Amphibolite Gneiss Research Articles

U–Pb geochronology of 3810–3630 Ma granitoid rocks south of the Isua greenstone belt, southern West Greenland

Abstract The oldest well preserved granitoid rocks on Earth envelope the Isua greenstone belt, southern West Greenland. The timing and nature of geological events that affected granitoid rocks south of the Isua belt (“southern gneisses”) are refined with new mapping and U–Pb geochronology. These results and previous work allow for a detailed comparison between the southern gneisses and similar granitoid rocks north of the arcuate Isua belt (“northern gneisses”). Zircons from tonalitic gneiss and meta-tonalite that yielded ages of 3810–3795 Ma are interpreted as having grown during igneous crystallisation of their protoliths based on the consistency of ages within and between samples, and the oscillatory internal zoning that is typical of magmatic zircons. Two dated tonalitic rocks are particularly important because they occur as concordant layers within supracrustal rocks: amphibolite with deformed pillow lava structures in the southwestern part of the Isua belt and quartz-rich gneiss (likely derived from a chemical sediment) that forms an enclave in the southern gneisses. Although these layers lack cross-cutting intrusive relationships, they are interpreted as having intruded as sheets into >3800 Ma supracrustal rocks. Most tonalite was converted into gneiss before intrusion of diorite dated at 3658.3±1.2 Ma and deformed again during or shortly after intrusion of 3650–3630 Ma granitic rocks, whereas a few small lenticular zones of meta-tonalite escaped nearly all strain. The timing of amphibolite facies metamorphism may be recorded by zircon and titanite growth in amphibolitic gneiss at 3632–3620 Ma. Other thermal events were recorded by zircon growth at ∼3550 and 2660 Ma, and titanite growth or Pb loss at ∼2650 and 2600 Ma. These new data and interpretations suggest that the southern gneisses underwent many geological events along with the northern gneisses, including >3658 Ma deformation, 3658 Ma diorite intrusion and 3655–3630 Ma crystallisation of granitic rocks during significant metamorphism and deformation. The major differences are that most tonalite in the southern gneisses is 100 my older than the northern tonalite, and the intensity of the Neoarchaean tectonothermal activity increased southward. Any large-scale juxtaposition of the southern gneisses against the northern gneisses, across the Isua belt, likely occurred before intrusion of the 3658 Ma diorite.

Testing the model of late Archean terrane accretion in southern West Greenland: a comparison of the timing of geological events across the Qarliit nunaat fault, Buksefjorden region

The prevailing tectonic model for the Archean gneiss complex of southern West Greenland holds that terranes with different metamorphic histories and protolith ages were juxtaposed between ∼2.80 and 2.72 Ga. This model was tested by using detailed mapping and U–Pb geochronology to compare the timing of geological events across the Qarliit nunaat fault, the boundary between terranes in the Buksefjorden region. The fault was interpreted as a terrane boundary mostly because it is a narrow zone of mylonitic rocks that separates granulite (or retrograde amphibolite) facies gneisses of the Tasiusarsuaq terrane (TT) from prograde amphibolite facies gneisses of the Akulleq terrane (AT), and it separates tonalitic gneisses of different age (3.87–3.60 and 2.80 Ga in the AT versus 2.92–2.86 Ga in the TT). Dating of similar homogeneous gneisses in each terrane yielded ages of 2843±3 Ma in the TT (tonalite) and 2824±2 Ma in the AT (granodiorite, Ikkattoq gneiss). Granulite facies metamorphism in the TT is thought to have coincided with intrusion of a 2805±2 Ma orthopyroxene pegmatite, and zircon and titanite from nearby amphibolitic gneiss yielded ages of 2779±2 and 2660±10 Ma, respectively. Metamorphic minerals in the AT are younger: zircon and titanite from amphibolitic gneiss are 2717±3 and 2510±10 Ma, respectively, and monazite from pelitic schist is 2732±5 and 2708±4 Ma. A concordant pegmatite layer that underwent a substantial amount of shearing along the Qarliit nunaat fault was dated at 2725±2. Other pegmatites that intruded relatively late in the structural history crystallized at 2710±2 in the TT and 2541±2 Ma in the AT. These results support the prevailing tectonic model by showing that (i) similar homogeneous gneisses in each terrane are different age; (ii) ∼2805 Ma granulite facies metamorphism in the TT predated 2740–2700 Ma amphibolite facies metamorphism in the AT; and (iii) shearing along the terrane boundary occurred after ∼2725 Ma.

The Chief Lake complex revisited, and the problem of correlation across the Grenville Front south of Sudbury, Ontario

South of Sudbury, the boundary between the Southern and Grenville provinces, the Grenville Front, is placed along the faulted southeastern edge the ca. 1.46 Ga Chief Lake complex (CLC). A southeast-dipping, thrust-sense mylonite zone lies in the immediate hanging wall of the Grenville Front boundary fault (GFBF), separating the CLC in the Southern Province from metasedimentary, granitoid, anorthositic and amphibolitic gneiss in the Grenville Province to the southeast. The GFBF also coincides with an abrupt break in the continuity of southeast-trending diabase dykes of the 1.24 Ga Sudbury swarm. The northern CLC is a composite intrusion whose units include, and are separated by, large rafts of the early Paleoproterozoic Huronian Supergroup in which relict stratigraphy outlines engulfed folds. Mafic rocks along the northwest contact of the CLC, previously interpreted as metasomatized Nipissing gabbro (2.22 Ga), are in fact a phase of the Chief Lake intrusive suite, commingled with granite. The northwest contact coincides in part with a steep, southeast-side-down, pre-intrusion fault that was later reactivated with reverse sense of displacement. The Chief Lake granitoid rocks become increasingly foliated southeast toward the Grenville Front, but two features suggest that at least some of the deformation of the Chief Lake intrusive suite was syn-emplacement: (1) the granitoid component of commingled rocks may be strongly foliated in places where the mafic component is undeformed, and (2) included Huronian metasedimentary rocks in general lack the constrictional fabric of their enveloping plutonic rocks. Sudbury dykes cut across the fabric of the CLC, but the facts that they are themselves buckled, locally metamorphosed to greenschist facies, and are offset by faults marked by ultramylonite and cataclasite attest to tectonism of Grenvillian age in the footwall for a limited distance northwest of the GFBF. Granitoid rocks dated at 1.74 and 1.47 Ga southwest of the study area, and Archean tonalitic rocks and 2.47 Ga anorthositic gabbro to the northeast, occur on both sides of the Grenville Front. In the study area, units of paragneiss in the hanging wall of the GFBF cannot be assigned to specific Huronian formations in the Southern Province footwall. However, although not proven, it is reasonable to correlate other hanging wall rocks, namely mylonitic gabbroic anorthosite, amphibolite, and granitic gneiss, to the 2.47 Ga East Bull Lake intrusive suite, Nipissing gabbro, and Chief Lake granite, respectively, in the Southern Province. Whereas Sudbury dykes do not intrude the mylonitic rocks immediately above the GFBF, they do reappear to the southeast as dyke segments and pods within mylonite-bounded panels where they cut earlier isoclinal folds and exhibit a progressive increase in metamorphic grade across a gradient shallower than that in the enclosing gneissic rocks. They document that much of the tectonic history recorded by their host rocks is pre-Grenvillian. Even so, there is no evidence that the Grenville Front was ever the locus of a suture.

<b>Earth Sciences</b><br><br>The geochemistry, tectonic setting and origin of the massive melanocratic amphibolite in the Ilesha schist belt, Southwestern Nigeria.

A massive melanocratic amphibolite, (MMA) occurs in Ilesha schist belt within a series of muscovite schists and amphibolite gneiss. Though metamorphosed, MMA shows no obvious textural deformation. Actinolite, tremolite, hornblende and boitite constitute the major minerals in MMA. Minor minerals in MMA include calcite, plagioclase and pyroxene while monazite, zircon and apatite form the accessory minerals. Chemical studies revealed that MMA contains low K2O and Na2O. Its Mg1, Cr and Ni contents are considerably lower than those of similar basalts derived from purely primitive mantle. Enrichment of LREE, negative Eu/Eu* anomaly and occurrence of monazite in its mineralogy are all indications that the precursor magma of MMA contains a sedimentary input. The plots of immobile trace (Zr, Ti, Nb, Y) and rare earth elements during the greenschist-amphibolite metamorphic grade for MMA show that it was derived from a low-K-Tholeiitic magma in a volcanic geotectonic setting (back arc basin). Global Journal of Pure and Applied Sciences Volume , No 1 January (2001) pp. 85-90

Mobility of components in metasomatic transformation and partial melting of gneisses: an example from Sri Lanka

Reaction textures, fluid inclusions, and metasomatic zoning coupled with thermodynamic calculations have allowed us to estimate the conditions under which a biotite–hornblende gneiss from the Kurunegala district, Sri Lanka [hornblende (NMg=38–42) + biotite (NMg=42–44) + plagioclase + quartz + K-feldspar + ilmenite + magnetite] was transformed into patches of charnockite along shear zones and foliation planes. Primary fluid inclusion data suggest that two immiscible fluids, an alkalic supercritical brine and almost pure CO2, coexisted during the charnockitisation event and subsequent post-peak metamorphic evolution of the charnockite. These metasomatic fluids migrated through the amphibolite gneiss along shear zones and into the wallrock under peak metamorphic conditions of 700–750 °C, 5–6 kbar, and aflH2O=0.52–0.59. This resulted in the formation of charnockite patches containing the assemblage orthopyroxene (NMg=45–48) + K-feldspar (Or70–80) + quartz + plagioclase (An28) in addition to K-feldspar microveins along quartz and plagioclase grain boundaries. Remnants of the CO2-rich fluid were trapped as separate fluid inclusions. The charnockite patches show the following metasomatic zonation patterns:

Taltson basement gneissic rocks: U–Pb and Nd isotopic constraints on the basement to the Paleoproterozoic Taltson magmatic zone, northeastern Alberta

The Taltson magmatic zone is a 300 km long, northerly trending belt of Archean to Paleoproterozoic granitic basement gneiss, amphibolite, supracrustal gneissic rocks, and voluminous Paleoproterozoic magmatic rocks underlying northeastern Alberta and the southwestern portion of the Northwest Territories. Taltson basement gneissic rocks exposed in the southern Taltson magmatic zone in northeastern Alberta are composed of a lithologically diverse suite of banded gneissic rocks, including amphibolitic gneiss, paragneiss, and metaplutonic gneiss. Metaplutonic gneissic rocks are predominant; six new U–Pb ages range from 2.4 to 2.1 Ga. Taltson basement also contains tectonic slices and layers of Archean gneissic rocks dated at 3.2 Ga, 3.1 Ga, and 2.6 Ga. εNd(2.2 Ga) values of –1.6 to –14.3 and TDM ages of 2.59–3.73 Ga from Taltson basement rocks (27 new analyses) overlap with values for the western Churchill Province and are interpreted to represent Paleoproterozoic recycling of the latter. The similarity in U–Pb ages and partial overlap in Nd isotopic compositions of the Taltson basement gneissic rocks and the Buffalo Head terrane are consistent with the two domains representing a single Paleoproterozoic crustal entity, which evolved from variable contributions of depleted mantle and preexisting Churchill Province crust. Greater contributions from a depleted mantle component in the Buffalo Head terrane are suggested by the Nd data. Taltson basement gneissic rocks and Buffalo Head terrane may represent earliest Paleoproterozoic magmatism associated with breakup of the western Churchill Province prior to Thelon–Taltson orogenesis.

Taltson basement gneissic rocks: U–Pb and Nd isotopic constraints on the basement to the Paleoproterozoic Taltson magmatic zone, northeastern Alberta

The Taltson magmatic zone is a 300 km long, northerly trending belt of Archean to Paleoproterozoic granitic basement gneiss, amphibolite, supracrustal gneissic rocks, and voluminous Paleoproterozoic magmatic rocks underlying northeastern Alberta and the southwestern portion of the Northwest Territories. Taltson basement gneissic rocks exposed in the southern Taltson magmatic zone in northeastern Alberta are composed of a lithologically diverse suite of banded gneissic rocks, including amphibolitic gneiss, paragneiss, and metaplutonic gneiss. Metaplutonic gneissic rocks are predominant; six new U–Pb ages range from 2.4 to 2.1 Ga. Taltson basement also contains tectonic slices and layers of Archean gneissic rocks dated at 3.2 Ga, 3.1 Ga, and 2.6 Ga. e Nd (2.2 Ga) values of –1.6 to –14.3 and T DM ages of 2.59–3.73 Ga from Taltson basement rocks (27 new analyses) overlap with values for the western Churchill Province and are interpreted to represent Paleoproterozoic recycling of the latter. The similarity in U–Pb ages and partial overlap in Nd isotopic compositions of the Taltson basement gneissic rocks and the Buffalo Head terrane are consistent with the two domains representing a single Paleoproterozoic crustal entity, which evolved from variable contributions of depleted mantle and preexisting Churchill Province crust. Greater contributions from a depleted mantle component in the Buffalo Head terrane are suggested by the Nd data. Taltson basement gneissic rocks and Buffalo Head terrane may represent earliest Paleoproterozoic magmatism associated with breakup of the western Churchill Province prior to Thelon– Taltson orogenesis.

U-Pb geochronologic constraints on Paleoproterozoic tectonism in the Monashee complex, Canadian Cordillera: Elucidating an overprinted geologic history

U-Pb isotopic data and field relationships are used to outline the Paleoproterozoic geologic history of the Monashee complex, the largest of three Paleoproterozoic exposures in the Canadian Cordillera and one of the westernmost exposures of crystalline basement in North America. The effects of Paleoproterozoic tectonism are best preserved in deep structural levels of the complex, where overprinting related to high-grade Cordilleran (early Tertiary) metamorphism and deformation was incomplete. Determination of precise crystallization ages is hindered by the U-Pb age discordance in the zircon, monazite, and titanite; the discordance is attributed to inherited Pb in some of the grains and to Pb loss, overgrowth, and recrystallization that occurred during one or more thermal overprints. Igneous crystallization ages are interpreted from the upper intercepts of linear arrays that are defined by three or more analyses. Varying degrees of confidence are attributed to the ages based on the probability of inheritance. Large bodies of augen orthogneiss and granodioritic orthogneiss yield precise igneous crystallization ages of 2077 ± 2 and 1862 ± 1 Ma, respectively. Crystallization ages of about 2.27 and 2.10 Ga are interpreted with less certainty from dioritic orthogneiss and granitic orthogneiss, respectively. Deformation associated with a migmatitic gneissosity occurred after intrusion of the 2077 ± 2 Ma augen gneiss, the youngest dated rock that contains the fabric, and before intrusion of a 1848 ± 3 Ma granite, the oldest confidently dated rock that postdates the fabric. Postdeformational pegmatite dikes are dated as 1845 ± 3 and 1836 ± 2 Ma. Metamorphism is interpreted as occurring during monazite growth at 2060 ± 1 Ma in pelitic schist and during titanite growth at ca. 1.85 Ga in amphibolitic gneiss. The gneisses are basement to an unconformably overlying cover sequence, the lower part of which was deposited prior to intrusion of a 1852 ± 4 Ma pegmatite. There is sufficient age resolution on the intrusive rocks to conclude that Monashee basement has affinities with other Paleoproterozoic exposures in the Cordillera and the western Canadian shield. The knowledge that a Paleoproterozoic geologic history can be elucidated through the Cordilleran overprint suggests that older histories may be preserved in the other Paleoproterozoic exposures in the Cordillera.

Late Jurassic Oceanic Crust and Upper Cretaceous Caribbean Plateau Picritic Basalts Exposed in the Duarte Igneous Complex, Hispaniola

Four distinct rock units have been recognized near El Aguacate, in the Janico–Juncalito–La Vega area of the Duarte complex (Dominican Republic): (1) serpentinites crosscut by numerous diabasic dikes, (2) basalts interbedded with Late Jurassic ribbon cherts, (3) picrites and ankaramites relatively enriched in incompatible trace elements, and (4) amphibolites and gneissic amphibolites chemically similar to Oceanic Plateau Basalts. Similar Ar‐Ar ages of late magmatic amphibole from a picrite, and hornblende from an amphibolite ($$86.1\pm 1.3$$ Ma and $$86.7\pm 1.6$$ Ma, respectively), suggest that the Duarte picrites are contemporaneous with the Deep Sea Drilling Program Leg 15 and Ocean Drilling Program Leg 126 basalts drilled from the Caribbean oceanic plateau. These basalts are associated with sediments containing Late Cretaceous faunas. Sr, Nd, and Pb data show that enriched picrites and amphibolites are isotopically similar to mafic lavas from previously described Caribbean plateau and Galápagos hotspot basalts. Major element, trace element, and lead isotopic features of Late Jurassic basalts and diabases are consistent with those of normal oceanic crust basalt. However, these basalts differ from typical N‐MORB because they have lower ϵNd ratios that plot within the range of Ocean Island Basalts. These rocks appear to represent remnants of the Caribbean Jurassic oceanic crust formed from an oceanic ridge possibly close to a hotspot. Later, they were tectonically juxtaposed with Late Cretaceous slices of the Caribbean‐Colombian plateau.

Geology of metasedimentary rocks and Late Cretaceous deformation history in the northern Valhalla complex, British Columbia

The Valhalla complex, a Cordilleran metamorphic core complex, is a domal culmination made up of gently dipping interlayered sheets of igneous and supracrustal rocks that were deformed and metamorphosed in the Middle Jurassic and Late Cretaceous, and exhumed by extensional faults in the Eocene. Mapping, fabric, and metamorphic studies of predominantly metasedimentary rocks in Valhalla and Passmore domes in the northern part of the complex, together with published geochronological data, reveal a significant Late Cretaceous tectonic history. This includes extensive magmatism, the culmination of upper amphibolite facies metamorphism (approx. 800°C and 8 GPa), migmatization, development of a dominant penetrative transposition foliation, and localization of strain on ductile thrust faults termed the Gwillim Creek shear zones. The Valhalla assemblage, a package of metasedimentary rocks in Valhalla and Passmore domes, comprises a heterogeneous sequence of pelitic schist, marble, calc-silicate gneiss, psammitic gneiss, metaconglomerate, quartzite, amphibolite gneiss, and ultramafic rocks. Based on the presence of distinct laterally continuous marker units and similar lithologic ordering, we propose that the Valhalla assemblage is correlative with part of the Palaeozoic North American stratigraphic succession. If this is correct, then the Valhalla assemblage represents an inverted sequence of strata that has been thinned by as much as 60%; thinning may have occurred during Late Cretaceous transposition foliation development. The Gwillim Creek shear zones, originally mapped in a restricted locality in Gwillim Creek, were found to merge into one broad, ductile shear zone beneath Valhalla dome and extend throughout the entire Valhalla complex. The general style and timing of Late Cretaceous deformation in the Valhalla complex is characteristic of that found throughout the Shuswap complex in a belt of rocks that were at mid-crustal levels during the Cretaceous. This zone is thought to have accommodated Cretaceous - Early Tertiary shortening in the eastern Cordillera, and is the ductile equivalent of the higher level Rocky Mountain thrust belt to the east.

The Late Cretaceous Duarte Complex and Siete Cabezas Formation (Hispaniola): Caribbean Oceanic Plateau

The Core of the Central Cordillera in Dominican Republic is composed of oceanic crustal fragments which form three distinct assemblages: the Duarte complex, the Siete Cabezas Formation and the serpentinized ultramafic belt. Three units are recognized in the Duarte complex: (i) basalts interbedded with Late Jurassic ribbon cherts; (ii) picrites and ankaramites, (iii) amphibolites and gneissic amphibolites. The Siete Cabezas Formation is composed of pillow basalts and pillow breccias associated with Upper Cretaceous pelagic sediments. Finally, the serpentinites are intruded by deformed diabase dykes. The Upper Jurassic pillow basalts and diabase dykes have major, trace element and lead isotopic compositions that are consistent with those of NMORB. However, the basalts differ from typical NMORB by lower ~Nd ratios which plot within the range of Ocean Island Basalts (OIB). The basalts and massive diabase of the Rio Verde complex are geochemically similar to the Upper Jurassic basalts and dykes. Thus, these rocks likely represent remnants of the Caribbean Jurassic oceanic crust formed from an oceanic ridge possibly close to a hotspot. The Duarte picrites and olivine-free ankaramites are generally intensely deformed and metamorphosed to greenschist facies. When devoided of any deformation, these rocks show preserved igneous textures and mineralogy with augite-diopside sometimes rimmed by Mg-hastingsite. Ar-Ar ages on separate Mg-hastingsite gave 86 4-1.4 Ma. This age is perfectly consistent with those of the basalts drilled from the Caribbean floor. The Duarte rocks are enriched in Nb-, Ta-, Ti-, and LREE like the Dumisseau tholeiites (Haiti) and the enriched basalts of Gorgona. Their eNd(T 86Ma) ratios range from +5.7 to 8.3 and are similar to those of OIB. Their eSr ratios (-17.7 to -4.6) are less homogeneous due to the hydrothermal alteration. Finally, their Pb isotopic ra t ios [(2~176 = 18.966-19.537] reflect the contribution of an HIMU-like enriched component. These isotopic ratios show that the Duarte picrites and the ankaramites are isotopicallyt similar to mafic lavas from previously described occurences of the Caribbean plateau (Costa Rica, Ha'it, Curagao and Gorgona). The amphibolites and gneissic amphibolites are composed of hornblende + plagioclase + epidote 4quartz. Most of the rocks have flat

Geochemistry of meta-igneous rocks from southern Ethiopia: a new insight into neoproterozoic tectonics of northeast Africa

Utilising geochemical data, various discriminant diagrams have been employed to establish the magma type and original tectonic environment for some Neoproterozoic amphibolites, ultrabasic rocks and gabbros of the Moyale area, southern Ethiopia. The gneissic amphibolites are found to have mixed geochemical characteristics indicative of island arc and/or ocean ridge basalts with tholeiitic composition whereas the porphyritic amphibolites show alkalic features with no clear tectonic setting. The ultrabasic and gabbroic units of the Moyale area are described in terms of their relation with mantle melts and parental material. The majority of ultrabasics relate to a cumulate origin and the gabbroic rocks appear as more differentiated magma from the same source. The mainly dunite bodies in the eastern sub-area at Moyale probably represent refractory residues left after variable degree of partial melting of a periodotite mantle. It is concluded that the gneissic amphibolites were probably part of an accreting arc associated with closure of a pre-existing oceanic basin. The ultrabasic and gabbroic rocks (together with the porphyritic amphibolite) are considered to be remnants of oceanic crust.

Characterization and tectonic evolution of a Mesoproterozoic island arc in the southern Grenville Orogen, Llano uplift, central Texas

Integrated field and U-Pb geochronologic data document the presence and tectonic evolution of a distinct lithotectonic terrane in the southeastern Llano uplift, here termed the Coal Creek Domain (CCD). The CCD consists of tonalitic and amphibolitic gneisses, foliated gabbroic, tonalitic, and granodioritic intrusions, and the structurally emplaced Coal Creek Serpentinite. The oldest rocks in the CCD consist of tonalitic gneisses with protolith crystallization ages of 1326 Ma and 1301 Ma and pre-1292 Ma amphibolitic gneisses (all ages are U-Pb zircon). They were intruded by dominantly mafic and intermediate with subordinate amounts of felsic low-K plutonic rocks between 1286 Ma and 1275 Ma. High-temperature metamorphism and local anatexis that resulted in the formation of gneisses and migmatites occurred between 1301 Ma and 1286 Ma and is suggested to have occurred at ∼1292 Ma. Metamorphism at 1256 Ma resulted in the growth of zircons in mafic rocks. The regional significance of this metamorphic event is unclear but it may be related to dynamothermal metamorphism of the CCD. The contemporaneous emplacement of low-K gabbro, tonalite, and granodiorite and their spatial association with serpentinite, combined with published geochemical data, strongly suggest that the CCD represents an ensimatic arc. The Coal Creek Serpentinite is interpreted as a fragment of oceanic crust that formed either basement to the arc, or in a related back-arc or intra-arc setting. Combined field, U-Pb geochronologic, isotopic, and geochemical data indicate that the magmatic, metamorphic, and structural history of the CCD differs from that of coeval rocks elsewhere in the Llano uplift. The boundary between the CCD and deformed and metamorphosed granitic and supracrustal rocks to the north is a zone of intense mylonitization and polyphase deformation. This zone is interpreted as the structural boundary between the arc terrane and units to the north.

Extrapolation of tectonic boundaries across the Labrador shelf: U–Pb geochronology of well samples

Near Saglek Fiord, a northerly trending boundary between the early Archean Saglek block and the middle Archean Hopedale block extends between drill sites which, respectively, sampled Uivak amphibolite gneiss with U–Pb zircon intercept ages of 3742 ± 12 and 2752 ± 42 Ma, and migmatitic Lister gneiss with concordant ages of [Formula: see text] for restite and [Formula: see text], for leucosome. Titanite ages of ca. 2508 Ma are common to both rocks. A nearby metapsammitic gneiss has detrital zircon and monazite ages of 2681 ± 5, 2700 ± 4, ca. 2730, and 2750 ± 2 Ma representing high-grade metamorphism related to the Hopedale–Saglek collision and metamorphic monazite of ca. 2560 Ma age representing metamorphism of the sediment during reactivation of the Saglek–Hopedale suture. Two hundred kilometres southeast, a gneissic granite records a protolith age of 3170 Ma and Late Proterozoic Pb loss. Near the Nain–Makkovik boundary, 1269 ± 4 Ma zircons indicate a significant extension of the Nain Platonic Suite. South of the Makkovik boundary, a foliated granite yielded an upper intercept age defining intrusion at 1895 ± 8 Ma and concordant 1872 ± 5 Ma titanite ages that date subsequent metamorphism. Discordant U–Pb ages from an alkali-feldspar granite also constrain crystallization to ca. 1890 Ma and together with the gneiss represent the previously defined Iggiuk event in the Kaipokok domain. Wells near the southerly end of the transect record 1801 ± 5, 1813 ± 3, and 1806 ± 8 Ma ages, respectively, that are typical of the synorogenic granitoid suite representing the Cape Harrison domain of southern Makkovik Province.

Tectonic burial, thrust emplacement, and extensional exhumation of the Cabot nappe in the Appalachian hinterland of Cape Breton Island, Canada

Silurian imbricate thrusting, Early Devonian high‐grade metamorphic nappe emplacement, and Devonian‐Carboniferous extensional denudation characterize deformation in the Appalachian hinterland of Cape Breton Island. Compressional deformation following Early Silurian arc volcanism features imbrication of Cambrian‐Precambrian basement rocks of Gondwana derivation with Ordovician‐Silurian cover sequences across thick zones of mylonite during south directed transport. High grade metamorphism and gneissic rocks of late Silurian age in the region indicate that significant tectonic burial and crustal thickening occurred as a result of the thrusting. Partial denudation of the high grade assemblages occurred during Early Devonian thrust emplacement of the Cabot nappe toward the northwest, along the Highlands Shear Zone. The nappe is characterized by an amphibolitic gneiss and high‐grade schist complex defining a large folded klippe above Silurian units. Kyanite is widespread within the nappe, and a distinctive feature of the thrust sheet is the dynamothermal metamorphism of cooler greenschist‐grade footwall rocks producing inverted isograds; staurolite is regionally distributed and occurs in pelitic units in the immediate footwall of the Highlands Shear Zone forming a discontinuous halo around the klippe. Greenschist‐grade footwall rocks are exposed in structural windows as a result of folding and faulting. Shear sense indicators along the margins of the Cabot nappe have been rotated into their present positions due to superposed folding, providing apparent movement directions for the nappe. Complete exhumation to surface occurred during Late Devonian extension along the low‐angle Margaree Shear Zone.

Reconnaissance Geochemical and Tectonic Study of the Meta-Igneous Rocks of the Central Structural Domain, Northeastern Brazil

The meta-igneous rocks in the Pianco-Alto Brigida foldbelt, Borborema Province (northeastern Brazil) comprises, in the studied area, greenschists, amphibolite, and felsic gneisses derived from basalt to rhyolite precursors. In chemical terms, the mafic rocks are divided into three subgroups. Group-A metabasites have MgO > 6% and TiO2 > 1.8%. All samples exhibit within-plate chemical characteristics, such as high Zr/Y (6 to 8) and enriched incompatible elements. They have a sloping REE-normalized pattern [(La/Yb)N = 8 to 20]. Group-B metabasic rocks have low TiO2 (< 1.8%) and low Nb/Y and Zr/Y ratios (0.05 to 0.5 and 2.5 to 4.0, respectively), which place them within the mid-oceanic ridge basalt field, with N-MORB characteristics. They show a less differentiated REE-normalized pattern [(La/Y)N = 0.5] than Group-A metabasites. Group-C metabasites have a Nb/Y ratio that is intermediate between Groups A and B, and lower Ti and Nb contents. The metafelsic rocks of Group D show a highly differentiated REE-norma...

Metamorphic events in the eastern Arunta Inlier, Part 2. NdSrAr isotopic constraints

In the Harts Ranges, the Central Province of the eastern Arunta Inlier is characterised by upper amphibolite facies ortho- and paragneisses. Previously published UPb dating of zircons, as well as new SmNd and RbSr data indicate that the intrusive ages of both mafic and felsic orthogneisses are in the range 1780-1740 Ma (Strangways Orogeny). These rocks had initial ϵ Nd values of 0 to − 3 and depleted mantle model ages of 2.5-2.1 Ga and they intruded supracrustal rocks which are now preserved as paragneisses with initial ϵ Nd of about − 15. SmNd and RbSr whole-rock isotope data indicate that the hornblende-rich amphibolitic gneisses and mylonites in the Harts Range Detachment Zone (Ding and James, 1989) were equilibrated at ∼ 1473 Ma (± 133) when the rocks experienced hydration, isotopic homogenisation and thrust faulting. This is the Anmatjira Event described elsewhere in the Arunta Inlier and is a predominantly structural influence with thrust faults and hydrous fluid flux. Accordingly the record of this event by radiogenic geochronometers has been diffuse, a problem also exacerbated by the probable influence on these rocks, of least one more subsequent lower-temperature amphibolite facies metamorphic event. In spite of the indications of ∼ 1470 Ma isotopic homogenisation of amphibolites in the Harts Ranges, RbSr isotopic analyses of all minerals and 40Ar 39Ar from hornblendes yield ages in the range 450-300 Ma. Using the SmNd system, a few garnet-hornblende pairs give 1400-1250 Ma ages, but several of these pairs also yield early Palaeozoic ages. These data suggest that the Central Province of the eastern Arunta Inlier also experienced an early Palaeozoic thermal event, having cooled from temperatures > 550°C during and immediately before the Carboniferous Alice Springs Orogeny. In the Southern Province, south of the Illogwa Shear Zone, micas record Mesoproterozoic ages (1500-1250 Ma) for the RbSr, KAr and ArAr systems, equivalent to the whole-rock ages obtained from the Central Province, to the north of the Illogwa Shear Zone. Clearly the exposed southern basement cooled to <350°C, presumably as a result of uplift and exhumation during the Anmatjira Event, when basement now exposed in the Central Province remained at depth, undergoing one or more additional amphibolite facies events. The spectacular contrast in cooling histories across the Illogwa Shear Zone emphasises that this is an important crustal scale tectonic structure.

Tectonics, alteration and the fractal distribution of hydrothermal veins in the lower ocean crust

Gabbros from Hole 735B preserve the igneous, structural and metamorphic history beneath the Southwest Indian Ridge down to middle amphibolite facies conditions. The gabbro was isolated from the zone of lithospheric necking by detachment faulting, unroofing and block uplift at the inside-corner high of the Atlantis II Transform. Lower crustal accretion, as preserved in the core, is a complex integration of igneous, hydrothermal and tectonic processes. Alteration began at anhydrous granulite conditions as surface faulting penetrated the brittle-ductile transition to form brittle-ductile shear zones. In the amphibolite facies, hydrous alteration around these zones was extensive, while undeformed sections remained virtually unaltered. Amphibole veins formed during the brittle-ductile deformation have a high fractal dimension, reflecting an unclustered distribution, consistent with high strain and cracking rates in the zone of lithospheric necking beneath the ridge. However, the fractal dimensions of the two major gneissic amphibolite zones in Hole 735B are different and suggest that they represent discrete fault zones formed at different times. Below middle amphibolite facies a dramatic drop in the extent of alteration reflects cooling, cracking and alteration under static conditions, similar to layered intrusions. Initially, fracture and vein formation exploited undeformed sections, where elastic strain accumulated during cooling from high temperatures was unrelieved by recrystallization. Plagioclase-diopside veins formed at this time have the same orientation as higher temperature veins, but a mineralogy reflecting much lower rock permeabilities, and a low fractal dimension, reflecting a tight clustering of veins, as would be anticipated for crack formation at low strain rates. Late zeolite and carbonate veins and irregular, smectite-lined, near-vertical cracks also have a very low fractal dimension, reflecting clustering appropriate to low strain, near static conditions. These, however, show no preference for undeformed sections of the core and are oriented with subvertical and subhorizontal maxima. This indicates crack formation under the present day stress field. Thus, elastic strain, due to cooling from high temperatures and extension during lithospheric necking, dissipated after plagioclase-diopside vein formation.

Polymetamorphic evolution of the Lewisian complex, NW Scotland, as recorded by U-Pb isotopic compositions of zircon, titanite and rutile

U-Pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the Lewisian complex and bordering Laxford Front reveal complex discordance patterns indicating multiple Late Archean and Early Proterozoic crystallization, overgrowth and Pb-loss events. The earliest stages in the evolution of the complex remain poorly resolved. Zircon ages of ≥2710 Ma date high-grade metamorphism and magmatism probably related to tectonic and magmatic accretion in a continental arc setting. A distinct event at 2490–2480 Ma, possibly initiated by metamorphism and deformation at high-grade conditions, caused amphibolitization of the granulites and emplacement of granitic pegmatites. This event can be correlated with development of Inverian shear zones and formation of granitoid layers along the Laxford Front. The emplacement of a younger generation of granitoid sheets during the Laxfordian event fromed hydrothermal titanite at ≥1754 Ma in gneisses south of the Laxford Bridge and partially reset older titanite at Scourie. Growth of secondary titanite and rutile also occurred during subsequent low-grade metamorphism at 1690–1670 Ma.

Maramungee; a Proterozoic Zn skarn in the Cloncurry District, Mount Isa Inlier, Queensland, Australia

The significant but subeconomic (1.8 Mt4.4% Zn) Maramungee deposit is 90 km south-southeast of Cloncurry in northwest Queensland. It lies in the Eastern fold belt of the Proterozoic Mount Isa inlier within rocks of the Maronan Supergroup that also hosts the large Pb-Zn-Ag deposits at Cannington and Pegmont. The Maramungee deposit is a product of a series of processes that occurred during the metamorphic evolution of the Eastern fold belt and the brittle structures that host sulfide mineralization are likely to have been active after the emplacement of the late to post-tectonic granites of the Williams batholith.Maramungee is hosted by upper amphibolite (sillimanite-K feldspar zone) gneisses, exceptionally iron-rich metabasites, and foliated granite. Mineralization was restricted to an elongate embayment in the margin of the Maramungee Granite pluton created by the antiformal part of a steeply inclined parasitic F 2 fold. A major photolinear feature lies immediately west of the hinge in both the granite and its host rocks. This structure was initiated by fabric intensification during attenuation of the adjacent fold limb. Mineralization was further localized by a heterogeneous altered rock package including (1) potassic rocks (generally foliated) with large amounts of microcline and/or biotite, (2) skarns (partly discordant) including both manganoan hedenbergite + Fe-Mn-rich grossularite + quartz + apatite rocks and Ca-rich almandine + or - clinopyroxene-bearing metabasites, and (3) a range of compositionally intermediate assemblages distinguished by the coexistence of Fe-Mn, Ca-, and K-bearing metamorphic silicates. Graphite commonly occurs at levels of several modal percent in the altered rocks but magnetite and zincian spinel are absent. The most strongly altered rocks have a chemical component derived from the metasedimentary gneisses and possibly reflect alteration of a unit which previously contained carbonates. This alteration would have involved reduction producing graphite coupled with differential addition of K, Fe, Mn, P, and F.The potassic and skarnoid rocks are distinctive components of a regional pattern of metasomatism that exploited steeply inclined D 2 structures. Early metasomatic fluids in equilibrium with graphite at Maramungee are unlikely to have been evolved from the magnetite-bearing pluton. Sulfides (mainly sphalerite, pyrrhotite, and pyrite) occur in veinlets and microbreccia matrices that overprint all the earlier rock types and alteration assemblages. They are associated with the development of retrograde (hydrous) and comparatively oxidized secondary minerals including Cl- and Fe-rich amphiboles, chlorite, scapolite, calcite, albite, epidote, and muscovite. Hematite is present in the youngest alteration assemblages. Retrograde phase relationships suggest that sulfide deposition was induced by reduction of the infiltrating fluid, initially occurred at temperatures of 450 degrees to 500 degrees C, and may have continued through a significant cooling interval.Zn is the only economically significant metal present, although Pb, Ag, Cu, Cd, and Sb are commonly anomalous and there are sporadically elevated amounts of Au, As, and Mo. Molybdenite is associated with fluorite and enrichments of Rb, Ba, U, Th, and light REE in the early potassic alteration. With the exception of Zn and Cd there is little overall correlation among the chalcophile elements. There is a crude zoning toward higher Cu/Zn ratios in the southern part of the prospect and there are sporadic occurrences of Cu-Fe sulfide mineralization that have no clear relationship to the main Zn zone associated with silicification, muscovite-chlorite alteration, and variably anomalous Au, Ag, Se, Co, REE, and U.