranes thrust toward the northwest between 1080 and 1060 Ma; (ii) residence at intermediate crustal levels, for a few tens of millions of years; and (iii) rapid exhumation by renewed thrusting that led to the emplacement of the high-pressure units over the northerly adjacent structural units of the Parautochthonous Belt.

The petrology of lower crustal xenoliths from the Executive Committee Range, Marie Byrd Land Volcanic Province, West Antarctica

Alkaline magmas from the late-Cenozoic Marie Byrd Land Volcanic Province, West Antarctica, have entrained lithospheric xenoliths which vary from spinel lherzolites to supracrustal rocks. Lower crustal xenoliths have been collected from the Executive Committee Range (Mounts Hampton in the north and Mount Sidley in the south) in central Marie Byrd Land, and their petrological characteristics together with preliminary geochemical data are discussed here. Granulite xenoliths include metaigneous gabbros and norites with varying proportions of clinopyroxene, spinel and either olivine or orthopyroxene. Pyroxenites occur together with granulites, which on the basis of their similar mineral assemblage, texture and composition are considered to be related to the granulites. The composition of xenoliths from Mounts Hampton and Sidley differ (e.g. Mount Sidley xenoliths have Mg# 32–80, are relatively LREE enriched and have 87Sr86Sr of 0.70286–0.70376 and 143Nd144Nd of 0.512864–0.512870, whereas Mount Hampton xenoliths have Mg# 68–78, are LREE depleted and have 87Sr86Sr of 0.70420–0.70458 and 143Nd144Nd of 0.512771–0.512819), defining a major lateral lower crustal discontinuity beneath the Executive Committee Range. Relict igneous textures and low abundances of incompatible elements indicate that the xenoliths initially formed as cumulates rather than as trapped melts. The xenolith suite differs in composition to the host rocks (Mount Sidley volcanics = 87Sr86Sr of 0.70300–0.70312 and 143Nd144Nd of 0.512814–0.512907) and cannot be co-genetic with them. They are interpreted here to represent the cumulates of mantle melts that evolved by crystal fractionation at lower crustal depths.

Geochemical Recognition of a Captured Back-Arc Basin Metabasaltic Complex, Southwestern Oregon

An extensive fault-bounded amphibolite terrane of Late Jurassic ($145 \pm 2 Ma$) metamorphic age occurring in the northeastern Klamath Mountains of southern Oregon has been recognized as the remnants of an ancient back-arc basin. In spite of thorough metamorphic recrystallization under amphibolite-facies conditions, the amphibolite locally displays relict igneous textures which suggest that the protoliths included basaltic dikes or sills, shallow diabase intrusions, and gabbros. The major- and minor-element chemistry of the amphibolite indicates that some alteration of original compositions has occurred. Nevertheless, certain elements (e.g. Zr, Ti, Y, Hf, P, Th, Ta, and the REE) were probably immobile and retain their original igneous abundances. Chondrite-normalized REE patterns are similar to those of mid-ocean ridge basalt (MORB): concave-downward and flat or slightly LREE-depleted, with abundances about six to 30 times chondritic values. Patterns are slightly LREE-enriched and HREE-depleted relative t...

Age and Sr isotopic signature of the Catoctin volcanic province: Implications for subcrustal mantle evolution

Variations in chemical and Sr isotopic signatures of a series of basaltic flows have been correlated with newly mapped stratigraphic sections in the upper Precambrian Catoctin flood basalts in the central Appalachians. Element mobility through fluid interaction is commonly observed in flows with a well-developed fabric, but some samples with more massive textures (commonly in the cores of thicker flows) still retain relict clinopyroxene and igneous textures and show only minor element redistribution. Five whole-rock samples with relict igneous textures and a clinopyroxene mineral separate yield an isochron with an age of 570 +/- 36 Ma (2sigma) and a strontium initial ratio of 0.7035 +/- 1. This age is consistent with recent paleontological evidence for a latest Precambrian time for Catoctin volcanism and, along with isotopic age determinations of 680-710 Ma for other rift-related magmatism, suggests either a two-stage or a very long, protracted episode of continental rifting. Paleogeographic reconstruction indicates that the original site of eruption for the lavas must have been at least 56 km to the east, in the same geographic location from which the Mesozoic Appalachian tholeiite terrane evolved. Variations in Sr isotopic signatures and differences in major-element abundances between these two sequences suggest mantle heterogeneitiesmore » that must have developed beneath the eastern North American craton during the interim 400 m.y.« less

Petrography and geochemistry of an unusual Fe-rich basaltic komatiite from Boston Township, northeastern Ontario

The Boston Creek Flow is a thick, Archean layered basaltic komatiite lava flow located 16 km south of Kirkland Lake, Ontario, within the Abitibi greenstone belt. It is superficially similar to the layered flows of Munro Township in that it consists of a basal pyroxenite overlain sequentially by peridotite, pyroxenite, gabbro, and pyroxene spinifex-textured rock and exhibits the typical cumulate komatiite crystallization sequence: olivine–clinopyroxene–plagioclase. The spinifex-textured unit is, however, unusually thick (33 m), and in comparison with other Archean lavas of similar magnesium content, the flow is Fe and Ti enriched and Al and Y depleted. CaO/Al2O3 and Al2O3/TiO2 ratios of 2–3 and 5, respectively, also distinguish it from other Archean magnesium-rich volcanic rocks.The preservation of typical tholeiitic chemistry in lavas underlying the Boston Creek Flow precludes metamorphism and metasomatism as explanations of the flow's enigmatic geochemistry. This is supported further by relict igneous textures exhibited by Ti-bearing oxides in the pyroxenite, gabbro, and spinifex-textured units of the flow. The presence of trellis-textured ilmenite lamellae in titanomagnetite, the textural evidence for symplectic intergrowth of titanomagnetite and clinopyroxene during crystallization, and the failure of assimilation models to explain the coupling of Fe and Ti enrichment with Al depletion suggest rather that the unusual geochemistry of the flow is a primary igneous feature. The Boston Creek Flow is more Al depleted and Fe and Ti enriched than Barberton and Minnesota Al-depleted komatiites (ADK) of the same magnesium number. It is therefore an Fe-rich, layered ADK.

Precambrian(?) Crystallization and Permian(?) Metamorphism of Hypersolvus Granite in the Avalonian Terrane of Rhode Island

The igneous and metamorphic rocks of southeastern Massachusetts and Rhode Island occupy one of the most poorly understood geologic terranes in the northern Appalachians. Interest in this terrane has been renewed with the realization that it may contain importtant evidence concerning the opening and closing of the Atlantic Ocean and its predecessors (Wilson, 1966, 1973) and that some of the rocks in this region may be similar to the Avalon zone of Newfoundland (Williams, 1976; Robinson, 1976; Rast and others, 1976). In this paper, we describe the mineralogy and petrology of some Precambrian (?) granites in Rhode Island. Despite superimposed deformation and metamorphism, it is possible to identify relict igneous textures and to show that these granites formed from iron-enriched, alkaline magmas at a high level in the crust. High-grade meta-morphism and deformation has caused complete recrystallization of the feldspars in parts of the granite. Some of this metamorphism occurred during Permian time, but the effects of earlier episodes cannot be evaluated. Geologic Setting We have studied the Scituate Granite Gneiss (SGG), Hope Valley Alaskite Gneiss (HVA), and the Ten Rod Granite Gneiss (TRG) in southern and central Rhode Island (Quinn, 1971). The age of these granites is probably upper Precambrian or lower Paleozoic based on several whole-rock Rb-Sr isochrons. Day (1968 and unpub. data) has made preliminary analyses that suggest an age of about 600 m.y. for the three units. These preliminary data are supplemented by reports of granites with similar ages from nearby areas (Fairbairn and others, 1967; Hills and Dasch, 1972; Smith and Giletti, 1977; Kovach and others, 1977).