Arc Granitoids Research Articles

Geology, geochemistry and petrogenesis of intrusive rocks of the Wallagga area, western Ethiopia

Crystalline rocks from the western Ethiopian Precambrian terrain comprise two major rock groups: (1) the often migmatised eastern and western high-grade gneisses; and (2) the central low-grade metavolcanosedimentary rocks. Granitoid bodies of different ages and compositions intrude these rocks. Field observations, petrographic investigations, and geochemical features support a two-fold classification of the granitoid rocks. The volcanic arc granitoids (VAG) are most common and include the Ujjukka granite and granodiorite, and the Dhagaa Booqa and Guttin K-feldspar megacrystic granites. The second and geographically more restricted group represents within-plate or anorogenic (A-type) granitoids, typified by the Tullu Kapii syenite. Geochemically, the Tullu Kapii syenite is characterised by moderate to high contents of SiO 2, total alkalis, Y, Nb, Ta, Hf, Zn, Zr, Ga, ΣREE, higher ratios of Fe 2O 3(total)/MgO, Rb Sr , Rb Ba , and lower contents of CaO, MgO, Sr, and Ba, compared to the other granitoids. The VAG group shows chondrite-normalised REE patterns with slightly enriched LREE and flat HREE patterns without significant Eu anomalies. In contrast, the Tullu Kapii A-type granitoid is characterised by a nearly horizontal REE pattern with variable negative Eu anomalies. The Ujjukka granite and granodiorite; and the Dhagaa Booqa and Guttin K-feldspar megacrystic granites originated in a two-step process, which involves batch equilibrium melting of basaltic or andesitic material producing tonalitic magma, followed by fractional crystallisation. The Tullu Kapii syenite was the product of partial melting of source rocks enriched in high field strength elements.

The late Archean Schreiber–Hemlo and White River–Dayohessarah greenstone belts, Superior Province: collages of oceanic plateaus, oceanic arcs, and subduction–accretion complexes

The late Archean (ca. 2.80–2.68 Ga) Schreiber–Hemlo and White River–Dayohessarah greenstone belts of the Superior Province, Canada, are supracrustal lithotectonic assemblages of ultramafic to tholeiitic basalt ocean plateau sequences, and tholeiitic to calc-alkaline volcanic arc sequences, and siliciclastic turbidites, collectively intruded by arc granitoids. The belts have undergone three major phases of deformation; two probably prior to, and one during the assembly of the southern Superior Province. Imbricated lithotectonic assemblages are often disrupted by syn-accretion strike-slip faults, suggesting that strike-slip faulting was an important aspect of greenstone belt evolution. Field relations, structural characteristics, and high-precision ICP–MS trace-element data obtained for representative lithologies of the Schreiber–Hemlo and White River–Dayohessarah greenstone belts suggest that they represent collages of oceanic plateaus, juvenile oceanic island arcs, in subduction–accretion complexes. Stratigraphic relationships, structural, and geochemical data from these Archean greenstone belts are consistent with a geodynamic evolution commencing with the initiation of a subduction zone at the margins of an oceanic plateau, similar to the modern Caribbean oceanic plateau and surrounding subduction–accretion complexes. All supracrustal assemblages include both ocean plateau and island-arc geochemical characteristics. The structural and geochemical characteristics of vertically and laterally dismembered supracrustal units of the Schreiber–Hemlo and White River–Dayohessarah greenstone belts cannot be explained either by a simple tectonic juxtaposition of lithotectonic assemblages with stratified volcanic and sedimentary units, or cyclic mafic to felsic bimodal volcanism models. A combination of out-of-sequence thrusting, and orogen-parallel strike-slip faulting of accreted ocean plateaus, oceanic arcs, and trench turbidites can account for the geological and geochemical characteristics of these greenstone belts. Following accretion, all supracrustal assemblages were multiply intruded by syn- to post-tectonic high-Al, and high-La/Yb n slab-derived trondhjemite–tonalite–granodiorite (TTG) plutons. The amalgamation processes of these lithotectonic assemblages are comparable to those of Phanerozoic subduction–accretion complexes, such as the Circum-Pacific, the western North American Cordilleran, and the Altaid orogenic belts, suggesting that subduction–accretion processes significantly contributed to the growth of the continental crust in the late Archean. The absence of blueschist and eclogite facies metamorphic rocks in Archean subduction–accretion complexes may be attributed to elevated thermal gradients and shallow-angle subduction. The melting of a hotter Archean mantle at ridges and in plumes would generate relatively small, hot, and hence shallowly subducting oceanic plates, promoting high-temperature metamorphism, migmatization, and slab melting. Larger, colder, Phanerozoic plates typically subduct at a steeper angle, generating high-pressure low-temperature conditions for blueschists and eclogites in the subduction zones, and low-La/Yb n granitoids from slab dehydration, and wedge melting. Metasedimentary subprovinces in the Superior Province, such as the Quetico and English River Subprovinces, have formerly been interpreted as accretionary complexes, outboard of the greenstone belt magmatic arcs. Here the greenstone–granitoid subprovinces are interpreted as collages of subduction–accretion complexes, island arcs and oceanic plateaus amalgamated at convergent plate margins, and the neighbouring metasedimentary subprovinces as foreland basins.

Tectonic settings of Archaean (3325-2775 Ma) crustal-supracrustal belts in the West Pilbara Block

The Archaean Pilbara Block comprises six fault-bounded tectonostratigraphic domains. The West Pilbara Block contains, from east to west, Domains 4, 5 and 6. Review of stratigraphic, magmatic, structural and geochronologic data, and new field mapping, indicates that the crustal and supracrustal rock records correlate to three megacycles. Megacycle IVU (3325-3135 Ma) is recorded by an intra-arc basin and arc granitoids in Domain 6, and a back-arc basin in Domains 4 and 5. The domains were an arc-continent orogen from ∼ 3200-3155 Ma, during which the outboard are setting was accreted to the back-arc continental margin. Although Domain 6 is allochthonous with respect to Domains 5 and 4, it was not exotic with respect to the tectonic settings of the Pilbara continent. Megacycle VL (3135-2955 Ma) is recorded by, in decreasing age: (i) a back-arc basin and arc granitoids in Domain 5: (ii) a deep-marine, retroarc or remnant-ocean basin in Domain 4 and a plutonic belt in Domain 5: (iii) a plutonic belt in Domains 5 and 6, and possibly also in Domain 4: and (iv) craton-wide deformation belts, coeval with the Whim Creek basin and granitoid plutons in Domain 5. The Whim Creek basin was a half-graben that developed at a releasing bend (or overstep) along the boundary fault to Domains 4 and 5, and, to the northeast. Domains 4 and 6. The basin was closed by sinistral transpression as the boundary fault between Domains 4 and 6 propagated to the southwest from the releasing bend, and thereby generating a restraining bend geometry. Fundamental strike-slip faults, which are interpreted to have been tectonic-escape responses to a cause external to the preserved Pilbara Block, juxtaposed domains craton-wide. The initial stage of Megacycle VU (2955-2775 Ma) is recorded by the eroded remnants of a rift province. Preserved elements include the Negri basin (Domain 5 only), mafic-ultramafic sub-volcanic layered intrusions, and granitic plutons and related high-level sills. Two periods of granitoid intrusion are preserved. Base-metal sulphide deposits, hosted by the Whim Creek basin, were probably sourced from extensional geofluids in the rift province. A plume origin is likely for the komatiitic parent magmas to sub-volcanic intrusions, and to basalts of the Negri basin. Plume magmatism is interpreted to have been coincident with, but not the cause of, lithospheric extension because plumerelated volcanism post-dates the first period of plutonic intrusion by ∼ 25 million years. Extension was followed by orogenic deformation that converted Domains 4, 5 and 6 to a hinterland fold-thrust belt. Deformation is interpreted to have been caused by collision orogeny during terrane accretion along the western Pilbara margin. Collision orogeny was the last event of a long history of Pacific-type tectonics along the margins of the Archaean Pilbara continent.

Pseudostratigraphy and origin of the Early Proterozoic Payson ophiolite, central Arizona

The Payson ophiolite (ca. 1.73 Ga), exposed within the Early Proterozoic orogenic belt of central Arizona, is a shallow-dipping, pseudostratigraphic sequence of gabbro, sheeted dikes, and submarine volcanic rocks, partly disjointed by later intrusion and deformation. An older basement complex occurs as roof pendants in the gabbro and screens in the sheeted dike complex and was tilted and eroded prior to development of the ophiolite. Gabbro-dike mingling and mutual intrusion in the transition from gabbro to sheeted dikes indicates that the dikes are rooted in the gabbro. Analysis of detailed measured sections of well-exposed sheeted dikes (600 m) reveals 90%–100% dikes with lateral variations in the proportions of tonalitic or dacitic dikes and basement screens. The transition from sheeted dikes to submarine basalts is abrupt and marked by a stratigraphically continuous zone of intensely altered rocks. The sheeted dike complex, pseudostratigraphic structure, and synmagmatic hydrothermal alteration of the Payson ophiolite are diagnostic of a crustal section formed by sea-floor spreading. The Payson ophiolite intrudes, and is intruded by, arc granitoids and is overlain by arc-derived volcanic and sedimentary rocks. A model of an intra-arc basin formed along an arc parallel strike-slip fault is proposed to account for (1) screens of older arc crust, (2) inferred arc-parallel extension, and (3) juxtaposition of the ophiolite with a distinct arc terrane prior to regional convergent deformation and accretion of the arc to North America during the Yavapai orogeny (ca. 1.70 Ga).

GRANITÓIDES SANTA LUZIA: REGISTRO DO MAGMATISMO GRANÍTICO BRASILIANO DO CINTURÃO ARAGUAIA NA REGIÃO DE PARAÍSO DO TOCANTINS (TO)

The Brasiliano granitic magmatism of the Araguaia Belt is expressed, from Xambioa region southwards, by a few isolated stocks (Ramal do Lontra granite, Presidente Kennedy granodiorite) that have been emplaced along the eastern margin of this megastructure. However, in the Paraiso do Tocantins region, the Brasiliano granite bodies appear to be much more abundant and constitute the informally denominated Santa Luzia Granitoids. They occur mainly as small stocks, dykes, veins and lenses intruded into the biotite schist and quartzites of the Estrondo Group (Upper Proterozoic), concordantly or not with the foliation of these metasediments. Such spatial relationship with the host rocks suggests that these granitoids are syn to late-tectonic. Petrographic and geochemical studies revealed that these rocks are peraluminous and consist essentially of granodiorites, monzogranites and pegmatoid granites which possibly resulted from differentiation of a single original magma. K/Rb and Rb/Sr ratios are fairly constant and indicate that Santa Luzia granitoids were originated from weakly fractionated magmas generated within the continental crust. The Ca and Sr contents as well as the 87 Sr/ 86 Sr ratios = 0.707 support this hypothesis and point out the gneissic basement (Colmeia Complex) as the principal source of these magmas. The presence of angular enclaves of schist in the granitoids and the sharp borders of these xenoliths suggest that the supracrustal metasediments contributed only subordinately to the formation of these magmas. Finally, these rocks show a trend similar to that of low Ca granites and plot within the field of the island arc granitoids, but close to the field of syn-collisional granitoids on a Rb-(Y+Nb) diagram. It is concluded that the Santa Luzia granitoids are syn-collisional.

Geochemistry, tectonic setting, and poly-orogenic history of Palaeoproterozoic basement rocks from the Caledonian fold belt of North-East Greenland

Major and trace element chemistry and Rb-Sr isotope data are presented for Palaeoproterozoic basement geneisses within the Caledonian fold belt of North-East Greenland. They indicate that these rocks represent remnants of a Palaeoproterozoic collisional orogen formed near the margin of an Archaean continent. Older tonalitic and granodioritic gneisses represent arc granitoids. Granitic rocks (sensu stricto) of two distinct kinds can be recognised: (1) collision-type granites; and (2) post-orogenic (“A-type”) granites, which may have formed during collapse of this orogen.It is suggested that the Palaeoproterozoic gneisses of North-East Greenland, together with coeval granitoid rocks in Scandinavia and North America, form a T-shaped arrangement of orogenic belts. Similar patterns are also recognised for the later Grenvillian and Caledonian orogenic episodes. Orogenic activity of different ages in North-East Greenland overlaps in space, resulting in very complex geological relationships.

Isotopic composition of lead in ore deposits of the Cordillera Oriental, southeastern Peru

Twenty-five new lead isotope data are provided for 23 sulfide and 2 oxide minerals from 20 ore deposits embracing the wide variety of metallic mineralization present in the Cordillera de Carabaya, southeastern Peru, segment of the central Andean Inner Arc domain. The deposits are subdivided into the following groups: (1) Cu + or - W + or - Mo + or - Sn veins hosted by Permian and Triassic, metaluminous to peraluminous granitoid plutons; (2) Pb-Zn-Ag-Cu veins spatially related to small, Upper Cretaceous stocks of granodioritic composition; (3) major Sn-Cu-Pb-Zn-Ag lodes associated with strongly peraluminous cordierite-biotite monzogranite stocks of mid-Tertiary age; (4) Sn, W, Cu, Pb, Zn, Ag, and Sb veins of undetermined but probably mid-Tertiary age; and (5) Fe + or - Mn + or - Ba manto deposits of poorly constrained age.Collectively, the isotopic data indicate a dominantly crustal lead component in all suites, with analyses plotting well above the 206 Pb/ 204 Pb- 207 Pb/ 204 Pb orogen growth curve of Zartman and Doe (1981); for several mineralized centers (Permo-Triassic and a probable mid-Tertiary Sb deposit), the data overlap the upper crust curve. Although lead isotope data for Cretaceous and mid-Tertiary (group 3) ores exhibit tight clustering in all Pb-Pb plots, the data for Permo-Triassic ores display a large spread and represent the most radiogenic compositions, with 206 Pb/ 204 Pb values ranging from 18.819 to 25.18. Isotopic analyses of acid leaches of sulfide ores from two mineralized centers of Permo-Triassic age define regression lines in 206 Pb/ 204 Pb- 207 Pb/ 204 Pb space and, in at least one case, indicate the presence of a 1,700-Ma radiogenic lead component. Data for group 4 mineralization, of probable mid-Tertiary age, plot near group 3 data in all Pb-Pb diagrams, providing some corrobofating evidence for their inferred mid-Tertiary age; the only exception is the Sb mineralization, which is relatively enriched in both 207 Pb and 208 Pb.The isotopic compositions cannot be accommodated by a single reservoir model because (1) the implied age difference (ca. 100-125 Ma) between groups 2 and 3 in Pb-Pb plots is greater than that for the host granites (ca. 55 Ma); and (2) group 1 mineralization, the oldest analyzed, contains the most radiogenic lead. Multiple source regions or complex mixing models are therefore predicated for the region as a whole.A comparison of the southeastern Peru ore lead data with those available for the metallo-genically similar northwestern Bolivian Sn-W-Ag province reveals broadly similar results for Tertiary Sn polymetallic ores but radically different compositions for (Permo-)Triassic mineralization. Thus, different source regions or processes are indicated for at least the older mineralization. In addition, whereas the Mesozoic and Cenozoic Bolivian ore lead compositions could be accommodated by a common evolving source reservoir(s) (Tilton et al., 1981), the Peruvian data cannot.The lead isotope compositions reported here overlap extensively with much of the Pb isotope data available for central Andean Main Arc mineralization, although our results lie toward the more radiogenic ends of defined fields. We therefore conclude that a model involving mixing of leads from a single source (mantle?) with various crustal reservoirs can account for the observed data spread in Pb-Pb plots. This is consistent with the petrochemistry of the Inner Arc granitoid igneous suites which indicates involvement of mantle-derived components in the anatectic granitoid rocks via magma mixing processes. An important implication of this study is that multiple reservoirs have been involved in the metallogenic evolution of the Central Andean tin belt, a conclusion arrived at earlier from petrologic studies of the associated igneous suites.

Tectonic discrimination of granitoids

Granitoids as categorized by tectonic environment are (1) island arc granitoids (IAG), (2) continental arc granitoids (CAG), (3) continental collision granitoids (CCG), (4) postorogenic granitoids (POG), (5) rift-related granitoids (RRG), (6) continental epeirogenic uplift granitoids (CEUG), and (7) oceanic plagiogranites (OP). Of these, the IAG, CAG, CCG, and POG are considered orogenic granitoids, and the RRG, CEUG, and OP are considered anorogenic granitoids. The discrimination of granitoids is based on the major-element chemistry. Various discrimination plots are presented which sequentially discriminate the different tectonic environments. OP are separated from all other granitoids on the K2O versus SiO2 plot. Discrimination between group I (IAG + CAG + CCG), group II (RRG + CEUG), and group III (POG) granitoids can be achieved by using plots of Al2O3 versus SiO2, FeO(T)/ [FeO(T) + MgO] versus SiO2, and AFM and ACF ternary diagrams. In the figures, group I and group II plot in individual fields. Identification of group III is different, in that group III does not have a unique field in which it plots. Group III is identified because it consistently displays characteristics of both group I and group II. Further discrimination within group I can be accomplished on the basis of Shand's index. Only CCG have A/CNK [AL2O3/(CaO + Na2O + K2O)] values greater than 1.15. It is not possible to discriminate between IAG and CAG. Further discrimination within group II is done using the TiO2 versus SiO2 plot. The proposed discrimination scheme is applied to the Proterozoic granitoids of the midcontinent of the United States. It is shown that the Arbuckle granitoids are not anorogenic as previously thought.

The geochemical characteristics of granitoids in contrasting arcs and comments on magma sources

Geochemical variations in modern subduction-related igneous suites with respect to arc ‘maturity’ in time and space are illustrated using data for both volcanic suites (basalt, andesite and dacite) and intrusive granitoid suites (diorite, tonalite/granodiorite and granite) from circum-Pacific arcs. Using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites: (a) subduction-zone enrichment of lithospheric mantle, locally coupled with crustal assimilation allied with fractional crystallization (AFC) in zones of thickened crust, all of which yield magmas with enhanced concentrations of the large-ion lithophile (LIL) elements K, Rb, Th, U, LREEs, etc; (b) with increasing distance from the active trench, contributions from within-plate sub-continental lithosphere producing mantle-derived magmas with enhanced levels of high-field strength (HFS) elements, among which Nb, Ta, Hf and Y are particularly distinctive. Thus, even for the evolved granitoids of intrusive arc series, ratios of HFS/LIL elements not significantly affected by crystal fractionation, such as (Ta, Nb)/(K, Rb, La), may throw some light on the origin of mafic-intermediate precursor magmas. In terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites. 1. Primitive, calcic arc granitoids with low LIL and HFS element abundances. 2. Normal, calc-alkaline continental arc granitoids with enhanced LIL element abundances and low HFS/LIL ratios. 3. Mature alkali-calcic arc granitoids with high levels of LIL and HFS elements and higher HFS/LIL ratios. 4. Back-arc/anorogenic alkaline granitoids with the highest levels of HFS elements.