Minor Metasedimentary Rocks Research Articles

Tectonic evolution of the South Pamir Orogen: Insights from the Permian to cretaceous magmatism

The Pamir orogen originated through the amalgamation of various arc terranes. Despite undergoing numerous studies, the subduction polarity and time regarding the accretion processes remain inadequately constrained. To address these issues, we carried out a detailed study of zircon and apatite U-Pb ages, zircon Hf isotopes, and whole-rock geochemistry for the Permian to Cretaceous gabbro, volcanic rocks and granitoids from the Southern Pamir and the Rushan-Pshart suture. Three stages of magmatism are identified, including 279–266 Ma gabbro and andesite to dacite, 209–208 Ma granitoids, and 108–103 Ma gabbro and granitoids, respectively. The Permian gabbro and basaltic andesite to dacite in Pshart Range are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), but are depleted in Ta, Nb, and Ti, suggesting formation from an enriched mantle source in a subduction zone. The Late Triassic I-type granitoids are typical arc calc-alkaline magmatic rocks with εHf(t) values varying from −8.7 to −3.5, which mainly sourced from the partial melting of mixed crustal sources predominately consisting mafic rocks with minor metasedimentary rocks. The Early Cretaceous Pshart granites are I-type granites with relatively low εHf(t) values ranging from −14.6 to −2.9, suggesting that they were crust-derived rocks. The Early Cretaceous Bazardara granites are identified as A2-type granites with the εHf(t) values ranging from −18.3 to +5.3, and were derived from mixing of mantle-derived and crust-derived magmas, or interaction between mantle melts and old crustal rocks. The early Cretaceous gabbro exhibits -MORB-like geochemical characteristics with flat REE pattern (La/YbN = 4.96), moderate enrichment of LILEs and weak depletion of Nb, Ta and Ti (Nb/LaPM = 0.94). Thus, the early Cretaceous A2-type granite, E-MORB-like gabbro and volcanic rocks were formed in an extensional setting. Our new data indicate that the southward subduction of the Rushan-Pshart oceanic slab started in the Early Permian and continued to the Late Triassic. Moreover, combining the previous data, we conclude that the Early Cretaceous granites and gabbro in the Southern Pamir formed in an extensional environment due to the southward rollback after northward near-flat subduction of the Neo-Tethyan oceanic lithosphere.

Metamorphism of pelites in the eastern Gangdese magmatic arc and its tectonic implications

冈底斯岩浆弧东端林芝地区出露的中-高级变质岩来自岩浆弧的中、下地壳，是研究岩浆弧深部组成与形成演化的窗口。本文对林芝布久地区西部产出的泥质片岩进行了岩石学和锆石U-Pb年代学研究。研究表明，含夕线石石榴石云母片岩的峰期矿物组合为石榴石+黑云母+斜长石+白云母+夕线石+石英+金红石，经历了角闪岩相变质作用和部分熔融，峰期变质温度和压力条件为~7.4kbar和~715℃。片岩的进变质和部分熔融作用很可能开始于~70Ma，退变质和熔体结晶作用发生在61~48Ma。本文和现有研究成果表明，冈底斯岩浆弧东端变质岩的变质条件存在明显的空间变化，角闪岩相和麻粒岩相变质带变质岩分别代表岩浆弧的中地壳和下地壳组成。同时，岩浆弧的下地壳主要由变质的基性和长英质岩浆岩组成，含少量变质沉积岩，而中地壳主要由变质的花岗质岩浆岩和变质沉积岩组成。我们认为在晚中生代-早新生代，印度大陆与亚洲大陆的碰撞和俯冲的新特提斯洋岩石圈的断离，引起了岩浆弧地壳构造缩短加厚和幔源岩浆增生，进而导致上地壳的沉积岩被埋藏到中、下地壳，并经历长期持续的高温变质和部分熔融作用。本研究不仅揭示出冈底斯岩浆弧经历了晚中生代-早新生代的变质作用，也为岩浆弧的地壳组成与空间变化提供了重要信息。;The Nyingchi area, eastern Gangdese magmatic arc, exposed middle-to high-grade metamorphic rocks, representing the middle and lower crustal components of the arc, and therefore, it provides a window into investigating the deep components, formation, and evolution of the arc. In this paper, we conduct a study of petrology and zircon U-Pb geochronology of pelitic schist from the west of the Bujiu, the Nyingchi area. The results show that the sillimanite-bearing garnet mica schist has the peak assemblage of garnet+biotite+plagioclase+muscovite+sillimanite+quartz+rutile. Phase equilibrium modeling and geothermobarometry reveal that the schists experienced amphibolite-facies metamorphism and partial melting under peak metamorphic conditions of ~7.4kbar and ~715℃. The prograde process and associated partial melting of these schists probably initiated at ~70Ma, and the retrograde process and melt crystallization occurred at 61~48Ma. Combined available results, we conclude that the metamorphic conditions of metamorphic rocks from the eastern Gangdese arc have obvious spatial variation, and amphibolite-facies and granulite-facies metamorphic belt correspond to the component of arc middle crust and lower crust, respectively. Moreover, the Gangdese arc lower crust is composed mostly of meta-mafic and felsic igneous rocks, including minor meta-sedimentary rocks, and meta-granitic igneous rocks and meta-sedimentary rocks are the main components of the middle crust. We suggest that the continental collision of Indo and Asia and the roll-back and breakoff of the subducted Neo-Tethyan slab during Late Mesozoic-Early Cenozoic, resulted in the shortening and thickening of the Gangdese arc crust and the accretion of voluminous mantle-derived magma, which in turn induced the sedimentary rocks from upper crust transported into the middle and lower crust and experienced long-lasting high-temperature metamorphism and partial melting. The present study does not only reveal that the eastern Gangdese arc experienced Late Mesozoic-Early Cenozoic metamorphism, but also provides a key constraint on the component and spatial variation of the arc crust.

High-pressure granulite-facies metamorphism in central Dronning Maud Land (East Antarctica): Implications for Gondwana assembly

Central Dronning Maud Land (DML; East Antarctica) is located in a key region of the Gondwana supercontinent. The Conradgebirge area (central DML) consists of orthogneisses, derived from both volcanic and plutonic protoliths, and minor metasedimentary rocks, intruded by Cambrian syn- to post-metamorphic plutons and dykes.Mafic-ultramafic boudins in the metavolcanic and metaplutonic gneisses from Conradgebirge consist of amphibolites and high-grade garnet-bearing pyroxene- and amphibole-rich granofels. They occur either as discontinuous levels or as pods boudinaged within highly-strained and strongly-migmatized gneisses. Bulk-rock major and trace-element compositions, together with geochemical discriminant diagrams (e.g., Th/Yb versus Ta/Yb and V versus Ti), suggest derivation from enriched mantle source for the mafic rocks boudinaged in metaplutonic gneisses, whereas a calc-alkaline signature is common for the mafic boudins in metavolcanic rocks. The microstructural study and P–T modelling of an ultramafic metagabbroic rock reveal a prograde metamorphic evolution from amphibolite-facies (ca. 0.5GPa; 500°C) up to high-P granulite-facies conditions (ca. 1.5–1.7GPa; 960–970°C). Partial melting is testified by “nanogranitoid” inclusions enclosed in garnet. An almost isothermal decompression down to ca. 0.4GPa and 750–850°C produced well-developed An+Opx-bearing symplectites around garnet. A final isobaric cooling at nearly 0.4GPa is testified by Grt coronas around high-T symplectites.The above reconstruction traces a clockwise loading-heating P-T evolution with a peak metamorphism at high-P granulite-facies conditions suggesting crustal thickening at nearly 570Ma, followed by a tectonically assisted rapid exhumation, and then, by an isobaric cooling. 40Ar-39Ar dating of amphibole and biotite at ~505–480Ma testify mineral re-equilibration at upper crustal level (T<650°C) during the isobaric cooling.This tectono-metamorphic scenario seems representative of the evolution resulting from the Neoproterozoic/Early Palaeozoic (600–500Ma) collision between parts of East- and West-Gondwana blocks that led to the final assembly of Gondwana.

Widespread Paleoproterozoic basement in the eastern Cathaysia Block: Evidence from metasedimentary rocks of the Pingtan–Dongshan metamorphic belt, in southeastern China

The Cathaysia Block in South China Block (SCB) is famous for its polyphase granitic magmatism and world-class mineralization, especially W, Sn and REE. Although isotopic data suggest that widespread igneous rocks were derived from Precambrian basement, outcrops of the basement rocks are rare in the Cathaysia Block. Therefore, the distribution of the Archean-Paleoproterozoic basement beneath Cathaysia Block is still unclear.The northeastern-striking Pingtan–Dongshan metamorphic belt (PDMB) is located along the coastal region of southeastern China, and consists mainly of voluminous granites and volcanic rocks with minor metasedimentary rocks. These metasedimentary rocks were intruded or captured by 160–143Ma granites. U–Pb dating of detrital and metamorphic zircons from seven metasedimentary rocks indicates that their protoliths were deposited between 183Ma and 160Ma, rather than pre-Devonian as previously proposed. Dating of metamorphic zircons suggests that the most important metamorphism took place at ca 97Ma, consistent with emplacement of A-type granites and mafic magmatism in the coastal region, and implying an extensional setting.Age spectra of detrital zircons in samples from the middle and southern PDMB exhibit a bimodal distribution with abundant Late Paleoproterozoic and Early Mesozoic zircons. In contrast, age spectra of detrital zircons from the northern PDMB display pronounced Neoproterozoic and Early Paleozoic age peaks as well as Late Paleoproterozoic and Early Mesozoic ones, suggesting a different provenance. The comparisons of age and Hf-isotope with the exposed rocks of the SCB suggest that the terrigenous material in these sediments was sourced from the Wuyi terrane, in the eastern Cathaysia Block.The age spectra of detrital zircons from Early Paleozoic to Late Mesozoic sedimentary rocks in the SE SCB show that late Paleoproterozoic (1.9–1.8Ga) detritus increases markedly in Middle Permian to Early Cretaceous sedimentary rocks. Comparison with zircon U–Pb–Hf isotopic data from exposed Paleoproterozoic rocks suggests that a Paleoproterozoic basement existed extensively beneath the eastern Cathaysia Block. It has been uplifted and eroded since Middle Permian time, and then covered by Late Cretaceous volcanic rocks. The uplift of the Paleoproterozoic basement was probably caused by the Late Paleozoic orogeny in the eastern Cathaysia Block.

Petrogenesis and tectonic setting of the Devonian Xiqin A-type granite in the northeastern Cathaysia Block, SE China

Most Silurian-Devonian granites in South China are S- or I-type granites, which are suggested to be petrogenetically related to the Wuyi-Yunkai orogeny. In this paper, we present the detailed LA-ICP-MS zircon U-Pb dating, major and trace element geochemical, and Nd-Hf isotopic data for Xiqin A-type granites in the northeastern Cathaysia Block, SE China. Zircon U-Pb dating results show that the Xiqin granites were emplaced at about 410Ma, indicating that they were generated at the end of Wuyi-Yunkai orogeny. These granites are high in K2O+Na2O (6.31–8.79wt%), high field strength elements (Zr+Nb+Ce+Y=427–699ppm), rare earth elements (total REE=221–361ppm) as well as high Ga/Al ratios (10,000Ga/Al=2.50–3.10), and show characteristics typical of A-type granites. εHf(t) values of the Xiqin granites mainly vary from −0.4 to −3.1 and yield Mesoproterozoic T2DM(Hf) (mainly ranging from 1.29 to 1.45Ga). The εNd(t) values are from −1.23 to −2.11 and T2DM(Nd) vary from 1.25 to 1.32Ga. These isotopic data suggest that the Xiqin granites were generated by partial melting of metavolcanic rocks with minor metasedimentary rocks in the lower crust. Our data on the Xiqin granites, coupled with previous studies of Silurian-Devonian magmatism, suggest that the tectonic regime had changed to a strongly post-collisional extension environment in the Wuyi-Yunkai orogen at least since 410Ma, and that delamination, which accounts for the change in stress from the compression to extension and asthenospheric upwelling during the early Paleozoic, plays a significant role in the generation of Xiqin A-type granites.

Structural setting and age of the Partridge Island block, southern New Brunswick, Canada: a link to the Cobequid Highlands of northern mainland Nova Scotia

The Partridge Island block is a newly identified tectonic element in the Saint John area of southern New Brunswick, located south of and in faulted contact with Proterozoic and Cambrian rocks of the Ganderian Brookville and Avalonian Caledonia terranes. It includes the Lorneville Group and Tiner Point complex. The Lorneville Group consists of interbedded volcanic and sedimentary rocks, subdivided into the Taylors Island Formation west of Saint John Harbour and West Beach Formation east of Saint John Harbour. A sample from thin rhyolite layers interbedded with basaltic flows of the Taylors Island Formation at Sheldon Point yielded a Late Devonian – Early Carboniferous U–Pb (zircon) age of 358.9 +6/–5 Ma. Petrological similarities indicate that all of the basaltic rocks of the Taylors Island and West Beach formations are of similar age and formed in a continental within-plate tectonic setting. West of Saint John Harbour, basaltic and sedimentary rocks of the Taylors Island Formation are increasingly deformed and mylonitic to the south, and in part tectonically interlayered with mylonitic granitoid rocks and minor metasedimentary rocks of the Tiner Point complex. Based on magnetic signatures, the deformed rocks of the Tiner Point complex can be traced through Partridge Island to the eastern side of Saint John Harbour, where together with the West Beach Formation, they occupy a thrust sheet above a redbed sequence of the mid-Carboniferous Balls Lake Formation. The Tiner Point complex includes leucotonalite and aegirine-bearing alkali-feldspar granite with A-type chemical affinity and Early Carboniferous U–Pb (zircon) ages of 353.6 ± 5.7 and 346.4 ± 0.7 Ma, respectively. Based on similarities in age, petrological characteristics, alteration, iron oxide – copper – gold (IOCG)-type mineralization, and deformation style, the Partridge Island block is correlated with Late Devonian – Early Carboniferous volcanic–sedimentary–plutonic rocks of the Cobequid Highlands in northern mainland Nova Scotia. Deformation was likely a result of dextral transpression along the Cobequid–Chedabucto fault zone during juxtaposition of the Meguma terrane.

P–T history and geochemical characteristics of mafic granulites and charnockites from west of Periya, North Kerala, southern India

The study region forms the northern part of Kerala (south India) and constitutes part of granulite–facies rocks of the exhumed Proterozoic south Indian Granulite Terrain (SGT). The SGT consists of a large variety of rock types with a wide range of mineral parageneses and chemical compositions, namely charnockite, mafic granulite, gneiss, schist, anorthosite, granite and minor meta-sedimentary rocks. Garnet–bearing mafic granulites occur as small enclaves within charnockites. We report for the first time P–T constraints on the prograde path preceding peak metamorphism in the northernmost part of Kerala. An increase of the Mg, Fe and decrease of Ca and Mn contents from the core towards the rim of garnet in the mafic granulites suggest prograde garnet growth. The prograde path was followed by peak metamorphism at a temperature of c. 800°C and a pressure of c. 7.5kbar as computed by isopleths of XMg garnet, XCa garnet and XAn plagioclase. The resorption of garnet in various reaction textures and the development of spectacular orthopyroxene–plagioclase, biotite–quartz and hornblende–plagioclase symplectites characterize the subsequent stages of metamorphism. The PT path is characteristically T-convex suggesting an isothermal decompression path and reflects rapid uplift followed by cooling of a tectonically thickened crust. Diffusion modeling of Fe–Mg exchange between garnet and hornblende suggests a near-peak cooling rate of 10–70°C/myr. Such cooling rates are too high to be accounted for by normal isostatic uplift and erosion and suggest that the terrain was tectonically exhumed. Charnockites are richer in SiO2 and lower in MgO and CaO when compared to mafic granulites. Their REE patterns are relatively flat and show prominent negative europium anomalies. The mafic granulites are metamorphosed tholeiitic basalts as revealed by major- and trace-element geochemistry.

U–Pb LA-SF-ICP-MS zircon geochronology of the Serbo-Macedonian Massif, Greece: palaeotectonic constraints for Gondwana-derived terranes in the Eastern Mediterranean

The Pirgadikia Terrane in northern Greece forms tectonic inliers within the Vardar suture zone bordering the Serbo-Macedonian Massif to the southwest. It comprises Cadomian basement rocks of volcanic-arc origin and very mature quartz-rich metasedimentary rocks. U–Pb laser ablation sector-field inductively-coupled plasma mass spectrometry analyses of detrital zircons from the latter reveal a marked input from a Cadomian–Pan-African source with minor contribution from Mesoproterozoic, Palaeoproterozoic and Archaean sources. The metasedimentary rocks are correlated with Ordovician overlap sequences at the northern margin of Gondwana on the basis of their maturity and zircon age spectra. The Pirgadikia Terrane can be best interpreted as a peri-Gondwana terrane of Avalonian origin, which was situated close to the Cadomian terranes in the Late Neoproterozoic–Early Palaeozoic, very much like the Istanbul Terrane. The second unit investigated is the Vertiskos Terrane, which constitutes the major part of the Serbo-Macedonian Massif in Greece. It comprises predominantly igneous rocks of Silurian age and minor metasedimentary rocks of unknown age and provenance. U–Pb analyses of detrital zircons from a garnetiferous mica schist of the Vertiskos Terrane indicate derivation from 550 to 1,150 Ma-old source rocks with a major Cadomian peak. This, combined with minor input of >1,950 Ma-old zircons and the absence of ages between ca. 1.2 and 1.7 Ga suggests a NW Africa source. The protolith age of the garnetiferous mica schist is presumably Early Ordovician. One sample of garnet-bearing biotite gneiss, interpreted as meta-igneous rock, comprises predominantly subhedral zircons of igneous origin with late Middle Ordovician to Silurian ages. We suggest that the rock association of the Vertiskos Terrane is part of an ancient active-margin succession of the Hun superterrane, comparable to successions of the Austro- and Intra-Alpine Terranes. The new data of this study provide evidence of occurrences of Avalonia- and Armorica-derived terranes in the Eastern Mediterranean and moreover help to clarify palaeogeographic reconstructions for the peri-Gondwana realm in the Early Palaeozoic.

Late Archean Lake Harris Komatiite, Central Gawler Craton, South Australia:Geologic Setting and Geochemistry

The Lake Harris Komatiite in the central Gawler craton of South Australia is the first documented komatiite outside the West Australian craton and the easternmost occurrence of such primitive ultramafic rocks in Australia. A U-Pb zircon age of ca. 2520 Ga for the komatiitic sequence indicates a previously unknown period of mantle-plume activity in the Late Archean. An integrated program of airborne magnetic surveys, gravity surveys, and core drilling was successful in defining the distribution and volcanic architecture of the komatiitic flows and associated greenstones through an extensive thin cover of Cenozoic alluvial sediments. Surface exposure of the komatiitic rocks is restricted to one small outcrop near Lake Harris. The greenstones form a series of subparallel east-northeast–trending sinuous magnetic highs flanked by large ovoid to elongate magnetic highs and lows that correlate with Archean-Proterozoic granitic bodies associated with province-wide shear systems, similar to the Archean greenstone terranes in the Yilgarn craton of Western Australia. The steeply dipping greenstone sequence was metamorphosed to middle amphibolite facies during the ca. 2440 Ma Sleafordian orogeny and sheared during the ca. 1700 Ma Kimban and ca. 1540 Ma Kararan orogenies. The greenstones consist of komatiite cumulates (43–32% MgO, anhydrous), high to low Mg komatiite (32–18% MgO), komatiitic and tholeiitic basalt (<18% MgO), pyroxenite cumulates, felsic volcanic rocks, minor metasedimentary rocks, pyroclastic rocks, and rare banded iron formation. They extend over 300 km in three subparallel belts that appear to be isoclinally folded around east-northeast axes and tectonically dismembered to the south by the Yerda shear zone. Komatiitic rocks have been confirmed by drilling in all three belts, but the absence of outcrop and structural complexities prevent detailed stratigraphic correlations within and between the belts. The komatiitic rocks display a range of quenched and cumulus textures defined by the different habits of olivine and its alteration products. Trace sulfides (pyrrhotite, chalcopyrite, pentlandite, pyrite, marcasite, polydymite violarite, heazelwoodite, millerite) form very small (0.01–0.2 mm) single-phase disseminated grains and coarser disaggregated grains. Their distribution largely reflects metamorphic and serpentinization processes, with high Ni/S ratios and probable sulfur loss from the more magnesian parts of the flows. Rare composite pyrrhotite-pentlandite-chalcopyrite blebs (0.1–0.5 mm) characterize some low Mg flows. Locally, there is supergene pyrite-marcasite and native copper-bornite-chalcocite(?) assemblage infilling of late low-temperature serpentine-chlorite veinlets. Thick ponded lava lake and distal composite sheet flow facies have been identified from different parts of the komatiitic sequences. Systematic whole-rock and mineral chemical trends indicate that despite the effects of recrystallization and reequilibration during amphibolite-facies metamorphism, the original magmatic geochemical profiles are largely preserved. The whole-rock data for the Lake Harris Komatiite does not show any obvious Ni depletion during fractionation but indicate a strong olivine control in dominantly sulfur undersaturated environments. Low sulfur (100–600 ppm S) and high Pd + Pt (5–30 ppb) contents, and Ti/Pd ratios of 2 to 4 × 10 5 for the komatiitic rocks are similar to sulfur-undersaturated Archean komatiites hosting Ni-Cu-PGE deposits, i.e., there is little evidence for sulfur saturation in the sampled komatiites. Identification of a pre-2.5 Ga source of sulfur in the substrate would be a positive indicator of potential sulfur saturation of the lavas elsewhere in the greenstone belt and a possible target for mineralization. The Lake Harris Komatiite has chemical (parent magma composition of 29% MgO, Al 2 O 3 /TiO 2 = 16, depleted light (L)REE and initial Nd isotope ( ϵ Nd = 2.8–3.0 at 2520 Ma) characteristics similar to typical Al-depleted Archean komatiites, and there is no clear evidence of chemical modification by processes associated with contemporary subduction. Coherent patterns of trace elements (Th, Nb, REE, Ti, Y, Zr, and P) and typical initial ϵ Nd signatures indicate a Late Archean komatiitic system involving a depleted mantle source and no obvious crustal contamination. This system probably exploited a lithosphere that was stretched and thinned by extension and/or thermal erosion in an intraplate environment. Dynamic tectonic environments involving mantle-plume activity, long-lived active plate margins, and widespread arc volcanism during the Late Archean are now favored for the evolution of the western Gawler craton.

Geological Setting of the West Meliadine Gold Deposits, Western Churchill Province, Nunavut, Canada

Abstract The West Meliadine area is underlain by structurally interleaved panels of mafic and minor ultramafic metavolcanic rocks and metasedimentary rocks that occur along the northern margin of the Neoarchean Rankin Inlet greenstone belt. Three structural and metamorphic domains are recognized: (1) the easterly Wesmeg domain; (2) the central Barracuda-Ridge domain; and (3) the westerly Peter Lake domain. The Wesmeg domain is characterized by a series of southeast-trending, north-dipping, foliation-parallel panels of greenschist facies mafic metavolcanic and metasedimentary rocks. The Barracuda-Ridge domain is comprised of greenschist to amphibolite facies mafic metavolcanic rocks that define an east-northeast-trending structural grain. The Peter Lake domain consists of amphibolite facies mafic metavolcanic rocks and minor metasedimentary rocks intruded by a monzonite pluton. West Meliadine hosts the economically significant Wesmeg gold deposits, as well as other important gold showings across the Barracuda-Ridge and Peter Lake domains. The geological setting of the Wesmeg gold deposits resembles that of a break or fault zone. The Pyke Break is a major geophysical discontinuity (&gt;65-km strike length) and is the first-order structural control on gold mineralization at West Meliadine. It is several kilometers wide and characterized by polyphase deformation and shear zone development accompanied by lode-gold mineralization. In general, gold concentration is related to quartz and iron-carbonate veining, iron sulfides (mainly arsenopyrite and pyrrhotite), and accompanying silicate alteration minerals that overprint favorable chemical and structural traps late in the history of deformation.

Age and Pb-Sr-Nd isotopic systematics of plutonic rocks from the Green Mountain magmatic arc, southeastern Wyoming: Isotopic characterization of a Paleoproterozoic island arc system

Three new U-Pb zircon ages and the Pb-Sr-Nd isotopic systematics of 24 whole-rock samples from mainly plutonic rocks of the Sierra Madre and Medicine Bow Mountains near the Colorado-Wyoming border help establish the Green Mountain magmatic arc as a Paleoproterozoic, variably eroded, island arc terrane. The Green Mountain magmatic arc, a terrane composed of variably metamorphosed volcanic and volcaniclastic rocks, minor metasedimentary rocks, high-grade gneisses, and plutons ranging from gabbro to granodiorite, was formed between ca. 1792 and 1744 Ma. It is the northernmost and oldest part of the Colorado province and is separated from Archean rocks to the north by the east-west-trending Cheyenne belt. New U-Pb zircon ages were determined for two dioritic samples of the Mullen Creek complex (1778 ±2 and 1778 ±17 Ma; an ultramafic/mafic layered intrusion) and for a sample of the Rambler granite (1771 ±3.4 Ma); both units are exposed in the Medicine Bow Mountains. A Sm-Nd internal isochron age of 1750 ±24 Ma (ϵ Nd i = + 3.8) was determined that is within error of the Sm-Nd whole-rock isochron age for the entire Lake Owen sample database (1775 ±45 Ma). Initial Nd signatures (+ 3.3 to 4.8) indicate that the bulk of the arc rocks was derived from a depleted mantle source at 1.78 Ga. Although the Rb-Sr systematics appear disturbed, data from extremely low Rb/Sr, non-hydrous, ultramafic layered units indicate an initial 87 Sr/ 86 Sr value of 0.7024. The range of initial Sr isotopic values for these rocks is elevated relative to depleted mantle sources at 1.78 Ga, an isotopic distinction of modern primitive oceanic island arc systems. The U-Pb data on the same mafic rock samples are consistent with the other isotopic results. The values define average initial Pb values of 206 Pb/ 204 Pb = 15.7 and 207 Pb/ 204 Pb = 15.3, indicative of a depleted mantle source at 1.78 Ga. Felsic plutonic arc rocks exhibit disturbed Pb and Sr isotopic behavior. They are characterized by the same depleted mantle signature with initial ϵ Nd values of ∼2.9–4.4, however, indicating little crustal contamination of source magmas for granites and precluding their derivation by subduction of Archean crustal components during collisional accretion of the arc.

Stratigraphic and structural relationships of multiply deformed Precambrian metamorphic rocks in the Rio Mora area, New Mexico

The contact between a Proterozoic greenstone belt and a Proterozoic quartzite terrane is exposed in the Rio Mora area, New Mexico. Metavolcanic rocks of the Vadito Group, forming the northern edge of the Pecos greenstone belt, crop out along the southern margin of the area. Minor meta-sedimentary rocks are interbedded. A thick section of quartzite, pelitic schist, and graphitic phyllite lies north of the Vadito rocks. This metasedimentary section can be correlated, bed for bed, with Ortega Group rocks of the nearby Picuris Range. The Vadito-Ortega contact appears to be conformable. All rocks are isoclinally folded about south-dipping axial planes, and so Ortega rocks are structurally below Vadito rocks. Most previous workers have accepted this structural superposition as evidence that the Vadito Group is younger. However, abundant cross-beds within Ortega Quartzite, which rests directly against the Vadito, and within quartzites of the overlying Rinconada Formation, show clearly that Ortega Group rocks are younger than Vadito Group rocks. The Vadito-Ortega contact occupies the southern limb of an overturned syncline. Metamorphic rocks have been deformed in two generations of tight to isoclinal folds, with subparallel axes plunging to the southwest. A single F1 syncline is exposed. Its axial plane is folded by an F2 antiform whose axial plane strikes N70°E and dips steeply to the south. Because the two folding events are nearly coaxial, the resulting map pattern appears similar to that formed from a single period of isoclinal folding. Only careful analysis of structural fabrics and structural facing relationships has revealed the multiply folded nature of the rocks.

The geochemical nature of the Archean Ancient Gneiss Complex and Granodiorite Suite, Swaziland: a preliminary study

Abstract The Ancient Gneiss Complex (AGC) of Swaziland, an Archean gray gneiss complex, lies southeast and south of the Barberton greenstone belt and includes the most structurally complex and highly metamorphosed portions of the eastern Kaapvaal craton. The AGC is not precisely dated but apparently is older than 3.4 Ga. The AGC consists of three major units: (a) a bimodal suite of closely interlayered siliceous, low-K gneisses and metabasalt; (b) homogeneous tonalite gneiss; and (c) interlayered siliceous microcline gneiss, metabasalt, and minor metasedimentary rocks — termed the metamorphite suite. A geologically younger gabbro-diorite-tonalite-trondhjemite suite, the Granodiorite Suite, is spatially associated with the AGC and intrusive into it. The bimodal suite consists largely of two types of low-K siliceous gneiss: one has SiO2 14%, low Rb/Sr ratios, and depleted heavy rare earth elements (REE's); the other has SiO2 > 75%, Al2O3 The siliceous gneisses of the metamorphic suite show low Al2O, K2O/Na2O ratios of about 1, high Rb/Sr ratios, moderate REE abundances and negative Eu anomalies. K/Rb ratios of siliceous gneisses of the bimodal suite are very low (∼130); of the tonalitic gneiss, low (∼225); of the siliceous gneiss of the metamorphite suite, moderate (∼300); and of the Granodiorite Suite, high (∼400). Rocks of the AGC differ geochemically in several ways from the siliceous volcanic and hypabyssal rocks of the Upper Onverwacht Group and from the diapirs of tonalite and trondhjemite that intrude the Swaziland Group.

Geologic Evolution of the Beartooth Mountains, Montana and Wyoming. Part 7. Structural Homogeneity of Gneisses in the Lonesome Mountain Area

The Beartooth Mountains represent an elongate block of Archean crystalline rocks (metamorphism dated at 2.7 b.y. by Rb-Sr) which was uplifted during the Laramide Revolution. The Lonesome Mountain area in the heart of the range consists mainly of folded tonalitic to granitic gneisses, migmatites, and ortho-amphibolites with minor metasedimentary rocks, small pegmatites, and orbicular granitoid rocks. Mafic intrusions were emplaced during at least four periods from the Archean to the Tertiary; two of these later underwent metamorphism. Analysis of compositional and mineral foliations, small fold axes, and mineral lineations in gneisses and migmatites defines statistical folds at stations and subdomains. These accurately reflect the geometry of large folds outlined by amphibolite sheets and axial trends constructed from field data. Presence of this pervasive fold system in heterogeneous migmatites and gneisses raises the question of whether the rocks had an intrusive or metasomatic origin. Independent studies of folds and fabric in the nearby Gardner Lake area by Harris, the Southern Beartooth Mountains area by Wise, the Line Creek area by Casella, and the Quad Creek-Wyoming Creek-Line Creek area by Rowan and Larsen where metasedimentary rocks are common show that mapped large folds and statistical large folds are coincident. Small folds in the Quad Creek-Wyoming Creek-Line Creek area are older than or formed simultaneously with the open, flexural large folds and were formed during metamorphism-metasomatism. Mesoscopic compositional foliations at stations there are not homogeneous with respect to the statistical large fold axis as they are in the Lonesome Mountain area. The geometry of large folds in the Lonesome Mountain area indicates dominant passive flow rather than flexural-flow and flexural-slip. Homogeneity of gneisses and migmatites on the mesoscopic scale is best explained by this mechanism of folding. Earlier structures would have been destroyed or transposed. Evidences for intrusive structures are measurable in inches or up to a few feet only, but phacolithic intrusion in the catazone cannot be ruled out by present criteria.

GEOLOGY OF THE MOUNT GARIBALDI MAP-AREA, SOUTHWESTERN BRITISH COLUMBIA, CANADA

The Mount Garibaldi map-area is in the southern part of the plutonic complex comprising the Coast Mountains of British Columbia. The oldest rocks of the map-area are metavolcanic and minor metasedimentary rocks of unknown age and structure. These are extensively invaded by a quartz diorite batholith, the earliest of the plutonic masses, which displays an unfoliated core, a margin with highly developed secondary foliation, and an intervening zone of primary foliation. Bedded rocks at least 20,000 feet thick, principally clastic sediments derived in part from the batholithic rocks, occur in fault blocks north of Garibaldi Lake. The rocks of one fault block rest uncomformably on the older quartz diorites and contain Upper Cretaceous fossils; those of the other fault blocks are thought to be slightly younger than the fossil-bearing beds. The bedded rocks have in turn been invaded by later quartz diorites which are partly contemporaneous with and partly later than the block faulting. Two post-tectonic and presumably post-Upper Cretaceous batholiths lie partly within the map-area.