Augen Granite Research Articles

Early Neoproterozoic magmatism in the Central Tianshan Block, China: Implications for its tectono-magmatic evolution and relationship to Rodinia

As an important ancient continental constituent of the Central Asian Orogenic Belt (CAOB), the Central Tianshan Block (CTB) is indispensable to improving our understanding of the early evolution and architectural framework of this orogenic belt. Here, we present new zircon U–Pb ages and Hf isotopes, as well as bulk-rock geochemistry, for amphibolite, gneissic granite, and augen granite from several localities in the CTB. Zircon U–Pb ages of ten samples demonstrate that the protolith of the amphibolite, gneissic granite, and augen granite formed in the early Neoproterozoic (from 905.9 ± 4.8 Ma to 940.0 ± 4.5 Ma). Geochemically, the early Neoproterozoic amphibolite is depleted in Nb, Ta, P, and Ti, but enriched in light REE (LREE) and large ion lithophile elements (LILE), similar to arc-related basalt. The gneissic granites from the Shardland area and the Alatage area, and gneissic granites and augen granites from the Xingxingxia area are enriched in SiO2, Na2O + K2O, LREE, and LILE, and depleted in heavy REE (HREE) and high field strength elements (HFSE), exhibiting an I-type granite affinity. Zircons from the amphibolite in the Shardland area exhibit positive εHf(t) values of +1.7 to +4.9 and TDM1 model ages of 1.27–1.39 Ga, indicating that the protolith crystallized from a mantle-derived magma. Zircons from the gneissic granites in the Shardland area, the Alatage area and the Xingxingxia area have εHf(t) values that range from –5.9 to +7.3 and TDM2 model ages of 1.33–2.17 Ga, indicating that the granites may have been derived from partial melting of ancient continental crust that contained compositionally variable juvenile material. These new results, combined with previous studies, indicate that early Neoproterozoic magmatic rocks are well-developed throughout the CTB and formed along a subduction-related continental arc. The CTB and other microcontinents of the southwestern CAOB may have been located proximal to the northern margin of the Tarim Craton during the early Neoproterozoic, thereby representing the active continental margin of the circum-Rodinia subduction–accretion system.

Tectonic evolution of circum-Rodinia subduction: Evidence from Neoproterozoic A-type granitic magmatism in the Central Tianshan Block, northwest China

Neoproterozoic igneous rocks associated with the assembly and configuration of the Rodinia supercontinent are widely distributed in the Central Tianshan Block, southwestern Central Asian Orogenic Belt (CAOB), northwest China. The Central Tianshan Block is an important geological unit in tectonic reconstructions of the CAOB and Rodinia. However, the early Neoproterozoic tectonic framework and evolution of the Central Tianshan Block are poorly understood. Here we present a geochronological, mineral and whole-rock geochemical, and Nd–Hf isotopic study of early Neoproterozoic augen granites from the Bingdaban area. In situ zircon U–Pb dating shows that the augen granites have crystallization ages of 1002–992 Ma. The augen granites have high SiO2 (70.46–76.24 wt%) and K2O (3.36–4.96 wt%) contents, and FeOT/(FeOT + MgO) ratios (0.76–0.84), low Mg# values (29–39), slightly enriched light rare earth element patterns, enriched large-ion lithophile elements, depleted high-field-strength elements (e.g., Nb, Ta, and Ti), and high 10,000 × Ga/Al ratios (2.3–8.3) and Zr + Nb + Ce + Y contents (256–408 ppm). The augen granites have geochemical features typical of A2-type granites, variable εNd(t) values of –1.64 to +6.87 and Nd model ages of 1.78–1.08 Ga, and zircon εHf(t) values of −1.96 to +6.42 with Hf model ages of 1.98–1.48 Ga. These data indicate the granites had a juvenile crustal-dominated source, with subordinate input of ancient crustal components, and formed in a subduction-related extensional setting. The extension might have been related to slab break-off beneath Rodinia. The early Neoproterozoic igneous rocks in the Central Tianshan and adjacent microcontinental blocks in the southwestern CAOB formed at or near an active continental margin, possibly part of the circum-Rodinian subduction zone during or after the assembly of Rodinia. Similar Neoproterozoic magmatism in the Central Tianshan and Yili blocks suggests that they had a comparable crustal evolution and a close tectonic and paleogeographic relationship during the Neoproterozoic. However, significant age and isotopic differences characterize the Central Tianshan–Yili blocks and Tarim Craton, suggesting that the Central Tianshan–Yili blocks were not separated from the Tarim Craton during the early Neoproterozoic.

Constraints on the evolution of the eastern margin of the Hearne craton: New data from the southwestern Rottenstone Domain, Trans-Hudson Orogen, Saskatchewan

The Rottenstone Domain (RD) is a complex lithotectonic supracrustal belt of the Paleoproterozoic Trans-Hudson Orogen in Saskatchewan, Canada, located between the Archean Hearne craton and the juvenile Paleoproterozoic Reindeer Zone (RZ). Its evolution and crustal affinity have been under considerable debate in the past few decades, with most interpretations considering it to be part of RZ, but some hinting that certain elements might have affinities to the Archean hinterland. On the basis of bedrock mapping, U-Pb, Sm-Nd and lithogeochemical data, the rocks of the southwestern RD were subdivided into several lithodemic entities, which were used to reconstruct a plate tectonic framework for the western part of the Trans-Hudson Orogen during Orosirian time.A U-Pb zircon crystallization age of 2518 ± 6 Ma and depleted mantle model age (TDM) of 2.8 Ga for an augen granite confirmed that the western part of RD is part of the Hearne craton. A newly discovered structural inlier at the eastern boundary of RD consists of a central unit of 2045 ± 9 Ma augen granite, with TDM ages between 2.8 and 2.6 Ga, that was intruded into gneissic granite with a TDM of 3.3 Ga. The augen granite was interpreted as representing a rift-related granite, emplaced into Archean rocks of the Hearne craton. Two lithotectonic assemblages, one dominated by feldspathic quartzite and psammite the other by migmatitic psammite-psammopelite, are in structural contact with the granitic inliers and have model ages of 2.6–2.4 Ga. One of the feldspathic quartzite samples contains detrital zircon age modes at 1910 and 1860 Ma, and lesser Archean modes. Both assemblages were likely deposited in a forearc basin to the 1.865–1.850 Ga Wathaman continental arc.Recognition of Archean components and metasedimentary assemblages with older model ages than those in the RZ to the east, suggest that this southern part of RD is more closely affiliated with the Hearne craton. The eastern edge of RD coincides with a mylonite zone (Howard Lake shear zone), which likely represents a fundamental, although multiply reactivated, structural break separating Archean granitoids and structurally overlying metasedimentary rocks from rocks of the more juvenile Reindeer Zone.

Micro‐fabrics and Genesis of Pyrite from Zhaoyuan Golden Deposits in Shandong Province, North China

AbstractAs a widely‐developed mineral in various types of rocks, pyrite features important geological information such as temperature and pressure during its formation and evolution (An et al., 2014; An et al., 2018). It is an important geological formation for studying the origins of rocks, mineralization, and deposits (Xue et al., 2014; An et al., 2016). The study of the genesis of gold deposits is a research hotspot in current geology and mineral deposits not only because of high economic value, but also because of important practical significance (An et al., 2017; Yang et al., 2014). Shandong Zhaoyuan, as the third largest gold production area in the world, has important research significance of the origin and generation of gold‐bearing minerals (Xue et al., 2014; Wen et al., 2014). In this study, pyrite from augen granite (figure 1a), syenite (figure 1b), basite (figure 1c) and gold ore (figure 1d) at Zhaoyuan gold deposit Shandong Province, was selected as the research object. The morphological and structural micro‐fabrics were analysed by polarized light microscopy (PLM) and Raman spectroscopy to reveal the source and process of gold further.

Application of pinch-and-swell structure rheology gauge to determine rock paleo-rheological parameters in Taili, western Liaoning, NE China

New results, in combination with previously published ones, reveal that when the Stress Exponent of the Competent layer (SEC) ranges from 1 to 10 (1<n<10), Pinch-and-Swell structure Rheology Gauge (PSRG) can only be available under the condition that the Viscosity ratio between the Competent layer and its corresponding Matrix layer (VCM) is larger than 10. Therefore, we made the attempt to calculate the viscosity ratio of pinch-and-swell structure of competent layer to the related matrix and stress exponent. Based on this knowledge, we applied this gauge to calculate SECs and VCMs of eight types of pinch-and-swell structures, which are widely developed in the Taili area of the west Liaoning Province in China. Statistical analysis of the SEC resulted in intervals of four types of competent layers, that is, Medium-scale Granitic coarse-to-pegmatitic Veins, Small-scale Augen Granite aplite Veins, Small-scale Granite aplite Veins, and Small-scale Augen Quartz–K-feldspar veins, with intervals of [3.50,4.63], [2.64,4.29], [2.70,3.51], and [2.50,3.36] respectively. The preferred intervals of VCM of the five types of pinch-and-swell structures, Small-scale Augen Granite aplite Veins+Fine-grained Biotite–Hornblende–plagioclase Gneiss, Medium-scale Granitic coarse-to-pegmatitic Veins+Fine-grained Biotite–Hornblende–plagioclase Gneiss, Small-scale Augen Granite aplite Veins+medium-to-fine-grained granitic gneiss, Medium-scale Granitic coarse-to-pegmatitic Veins+medium-to-fine-grained granitic gneiss, and Small-scale Augen Granite aplite Veins+fine-grained biotite–plagioclase gneiss, are [19.98,62.51], [15.90,61.17], [26.72,93.27], [22.21,107.26], and [76.33,309.39] respectively. The similarities between these calculated SEC statistical preferred intervals and the physical experimental results verify the validity of the PSRG. The competent layers of the pinch-and-swell structures were presented in this study as power-law flow with SEC values that increased with the thickness of the layer. Grain-size plays an important role in the rheology of pinch-and-swell structures. The results offer a case for the application of PSRG and determine the key rock rheological parameters of North China Craton for future related studies.

New SHRIMP U–Pb zircon ages of granitic rocks in the Hida Belt, Japan: Implications for tectonic correlation with Jiamushi massif

AbstractGranitoids in the Hida region of Japan encompass two main rock types: younger type‐1 granites and older type‐2 granites. Sensitive high mass‐resolution ion microprobe (SHRIMP) U–Pb zircon dating of older type‐2 granites collected from the Tateyama area show similar ages of 245 ± 2 Ma and 248 ± 5 Ma for two gneissose granites, while a significantly younger intrusion age of 197 ± 3 Ma was determined for the younger type‐1 granites collected from the Hayatsukigawa River which belongs to the Okumayama pluton. A felsic gneiss sample (07HI‐3) collected from the right bank of the Hayatsukigawa River yielded multiple complex ages at 330 ± 6 Ma, indicating the timing of the Hida regional tectono‐thermal events that formed the Hida gneisses; 243 ± 8 Ma, representing the timing of intrusion of the augen granite; and 220 Ma, indicating the timing of regional dextral ductile shearing that caused a repeated recrystallization of metamorphic rocks in the study area. Considering the geochronological data, the rock types and assemblages, basement, and Sr–Nd isotopic constraints, we propose that the Hida Belt separated from the Jiamushi massif, which is located in the eastern margin of the Central Asian Orogenic Belt.

The Arrowsmith orogeny: Geochronological and thermobarometric constraints on its extent and tectonic setting in the Rae craton, with implications for pre-Nuna supercontinent reconstruction

Geochronological and thermobarometric data for two widely spaced and previously unstudied areas of the Rae craton, Canada provide important temporal and spatial links between previously investigated areas in the northern and southern Rae craton that had been interpreted to have been reworked during the ca. 2.5–2.3Ga Arrowsmith orogeny. On southern Boothia Peninsula, upper amphibolite- to granulite-facies metamorphism, dated by in situ SHRIMP U–Pb analysis of monazite, occurred at ca. 2.64–2.60, 2.54, 2.50–2.49, 2.44, and 2.37–2.34Ga, with retrograde monazite growth at 2.27Ga. SHRIMP U–Pb ages of zircon in two metasedimentary rocks provide a further indication of metamorphism at ca. 2.54 and 2.50Ga, which also corresponds to the age determined for a K-feldspar augen granite. Textural relationships indicate that the regional foliation formed between ca. 2.54Ga and 2.50–2.49Ga. Peak metamorphic conditions of ∼700°C and 5–6kbar are interpreted to have been reached during the 2.37–2.34Ga event. In the McCann–McArthur Lake region of the southwestern Rae craton, upper amphibolite- to granulite-facies metamorphism is dated by in situ SHRIMP monazite ages of ca. 2.53, 2.47, and 2.37–2.28Ga. Textural relationships indicate deformation initiated by ca. 2.37Ga and terminated by ca. 2.30Ga. Metamorphic conditions of ∼5kbar and 700–735°C were quenched at ca. 2.3Ga following peak conditions of ∼7kbar and 800°C.The evidence for regional deformation at 2.54–2.49Ga on southern Boothia Peninsula supports an Andean-type accretionary margin setting for the Arrowsmith orogeny. A collisional event may have terminated high-grade metamorphism in the northern region by ca. 2.34Ga while metamorphism associated with continental arc magmatism continued in the south until at least ca. 2.28Ga. These combined data demonstrate the continuity and broad extent of the Arrowsmith orogen along the western flank of the Rae craton, extending ∼1500km from northern Saskatchewan to northern Baffin Island. The broad extent of this globally rare orogenic event provides an important piercing point for supercontinent reconstructions which are discussed in a companion paper. The inferred convergent tectonic activity between ca. 2.54 and 2.28Ga is interpreted to have occurred on the flank of a growing supercontinent, a setting that could account for an apparent slowdown, but not shutdown in global tectonics.

Mid- to late Paleozoic K-feldspar augen granitoids of the Yukon-Tanana terrane, Yukon, Canada: Implications for crustal growth and tectonic evolution of the northern Cordillera

Potassic feldspar-bearing augen granitoids are a fundamental component of the architecture of the Yukon-Tanana terrane and the ancient Pacific margin of the northern Cordillera. These augen granitoids form a belt that extends from Alaska to southeast Yukon Territory, vary in age, and provide probes of the crustal evolution and tectonic history of the Yukon-Tanana terrane and ancient Pacific margin of North America in the Paleozoic. We present results of an integrated field mapping, geochemical, Sm-Nd tracer isotopic, and U-Pb zircon geochronologic study of the augen granitoids in the Stewart River area in an attempt to understand their role in the crustal evolution and tectonic history of the Yukon-Tanana terrane and ancient Pacific margin of North America. Augen granitoids of the Stewart River area are of three distinct ages: Late Devonian, early Mississippian, and Permian. U-Pb zircon geochronology of these augen granitoids has yielded ages of 362.1 ± 2.7 Ma (Stewart River augen granite), 347.5 ± 0.7 Ma (Mount Burnham augen granite), and 264.8 ± 3.7 Ma (Wounded Moose augen granite). All of the augen granitoids, regardless of age, have negative ϵ Ndt values (−2.0 to −15.3) and Proterozoic-Archean depleted-mantle model ages (T DM = 1.37–2.56 Ga). These geochemical and isotopic attributes, coupled with the presence of inherited zircon with Precambrian ages, suggestthatthesegranitoidsaretheproductof crustal melting and crust-mantle mixing during three different cycles of arc magmatism in the Paleozoic. Furthermore, these granitoids represent net crustal recycling along the ancient Pacific margin of North America in the Paleozoic. Importantly, however, there are minor secular variations in crustal recycling, and the younger Permian augen granitoids exhibit higher ϵ Ndt , Nb/Ta, V/Yb, and Sc/Yb, consistent with a greater juvenile component in their genesis. This juvenile component is probably due to assimilation of underplated mafic material derived from older early Mississippian Yukon-Tanana terrane arc magmatism and/or a greater mantle component due to enhanced infiltration of underplated mafic material into augen granitoid magma chambers through rheologically weak crust associated with Permian subduction. The older Late Devonian and early Mississippian augen granitoid suites represent two pulses of Yukon-Tanana terrane arc magmatic activity that developed in response to east-dipping subduction along the western edge of the North America craton in the mid-Paleozoic. This east-dipping Yukon-Tanana terrane arc system continued to evolve throughout the Mississippian to Early Permian and was coincident with the development of the Slide Mountain backarc basin that formed between the Yukon-Tanana terrane arc system and the North American craton; this east-dipping arc-backarc system continued until ca. 275 Ma. After ca. 275 Ma, the east-dipping arc and backarc magmatism ceased and was replaced by ca. 270–269 Ma high-pressure metamorphism and the establishment of a new subduction zone that formed in response to the closure of the Slide Mountain backarc basin. The Permian augen granitoids from the Stewart River are the magmatic record of this new west-dipping subduction zone. Although there are subtle variations, the petrogenetic and tectonic histories of the three suites of augen granitoids in the Stewart River area are remarkably similar and attest to the constancy of magmatic and tectonic processes that occurred along the ancient Pacific margin of North America in the Paleozoic.

U–Pb geochronology of late Neoproterozoic augen granites in the Moine Supergroup, NW Scotland: dating of rift-related, felsic magmatism during supercontinent break-up?

Within the Caledonides of Caithness, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup are intruded by minor sheets of strongly deformed granite. U–Pb sensitive high-resolution ion microprobe zircon ages of 599 ± 9 Ma (Berriedale augen granite) and 588 ± 8 Ma (Braeval augen granite) are interpreted to date emplacement during the late Neoproterozoic. These augen granites are therefore unrelated to either Knoydartian ( c . 870–790 Ma) or Caledonian ( c . 470–420 Ma) orogenesis in the Scottish Highlands. Intrusion was, however, broadly contemporaneous with late Neoproterozoic extension of the Laurentian margin during continental break-up and the opening of the Iapetus Ocean. In the context of evidence for rift-related mafic magmatism within the Dalradian basin in Scotland at c . 600 Ma, as well as contemporaneous anorogenic magmatism along the margins of the developing Iapetus Ocean in the Appalachians, we propose that the protoliths of the augen granites were probably emplaced during continental break-up. The new data broaden the extent of this late Neoproterozoic magmatic event in the Scottish Highlands: other deformed granites that are at present undated but have been assumed to be of Caledonian age therefore require reinvestigation.

Evidence for Neoproterozoic orogenesis and early high temperature Scandian deformation events in the southern East Greenland Caledonides

Integrated field structural studies and SHRIMP U–Pb zircon and monazite dating have been undertaken in Renland, west of Scoresby Sund district in the southern part of the East Greenland Caledonides. Southwest Renland is dominated by metasedimentary rocks correlated with the Krummedal supracrustal succession of East Greenland and which on Renland were intruded by augen granites. Krummedal psammite from Renland yielded a spectrum of Mesoproterozoic to Palaeoproterozoic detrital U–Pb zircon dates, the youngest of which indicate deposition of the psammite occurred c. 1000 Ma ago, thus post-dating Grenvillian continent–continent collision in North American Laurentia. These Krummedal metasediments were deformed into regional nappe-scale folds prior to metamorphism, crustal anatexis and genesis of augen granites; an example of the latter has been dated at 915±18 Ma (U–Pb zircon). This demonstrates early Neoproterozoic high-temperature tectono-metamorphism affecting rocks within the southern East Greenland Caledonides, broadly contemporaneous with similar rocks farther north in East Greenland and with Sveconorwegian events on Baltica. Still in southwestern Renland, a later thermal event led to development of uppermost amphibolite to granulite facies metamorphic assemblages, veins and patches of in situ garnetiferous melt-bearing neosome in both metasediments (432±6 Ma, U–Pb zircon) and in the augen granites, and contemporaneous biotite-bearing granite sheets in top-down-to-the-E extensional shear zones (434±5 Ma, U–Pb zircon). Monazites from southwestern Renland record Caledonian thermal events as late as 410−400 Ma. In contrast, southeastern Renland is dominated by quartzofeldspathic migmatites with a strongly Caledonian signature but enclosing relicts of augen granite and retrogressed granulite facies psammitic and pelitic metasediment. There is also a sequence of Caledonian granitoid intrusions. Two samples from a hypersthene monzonite intrusion yielded U–Pb zircon dates of 424±8 Ma and 424±6 Ma. This pluton shows the marginal effects of the regional migmatization and was intruded early in the sequence of granitoid emplacement. An amphibolite facies migmatite, textural evidence from which suggests that it had never hosted granulite facies assemblages, records zircon growth at 423±6 Ma, and closure of monazite by 402±10 Ma. High grade metamorphism, and the protracted sequence of granitoid emplacement and still younger thermal events which together span the period between 430 and 400 Ma may, in part, reflect complicated lithospheric dynamics associated with subduction outboard of the Laurentian margin. Crustal segments carrying the relict evidence of Neoproterozoic and early Caledonian events must then quickly have been thrust northwestwards in foreland-propagating, northwesterly directed thrusts over Cambro-Ordovician platformal sequences on the Laurentian margin. This records the final closure of Iapetus, encroachment of Baltica and continent–continent collision from late Llandovery times (425–430 Ma).

Sm‐Nd, Pb‐Pb and Rb‐Sr Isotopic Dating and Its Dynamic Implications for the Proterozoic Augen Granite in the Yunkai Area, Western Guangdong Province

AbstractThe samples of ductile‐rheologic deformational augen granite from the Yunkai uplift area, western Guangdong province, were determined by the whole‐rock Sm‐Nd, Pb‐Pb and Rb‐Sr isotopic dating to have an Sm‐Nd isochron age of 1414±68 Ma, a Pb‐Pb isochron age of 1388±90 Ma and a Rb‐Sr isochron age of 490±36 Ma. The first two ages are interpreted as the formation age of this suite of granite and the last age represents the timing of the tectono‐thermal event of Caledonian ductile‐rheologic shear partial melting. It is indicated that in the study area not only an orogeny took place in the Caledonian, but also a more important tectono‐magmatic activity occurred in the Meso‐proterozoic there, which may be related to the subduction‐collision between the Yangtze block and Cathaysia block.

Episodic tectono-thermal activity in the southern part of the East Greenland Caledonides

Isotopic data from the Renland augen granites of the Scoresby Sund region (Figs 1, 2) provided some of the first convincing support for relicts of potentially Grenvillian tectono-thermal activity within the East Greenland Caledonides. In Renland, Chadwick (1975) showed the presence of major bodies of augen granite (Fig. 2) interpreted by Steiger et al. (1979), on the basis of Rb–Sr whole rock and U–Pb zircon age determinations, to have been emplaced about 1000 Ma ago.

The Itsaq Gneiss Complex of southern West Greenland; the world's most extensive record of early crustal evolution (3900-3600 Ma)

The Itsaq Gneiss Complex of southern West Greenland contains the best-preserved occurrences of ⩾ 3600 Ma crust. Its known area is ∼ 3000 km 2 with almost continuous exposure in some places. SHRIMP UPb zircon geochronology shows that the gneiss complex had a complicated early history, having been added to, and modified in, several events starting at ∼ 3900 Ma. Supracrustal, mafic and ultramafic rocks comprise approximately 10% of the complex and range in age from ⩾ 3870 to ∼ 3600 Ma. A large portion of the Isua supracrustal belt and some other bodies contain sequences of LREE-enriched, mafic (locally pillow-structured) to felsic volcanic and volcaniclastic rocks (some deposited from turbidity currents) and abundant diverse chemical sediments. These sequences might have formed in an environment analogous to present-day volcanic arcs. Other (mostly ⩾ 3800 Ma?) units dominated by LREE-depleted, high Ti/Zr mafic rocks (as found in komatiites and komatiitic basalts free of crustal contamination) with layers of banded iron formation might have been derived from volcanic edifices formed as a result of plume activity. Only in the youngest supracrustal sequences (∼ 3600 Ma) are detrital sediments derived from mixed-provenance clastic sources an important component. Layered anorthosite, gabbro and peridotite units (some ⩾ 3800 Ma) derived from large deep crustal intrusions are also widespread. In addition there are massive dunites and harzburgites. In the rare cases where their original mineralogy and texture are preserved, these contain aluminous spinel, indicating equilibration in the lowermost crust or upper mantle. Approximately 90% of the complex consists of grey gneisses, the dominant protoliths having been tonalites and less abundant trondhjemites, quartz-diorites, diorites and granodiorites. The protoliths were intruded in several events starting at ∼ 3870 Ma. Like other suites of Archaean grey gneisses, they were formed by partial melting, probably in an arc environment, of buried (subducted?) amphibolite, leaving residual hornblende ± garnet. Granites form approximately 10% of the complex. The oldest are 3650 Ma leucogranites, which probably formed by deep crustal anatexis of predominantly tonalitic gneisses. There are also ∼ 3625 Ma augen granites and ferrogabbros, whose chemistry resembles that of some A-type, within-plate granite suites. The evolution of the Itsaq gneiss complex is marked by increasing compositional diversity with time. Pre-3650 Ma lithologies are predominantly mafic and ultramafic rocks, tonalites and diorites. The first recorded regional metamorphic event occurred at 3650 Ma, and was marked by localised partial melting and intrusion of leucogranites. This might record crustal thickening, brought about by collision of different blocks of tonalite-dominated crust. A further thermal event occurred just after 3600 Ma, as shown by PbPb titanite and feldspar ages and local intrusion of granites. Deposition of sediments at ∼ 3600 Ma which were derived from mixed-age sources suggests their derivation from an extensive block of “continental” crust.

Origin and U–Pb geochronology of amphibolite-facies metamorphic rocks, Miramichi Highlands, New Brunswick

The Miramichi Highlands of New Brunswick are underlain by subgreenschist- to greenschist-facies sedimentary and volcanic rocks of the Cambro-Ordovician Tetagouche Group and by amphibolite-facies paragneisses, amphibolites, and felsic orthogneisses of the Trousers Lake and Sisson Brook suites. New field, geochemical, and geochronologic data for the amphibolites and felsic orthogneisses suggest that they are high-grade metamorphic equivalents of the Tetagouche volcanic rocks and their associated intrusions.Amphibolites in the Miramichi Highlands occur as striped and unstriped varieties that possess chemical characteristics indicative of an igneous origin. However, the two types are compositionally distinct: the striped amphibolites resemble volcanic-arc tholeiites, whereas the unstriped amphibolites are like within-plate tholeiites. The geochemically inferred tectonic origin of these amphibolites is compatible with a recently proposed intracontinental back-arc tectonic setting for the Tetagouche Group.Felsic orthogneisses (Fox Ridge augen granite and Trousers Lake felsic orthogneiss) exhibit concordant contacts with the unstriped amphibolites. U–Pb zircon ages for the Fox Ridge augen granite [Formula: see text] and Trousers Lake felsic orthogneisses [Formula: see text] indicate a Late Ordovician intrusive event. Thus, there is no evidence for Precambrian granite and orthogneiss in the Miramichi Highlands, as had been previously inferred from a correlation with purported Precambrian rocks in the Gander Zone of Newfoundland. The age of the unstriped amphibolites is interpreted as being the same as that of the felsic orthogneisses because these two rock types always exhibit close relationships in the field. The age of the striped amphibolites is less certain, although a correlation with Ordovician basalts of the Tetagouche Group is consistent with their field relationships and tectonic setting.

Polyorogenic Nature of the Southern Caledonian Fold Belt in East Greenland: An Isotopic Age Study

The southern part of the Caledonian fold belt in East Greenland comprises a complex of pre-Caledonian rock units as demonstrated by numerous age determinations and interpretation of field work. U-Pb isotopic age determinations on zircons, Rb-Sr analyses of whole-rock and mineral samples and one K-Ar mineral age are presented. They date some of the key rock units fundamental to the unravelling of the development of this part of the fold belt, and support the polyorogenic nature of the Caledonian fold belt. Three main groups and two orogenic events with related migmatizations are observed: (1) Archean basement: early intrusive granites and basic dikes imply a minimum age of 2500 m.y. for the basement; (2) Proterozoic intrusions: bodies and sheets of augen granites were emplaced about 1000 m.y. before present. The intrusions coincide with the first (i.e. Proterozoic) orogenic event as well as the first migmatization recognized in the area; (3) Caledonian intrusives: a massive hypersthene monzonite sheet representing an early phase of the second (i.e. Caledonian) orogeny, was emplaced about 475 m.y. ago. Younger granitic stocks as well as sheets of granite associated with the second migmatization were formed in the period between 425 and 400 m.y. ago.

Rb-Sr ages of intrusive plutonic rocks from the Stora Le—Marstrand belt in Orust, SW Sweden

Rb-Sr whole-rock isochrons are reported for the Hälleviksstrand amphibolite (1432 ± 92 Ma) and the Assmunderöd-Myckleby augen granite (1379 ± 46 Ma) which intrude the Stora Le—Marstrand migmatite belt of Orust, SW Sweden. These ages provide a younger limit for the age of the main migmatisation and an older limit for the emplacement of minor silicic and mafic intrusions and subsequent regional metamorphism and deformation. They overlap with published ages from granitic intrusions in the adjacent Åmål belt and provide the basis for a correlation between the two belts.

Isotopic studies on a Precambrian geochronologic boundary, Sierra Madre Mountains, Wyoming

Geochronologic evidence indicates that a profound discontinuity separating two discrete Precambrian provinces parallels the Wyoming-Colorado border. This east-trending boundary transects the Sierra Madre Range in southern Wyoming adjacent to a fault-bounded sequence of folded middle Precambrian sedimentary rocks. Precambrian rocks of the Sierra Madre Mountains reveal a series of igneous and sedimentary events that occurred near the probable edge of the Archean continental nucleus between 2,600 and 1,600 m.y. B.P. North of the boundary, Archean quartz-biotite gneiss, probably 2,800 to 2,900 m.y. old, grades eastward into a northwest-trending antiform of microcline augen gneiss and granite. The Rb-Sr isochron of the granite gneiss complex yields an age of 2,560 m.y. and an initial Sr 87 /Sr 86 intercept of 0.7000. Geochemical evidence suggests that the quartz-biotite gneiss may have evolved from a supracrustal dacitic protolith formed by anatexis of a basaltic amphibolite. Calc-alkalic volcanic flows, tuffs, and volcanogenic sediments and a 4-km-thick sequence of conglomerate, quartzite, and limestone were deposited on the gneiss basement after 2,560 m.y. B.P. A period of igneous activity and tectonism between 1,650 and 1,900 m.y. B.P. resulted in the metamorphism and folding of the sediments into an east-trending anticlinorium of isoclinal folds. The Sierra Madre south of this metasedimentary structure consists principally of a composite batholith of plutons ranging from gabbro to granite. The Encampment River Granodiorite, the oldest intrusion, is a gneissic unit that was probably derived by partial melting and remobilization of the polycyclic quartz-biotite gneiss, as suggested by isotopic and rare-earth-element data. This thermal reactivation occurred between 1,900 and 2,000 m.y. B.P. The majority of the batholith is made up of a distinctive red granite that yields an isochron age of 1,680 m.y. and an initial Sr 87 /Sr 86 ratio of 0.7024. A white quartz monzonite discordantly intrudes the granite and hornblende gneiss country rock. Isochron ages and initial Sr 87 /Sr 86 intercepts of the monzonite and gneiss are 1,680 m.y. and 0.7020 for the quartz monzonite and 1,880 m.y. and 0.7015 for the country rock. At least three distinct generations of mafic dikes are present in the Sierra Madre; field relations with dated units indicate ages of ∼2700 m.y., 1,800 to 2,650 m.y., and ∼1,680 m.y.

The structure of south Renland, Scoresby Sund. With special reference to the tectono-metamorphic evolution of a southern internal part of the Caledonides of East Greenland

Renland occupies an internal position within the southern extreme of the outcrop of the Caledonian mobile belt of East Greenland exposed between latitudes 70° and 82° N. In south-west Renland migmatised paragneisses derived from sediments comparable to the late Precambrian Lower Eleonore Bay Group form a multilayered sequence with a minimum thickness of 1500 m. The migmatites are interleaved with thick concordant sheets of garnetiferous augen granite, the formation of which may be linked with the low-pressure granulite or transitional amphibolite-granulite facies conditions attained during migmatisation of the paragneisses. These conditions persisted during the folding together of paragneisses and granites into regional structures of nappe dimensions which had a north or north-west direction of transport. Refolding of the nappes under continued high-grade conditions gave rise to structures locally coaxial with nappe axes. Reversals of facing of nappes occur in backfolds. Linear fabrics of sillimanite and biotite and prolate ellipsoidal augen of feldspar are parallel to fold axes and show that constrictional deformation dominated the later stages of the nappe phase and the refolding event. The constriction is attributed to compressing of rocks in south-west Renland between nappes advancing from the south and a rising mass of granite and basement gneisses in the north. Intrusion of concordant sheets of biotite-rich hypersthene monzonite (mangerite) followed the nappe deformation in south-east Renland. The principal sheet, which is 500 m thick, forms the rim to part of a lopolithic basin. Thinner sheets of monzonite injected into migmatites within the basin have been disrupted by further migmatisation and granitisation. Stable assemblages in pyribolite restite suggest this later event, which was restricted largely to the basin, attained conditions of hornblende-granulite facies. Open warps attributed to monzonite injection and the basin formation are superimposed on nappes west of the principal sheet. Normal faults with downthrow to east and west relate to the formation of troughs filled with Upper Palaeozoic and Mesozoic sediments in the Scoresby Sund region. The distribution of the faults suggests Renland was a horst area in Upper Palaeozoic times. Tertiary igneous activity in south Renland is represented by rare dykes of olivine dolerite and scattered plugs of pyroxenite which locally contain large blocks of host gneisses.

Preliminary account of the geology of south-east Renland, Scoresby Sund, East Greenland

South-east Renland occupies an internal position in the Caledonian orogenic belt of East Greenland. The area comprises migmatised paragneisses ofCaledonian supracrustal origin intruded by concordant sheets of garnetiferous augen granite in the west. The gneisses and granites are folded in large-scale, tight to isoc1inal folds. Intermediate igneous rocks with charnockitic affinity were intruded after the folding. The principal intrusion is an extensive, concordant sheet delimiting an area of gneisses in the east that have undergone further migmatisation and granite intrusion. Prominent among the many fractures are normal faults with downthrow to the east. No indications of economic mineralisation were observed.

Plutonic development of the Ilordleq area, South Greenland. Part I: Chronology, and the occurrence and recognition of metamorphosed basic dykes

Evidence is presented which shows the rocks of Ilordleq to be the products of two distinct episodes of plutonic (Read, 1957, p.165) activity. In the first of these episodes the mise-en-place of a large area of synkinematic granitic rocks was affected. A NE striking belt of pre-granite supracrustal rocks is preserved in the north of the area, and isolated remnants of the supracrustal rocks remain as enclaves in the granite. Cutting the metasediments and metavolcanics of the supracrustal group are numerous basic dykes which pre-date the granite. The first plutonic episode was followed by a further period of emplacement of basic dykes, which cut both supracrustal and granitic rocks. Indications of original chilled margins in these dykes, together with their regular form, suggest a marked change in conditions from those prevailing in the preceding plutonic episode. The second period of plutonic activity which followed resulted in a reactivation of much of the granite previously formed, especially in the northern part of the area where basic dykes were deformed and became either disrupted or were migmatised by kinematically controlled metasomatism. The surrounding granite shows evidence of flow in the solid state and was evidently approaching a state in which diapiric movement was possible. In a less reactivated part of the area, porphyroblastic microcline augen granites were formed in those places where the supracrustal enclaves were most strongly deformed. Deformation during each plutonic episode took place by homogeneous strain with resulting change in shape rather than tectonic transport; minor structures in supracrustal, granitic and dyke rocks are homologous. Principal stress axes were similarly oriented during both plutonic episodes. A further period of emplacement of basic dykes took place in the closing stages of the second period of plutonic activity, while the country rocks were still at elevated temperatures and during the operation, at least intermittently, of compressive forces. A conjugate set of shear fractures controlled the emplacement of these dykes; movement along the fractures and emplacement of dykes were essentially contemporaneous and gave rise to 'differentiation by deformation' of the partially consolidated dyke magma, sigmoid foliation of the dykes, and other distinctive features. Further swarms of basic dykes were subsequently emplaced during the Gardar volcanic episode (ca. 1150 x 106 years). Reference is made to published criteria for the recognition of relict dykes, replacement dykes, and dykes emplaced in hot granite, and the possibility of confusion with dykes in reactivated granites is indicated. The suitability of the orogenic cycle as the primary unit of Precambrian chronology is questioned and a less interpretative unit advocated. The Ilordleq chronology is compared with those established by the writer's colleagues elsewhere in South Greenland and possible alternatives suggested to some current correlations. The similarity of many features of the Ilordleq rocks to those of the Lewisian rocks of Scotland is referred to, and the concept of a migrating front of plutonic activity shown to be not applicable to Ilordleq.