Extensional collapse of the Gondwana orogen: Evidence from Cambrian mafic magmatism in the Trivandrum Block,southern India

The assembly of Late Neoproterozoice Cambrian supercontinent Gondwana involved prolonged subduction and accretion generating arc magmatic and accretionary complexes, culminating in collision and formation of high grade metamorphic orogens. Here we report evidence for mafic magmatism associated with post-collisional extension from a suite of gabbroic rocks in the Trivandrum Block of southern Indian Gondwana fragment. Our petrological and geochemical data on these gabbroic suite show that they are analogous to high Fe tholeiitic basalts with evolution of the parental melts dominantly controlled by fractional crystallization. They display enrichment of LILE and LREE and depletion of HFSE with negative anomalies at Zre Hf and Ti corresponding to subduction zone magmatic regime. The tectonic affinity of the gabbros coupled with their geochemical features endorse a heterogeneous mantle source with collective melt contributions from sub-slab asthenospheric mantle upwelling through slab break-off and arc-related metasomatized mantle wedge, with magma emplacement in subduction to post-collisional intraplate settings. The high Nb contents and positive Nbe Ta anomalies of the rocks are attributed to inflow of asthenospheric melts containing ancient recycled subducted slab components and/or fusion of subducted slab materials owing to upwelling of hot asthenosphere. Zircon grains from the gabbros show magmatic crystallization texture with low U and Pb content. The LA-ICPMS analyses show ²⁰⁶ Pb/²³⁸ U mean ages in the range of 507-494 Ma suggesting Cambrian mafic magmatism. The post-collisional mafic magmatism identified in our study provides new insights into mantle dynamics during the waning stage of the birth of a supercontinent.     

Crustal recycling through intraplate magmatism: Evidence from the Trans-North China Orogen

The North China Craton (NCC) preserves the history of crustal growth and craton formation during the early Precambrian followed by extensive lithospheric thinning and craton destruction in the Mesozoic. Here we present evidence for magma mixing and mingling associated with the Mesozoic tectonic processes from the Central NCC, along the Trans-North China Orogen, a paleo suture along which the Eastern and Western Blocks were amalgamated at end of Paleoproterozoic. Our investigations focus on two granitoids – the Chiwawu and the Mapeng plutons. Typical signatures for the interaction of mafic and felsic magmas are observed in these plutons such as: (1) the presence of diorite enclaves; (2) flow structures; (3) schlierens; (4) varying degrees of hybridization; and (5) macro-, and micro-textures. Porphyritic feldspar crystals show numerous mineral inclusions as well as rapakivi and anti-rapakivi textures. We present bulk chemistry, zircon U–Pb geochronology and REE data, and Lu–Hf isotopes on the granitoids, diorite enclaves, and surrounding basement rocks to constrain the timing of intraplate magmatism and processes of interaction between felsic and mafic magmas. Our LA-ICP-MS zircon U–Pb data show that the pophyritic granodiorite was emplaced at 129.7 ± 1.0 Ma. The diorite enclaves within this granodiorite show identical ages (128.2 ± 1.5 Ma). The basement TTG (tonalite–trondhjemite–granodiorite) gneisses formed at ca. 2.5 Ga coinciding with the major period of crustal accretion in the NCC. The 1.85 Ga age from zircons in the gabbro with positive Hf isotope signature may be related to mantle magmatism during post-collisional extension following the assembly of the Western and Eastern Blocks of the NCC along the Trans-North China Orogen. Our Hf isotope data indicate that the Neoarchean–Paleoproterozoic basement rocks were derived from complex sources of both juvenile magmas and reworked ancient crust, whereas the magma source for the Mesozoic units are dominantly reworked basement rocks. Our study provides a window to intraplate magmatism triggered by mantle upwelling beneath a paleosuture in the North China Craton.     

Petrogenesis of the Palaeoproterozoic Xishankou pluton,northern Tarim block,northwest China: implications for assembly of the supercontinent Columbia

The Tarim block, one of the largest cratons in China, records an important part of the Proterozoic crustal evolution of the Earth. Many previous studies have focused on the Neoproterozoic magmatism and tectonic evolution of this block in relation to the break-up of Rodinia, although relatively little is known about its earlier tectono-magmatic history. In this article, we present detailed petrographic, geochronologic, whole-rock geochemical, and in situ zircon Hf isotope data for the pre-Neoproterozoic Xishankou granitoid pluton (XBP), one of several blue quartz-bearing granitoid intrusions well exposed in the Quruqtagh area, and discuss these intrusions in terms of their tectonic environment. Zircon LA-ICP-MS dating indicates that gneissic quartz diorite and granodiorite of the XBP crystallized at 1934 ± 13 and 1944 ± 19 Ma, respectively. Both underwent metamorphism essentially coeval with emplacement, a time that is compatible with the globally distributed 2.1–1.8 Ga crustal amalgamation during formation of the supercontinent Columbia. Petrographic and geochemical evidence suggest that the XBP is a continental-arc-type granite and may have been generated by the partial melting of Archaean thickened lower crust; this would suggest that the northern Tarim block was a continental-type arc at ca. 1940 Ma. Our new data, together with previous regional geological studies, indicate that a series of Palaeoproterozoic (ca. 2.0–1.8 Ga) tectono-magmatic events occurred in the northern Tarim attending the assembly of Columbia.     

High-precision geochronology of the Early Cretaceous Yingcheng Formation and its stratigraphic implications for Songliao Basin,China

The Songliao Basin in Northeast Asia is the largest and longest-lived rift basin and preserves a near-continuous continental succession of the most of the Cretaceous period, providing great material to investigate the adaption of the terrestrial systems to the Cretaceous greenhouse climate and tectonic events. However, the paucity of precise and accurate radioisotopic ages from the Early Cretaceous strata of the Songliao Basin has greatly held back the temporal and causal correlation of the continental records to the global Early Cretaceous records. Three tuff layers intercalated in the Yingcheng Formation have been intercepted by the SK-2 borehole, which offer excellent materials for radioisotopic dating and calibration of the chronostratigraphy of the Lower Cretaceous sequence of Songliao Basin. Moreover, the Yingcheng Formation recorded the largest and the last of the two major volcanic events in Songliao Basin, which also represents a turning point in the basin evolution history of Songliao from syn-rift stage to post-rift stage. Here we report high-precision U–Pb zircon geochronology by the CA-ID-TIMS technique on three tuff samples from the Yingcheng Formation of the SK-2 borehole in the Songliao Basin to construct a greatly improved, absolute age framework for the Yingcheng Formation and provide crucial age constraints for the Songliao Lower Cretaceous Strata. The new CA-ID-TIMS geochronology constrained the Yingcheng Formation at 102.571 + 0.320/?2.346 Ma to ca. 113 Ma, correlating to the Albian Stage. Combined with the previous published Songliao geochronology, the Quantou Formation is constrained to between 96.442 + 0.475/?0.086 Ma and 91.923 + 0.475/?0.086 Ma; the Denglouku Formation is constrained to between 102.571 + 0.320/?2.346 Ma and 96.442 + 0.475/?0.086 Ma; the age of the Shahezi Formation is estimated at ca. 113 Ma to ca. 118 Ma, which could extend to ca. 125 Ma in some locations in Songliao Basin. The major unconformity between the Yingcheng Formation and the Denglouku Formation, which represents the transition of the basin from syn-rift to post-rift is thus confined to between 102.571 + 0.320/?2.346 Ma and 96.442 + 0.475/?0.086 Ma. This is roughly contemporaneous with the change in the direction of the paleo-Pacific plate motion from west-southwest to north or northwest in mid-Cretaceous, suggesting their possible connections.     

Geochronology and geochemistry of mafic dykes in the Helanshan complex: Implications for Mesozoic tectonics in the North China Craton

The Helanshan tectonic belt(HTB) is a major tectonic divide between the Alxa and Ordos blocks in the North China Craton. The geochronology and petrogenesis of the mafic dykes in the northern HTB are keys to understanding the tectonic evolution of this belt. The mafic dykes, intruded into the Neoarchean-Paleoproterozoic metamorphic basement, are mainly composed of diabase with a mineral assemblage of plagioclase(45%-60%), pyroxene(25%-35%), minor quartz and Fe-Ti oxides. The LA-ICPMS U-Pb analysis of zircon grains from representative dykes yield a weighted mean age of 206 ± 1.9 Ma, which represents the crystallization age of the dyke. The diabases show high contents of Fe_2 O_3~T(11.88-17.55 wt.%), low contents of SiO_2(45.65-50.95 wt.%) and MgO(3.31-5.50 wt.%) with low Mg#(=100×MgO/(MgO + FeO) atomic ration) of 33-44. They are characterized by enrichment of light rare earth elements(LREEs) and large ion lithophile elements(LILEs)(e.g., Rb, Ba and Pb), and slight depletion of high field strength elements(HFSEs). These features suggest that the magma has undergone extensive fractionation of olivine and pyroxene but only minor crustal contamination during its evolution. Their high Sm contents and La/Sm ratios, and low Sm/Yb ratios indicate that magma from which the dykes formed was derived from low degree(about 5%) partial melting of an enriched garnet + spinel lherzolite mantle source. Together with regional geology, these geochemical and geochronological data suggest that the mafic dykes in the HTB were formed in an intracontinental extensional setting during the late Triassic.     

Timing and tectonic significance of Paleozoic magmatism in the Sakar unit of the Sakar-Strandzha Zone,SE Bulgaria

ABSTRACT

Palaeozoic granitoids and meta-granitoids dominate the metamorphic basement of the Sakar unit of the Sakar-Strandzha Zone (SASTZ) in southeast Bulgaria. In this article, we present new whole-rock geochemical data and U–Pb zircon geochronology for the Sakar unit granitoids. The igneous minerals and textures are preserved, except the meta-granitoids that experienced a weak amphibolite-facies overprint. Geochemistry reveals compositions of peraluminous high-K calc-alkaline I- to S-type granitoids of volcanic arc origin. A major group of LILE-LREE-enriched granitoids and meta-granitoids and a single HFSE-HREE-enriched meta-granitoid are distinguished. U–Pb geochronology has yielded crystallization ages between 305 and 295 Ma for the major group granitoids and a ca. 462 Ma crystallization age of HFSE-HREE-enriched meta-granitoid. Late Palaeozoic granitoids of the Sakar unit show similar compositions and a similar tectonic setting when compared to other granitoids of the SASTZ, confirming a uniform region-wide tectono-magmatic event. As the Late Carboniferous-Permian magmatic arc components extend across the SASTZ, they trace the time-correspondent active continental margin along the Eurasian plate during subduction of the Palaeotethys oceanic lithosphere. The late Palaeozoic Eurasian active continental margin magmatic arc evolution of the SASTZ can be extended into the Serbo-Macedonian-Rhodope zones to the west, where time equivalent meta-granitoids support the same geodynamic context.     

Provenance of middle to late Palaeozoic sediments in the northeastern Colombian Andes: implications for Pangea reconstruction

ABSTRACT

Global-scale Palaeozoic plate tectonic reconstructions have suggested that Laurentia was obliquely approaching against the northwestern margin of Gondwana until the final agglutination of Pangea. In this contribution integrated petrographic analysis, heavy mineral analysis, and tourmaline geochemistry were done, and U–Pb detrital zircon geochronology was obtained, in late Palaeozoic sedimentary and meta-sedimentary units from the Floresta and Santander Massifs in the Eastern Colombian Andes in order to constrain their provenance and related it with the magmatic, sedimentary, and deformational record of the Gondwana–Laurentia convergence until the late Carboniferous to Permian formation of Pangea. Late Devonian to early Carboniferous sandstones from the Floresta Massif changed from sublithoarenites to lithoarenites, tracking the progressive uplift and unroofing of sedimentary and metamorphic rocks, with associated volcanic activity. The U–Pb detrital zircon geochronology from the sedimentary and metasedimentary of Floresta and Santander documents Mesoproterozoic and Palaeoproterozoic sources, and younger Ordovician to Silurian age populations, that can be related to the early to middle Palaeozoic plutonic rocks and the Amazon Craton. The limited Silurian to Early Devonian detrital ages that contrast with the more significant Middle to Late Devonian zircons that document the erosion of contemporaneous magmatic sources formed after a late Silurian to Early Devonian reduction on the magmatic activity along the proto-Andean margin. These rocks were apparently deformed and metamorphosed between the late Carboniferous and the early Permian. It is suggested that the filling and deformation record of these rocks documented the changes in plate convergence obliquity at the western margin of Gondwana associated with the migration of Laurentia until its final position in Pangea. Between the late Carboniferous and the early Permian, peri-Gondwanan continental terranes also collided with the continental margin. Over-imposed Mesozoic tectonics have contributed to the final redistribution of these terranes to their current position.

Abbreviations:LA: laser ablation inductively couple mass spectrometer; CL: cathodoluminiscence     

Early Carboniferous anorogenic magmatism in the Levant: implications for rifting in northern Gondwana

The Variscan orogenesis in Europe peaked during the Late Devonian–Early Carboniferous times when Gondwanan terranes collided with Laurasia. Hitherto it has been thought that Carboniferous tectonics in northern Arabia and the adjacent parts of NE Africa were broad swells (‘arches’) and depressions (‘basins’) that formed as a far-field contractional effect of the Variscan compression. The discovery of a 351 ± 3 Ma (U–Pb in zircon) within-plate felsic volcanism in the Helez borehole, southern coastal Israel, suggests that the Levant Arch is, instead, extensional in origin. Felsic volcanism was associated with gabbro underplating of the crust, an extreme (~50°C/km) crustal thermal gradient, major uplift, and truncation of the ≥2.5 km section. Taken together with the recent discovery of the ~340 Ma oceanic crust in the Eastern Mediterranean, the Levant Arch is interpreted as an uplifted shoulder of a rift, preceding ocean spreading.     

Late Mesozoic high-K calc-alkaline magmatism in Southeast China: the Tongling example

ABSTRACT

We report new zircon U–Pb ages, Hf isotopic and geochemical results for the Tongling granitic plutons of Southeast China. SHRIMP U–Pb ages for the Miaojia quartz monzodiorite porphyrite,the Tianebaodan and Tongguanshan quartz monzodiorites, the Xinqiaotou granodiorite porphyry, and the Shatanjiao and Nanhongchong granodiorite are 143 ± 2, 141 ± 1 and 142 ± 1, 147 ± 1, and 145 ± 1 and 139 ± 1 Ma, respectively. Combined with previous geochronological data, our results indicate that the porphyritic rocks are older than rocks of the same type lacking porphyritic texture. Geochemically, these high-K calc-alkaline intrusive rocks are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Lu–Hf isotopic compositions of zircons from the high-K calc-alkaline (HKCA) rocks have ε_Hf(t) values of magmatic 139–147 Ma zircons from ?8.1 to ?25.6, with two-stage model ages (t_DM2) of 1.71–2.67 Ga, whereas ε_Hf(t) values of inherited 582–844 Ma zircons range from 5.4 to ?9.5, with t_DM2 of 1.39–2.22 Ma, younger than t_DM2 values of igneous zircon, indicating that newly formed mantle material was added to the continental crust of the Yangtze Block. Moreover, ε_Hf(t) values of inherited zircon cores older than 1000 Ma are from ?7.8 to ?26, similar to magmatic zircons, and the t_DM2 values are all greater than 3.0 Ga (3.16–3.75 Ga), reflecting partial melting of ancient sialic material. We conclude that the plutonic melts were derived from both the enriched mantle and the ancient crust. The HKCA Tongling intrusions coincide temporally with the J₃–K₁ magmatic event that was widespread in Southeast China. This igneous activity may have accompanied sinistral slip along the Tan-Lu fault due to oblique subduction of the Palaeo-Pacific plate.     

An uncommon episode of mafic magmatism at 1347 Ma in the Mesoproterozoic Telemark supracrustals, Sveconorwegian orogen—Implications for stratigraphy and tectonic evolution

The Mesoproterozoic Telemark supracrustals in southern Norway comprise two major assemblages of bimodal volcanic and clastic metasedimentary rocks. The older Vestfjorddalen supergroup evolved from A-type, ca. 1500 Ma continental felsic volcanism, via within-plate type basaltic volcanism, into open sea siliciclastic sedimentation, and produced an at least 5 km thick, quartzite-dominated sequence, the Vindeggen group. It overlies a basement formed by just slightly older, 1550–1500 Ma mature arc rocks. The younger, 1170–1140 Ma Sveconorwegian supergroup was characterized by bimodal volcanism, associated with plutonism, and with several intervening periods of clastic sedimentation. The metadiabase dated in this study cuts the Vindeggen group at the top of the older supergroup and is itself delimited by an unconformity at the bottom of the younger supergroup. The 1347 ± 4 Ma age, obtained by ID-TIMS analysis of zircon, defines a mimimum age for deposition of the Vindeggen group. The age is unique in the regional context but in general terms it fits a pattern of episodic and locally intense magmatism that characterized the Mesoproterozoic development of the margins of Proto-Baltica and -Laurentia and has been related to the evolution of a long-lived convergent margin. The similarities between some of these terranes and distinctiveness from others, in both orogens, may indicate outboard evolution of the Telemarkia and Frontenac terranes before their aggregation within the Sveconorwegian–Grenvillian orogen.     

Soil-geochemistry,radiometric and soil gas helium studies in the uranium mineralized zone of Tumallapalle,Cuddapah Basin,Andhra Pradesh

A geochemical soil sampling survey undertaken at Tumallpalle uranium mineralized zone Cuddapah district, has confirmed the presence of uranium anomalies in soils. Bulk soil samples were collected at every 20 m along the traverse from approximately 30 cm below the surface and were assayed for uranium by x-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). The uranium anomalies detected by the insitu radiometric survey show a correlation with the helium highs. The soil gas helium studies have aided in delineating the subsurface extension of the deposit. This study endeavors for an integration of different techniques in a known area which could probably aid in delineating uranium zones for optimal exploitation in the future exploration programmes.     

Geochronological evidence and tectonic significance of Carboniferous magmatism in the southwest Trabzon area,eastern Pontides,Turkey

The northern and southern zones of the eastern Pontides (northeast Turkey) contain numerous plutons of varying ages and compositions. Geochemical and isotopic results on two Hercynian granitoid bodies located in the northern zone of the eastern Pontides allow a proper reconstruction of their origin for the first time. The intrusive rocks comprise four distinct bodies, two of which we investigated in detail. Based on LA–ICP–MS U–Pb zircon dating, the Derinoba and Kayadibi granites have similar ²⁰⁶Pb/²³⁸U versus ²⁰⁷Pb/²³⁵U Concordia ages of 311.1 ± 2.0 and 317.2 ± 3.5 million years for the former and 303.8 ± 1.5 million years for the latter. Aluminium saturation index values of both granites are between 0.95 and 1.35, indicating dominant peraluminous melt compositions. Both intrusions have high SiO₂ (74–77 wt.%) contents and show high-K calc-alkaline and I- to S-type characteristics. Primitive mantle-normalized element diagrams display enrichment in K, Rb, Th, and U, and depletion in Ba, Nb, Ta, Sr, P, and Ti. Chondrite-normalized rare earth element patterns are characterized by concave-upward shapes and pronounced negative Eu anomalies with La_cn/Yb_cn?=?4.6–9.7 and Eu_cn/Eu*?=?0.11–0.59 (Derinoba), and La_cn/Yb_cn?=?2.7–5.5 and Eu_cn/Eu*?=?0.31–0.37 (Kayadibi). These features imply crystal-melt fractionation of plagioclase and K-feldspar without significant involvement of garnet. The Derinoba samples have initial ?Nd values between –6.1 and –7.1 with Nd model ages and T _DM between 1.56 and 2.15 thousand million years. The Kayadibi samples show higher initial ?Nd_(I) values, –4.5 to –6.2, with Nd model ages between 1.50 and 1.72 thousand million years. This study demonstrates that the Sr isotope ratios generally display negative correlation with Nd isotopes; Sr isotope ratios were lowered in some samples by hydrothermal interaction or alteration. Isotopic and petrological data suggest that both granites were produced by the partial melting of early Palaeozoic lower crustal rocks, with minor contribution from the mantle. Collectively, these rocks represent a late stage of Hercynian magmatism in the eastern Pontides.     

Petrogenesis of the Mairupt microgranite: A witness of an Uppermost Silurian magmatism in the Rocroi Inlier,Ardenne Allochton

Magmatism in the Rocroi inlier (Ardenne Allochton, southeastern Avalonia during eo-Hercynian times) consists of a swarm of bimodal dykes (diabase and/or microgranite) emplaced in Middle to Upper Cambrian siliciclastics (Revin Group). Felsic volcanites interbedded within the Upper Silurian/Lower Devonian transgressive strata on the eastern edge of the inlier were interpreted as belonging to the same magmatic event. This was subsequently invalidated by zircon U–Pb dating of the Mairupt and Grande Commune magmatic rocks, which yielded an Upper Devonian age. Here we report a reevaluation of the age of the Mairupt microgranite based on LA–ICP–MS in situ U–Pb zircon geochronology, which yields a concordant age of 420.5 ± 2.9 Ma (Late Silurian/Early Devonian). This new dating restores the consistency between the different magmatic occurrences in the Rocroi inlier. The geochemical and petrographical data furthermore indicate a major crustal contribution, which fits well within the context of crust thinning of the Ardenne margin (southeastern Avalonia) in the transtensional Rheno-Hercynian back-arc basin.     

Late syn-to post-collisional magmatism in Madagascar:The genesis of the Ambalavao and Maevarano Suites

The East African Orogen involves a collage of Proterozoic microcontinents and arc terranes that became wedged between older cratonic blocks during the assembly of Gondwana.The Ediacaran-Cambrian Ambalavao and Maevarano Suites in Madagascar were emplaced during the waning orogenic stages and consist of weakly deformed to undeformed plutonic rocks and dykes of mainly porphyritic granite but also gabbro,diorite and charnockite.U-Pb geochronological data date emplacement of the Ambalavao Suite to between ca.580 Ma and 540 Ma and the Maevarano Suite to between ca.537 Ma and522 Ma.Major and trace element concentrations are consistent with emplacement in a syn-to postcollisional tectonic setting as A-type(anorogenic) suites.Oxygen(δ~(18)O of 5.27‰-7.45‰) and hafnium(ε_(Hf)(t) of-27.8 to-12.3) isotopic data from plutons in the Itremo and Antananarivo Domains are consistent with incorporation of an ancient crustal source.More primitive δ~(18)O(5.27‰-5.32‰) andε_(Hf)(t)(+0.0 to+0.2) isotopic values recorded in samples collected from the Ikalamavony Domain demonstrate the isotopic variation of basement sources present in the Malagasy crust.The Hf isotopic composition of Malagasy zircon are unlike more juvenile Ediacaran-Cambrian zircon sou rces elsewhere in the East African Orogen and,as such,Madagascar represents a distinct and identifiable detrital zircon source region in Phanerozoic sedimentary provenance studies.Taken together,these data indicate that high-T crustal anatexis,crustal assimilation and interaction of crustal material with mantle-derived melts were the processes operating during magma emplacement.This magmatism was coeval with polyphase deformation throughout Madagascar during the amalgamation of Gondwana and magmatism is interpreted to reflect lithospheric delamination of an extensive orogenic plateau.     

Geochronological data from TTG-type rock associations of the Arroio dos Ratos Complex and implications for crustal evolution of southernmost Brazil in Paleoproterozoic times

U–Pb isotope analyses by LA-MC-ICPMS (Laser Ablation – Multi Collector – Inductively Coupled Plasma Mass Spectrometry) in zircon crystals from metatonalites, tonalites and granodiorite gneiss from the Arroio dos Ratos Complex (ARC) early magmatism in southernmost Brazil are presented. The ARC is located in the eastern portion of the Sul-rio-grandense Shield, occurring as septa and roof pendants on granitoids emplaced along the Southern Brazilian Shear Belt (SBSB). The SBSB corresponds to a translithospheric structure composed of several anastomosed shear zones of dominantly transcurrent kinematics whose syntectonic magmatism, of Neoproterozoic age, is characteristic of post-collisional environments. The studied rocks comprise TTG-type associations with coeval mafic magmatism, deformed and metamorphosed within a ductile shear zone. Zircon crystals obtained from six samples are interpreted as igneous given that the crystals are subhedral to euhedral, bipyramidal, with concentric zonation, have ratios Th/U between 0.13 and 0.81 and have restricted evidence of overgrowth. The oldest Association 1 (A1) has structures compatible with recrystallization under conditions of high temperature and an igneous age of 2148 ± 33 Ma, obtained in a metatonalite. The rocks of Association 2 (A2) have similar compositions, although with a more significant coeval mafic fraction. They are intrusive into A1 and also show high-temperature recrystallization features. However, they are less deformed and partly preserve their primary, igneous fabric. The igneous ages obtained from two A2 tonalites are 2150 ± 28 Ma and 2136 ± 27 Ma. Association 3 (A3) is represented by tonalitic to granodioritic gneisses whose structure, composition and metamorphic features are similar to those of A1 rocks, except for the absence of coeval mafic magmas in the former. Local features resulting from partial melting are present in A3 rocks. Three samples from A3 were dated. A tonalitic gneiss gives igneous age of 2099 ± 10 Ma and two granodioritic gneisses give igneous ages of 2081 ± 7 Ma and 2077 ± 13 Ma. Restricted to A1, inheritance is represented by one subhedral, zoned, gently rounded zircon crystal interpreted as igneous, of 2732 ± 40 Ma (²⁰⁷Pb/²⁰⁶Pb age), with discordance of 9% and ²³²Th/²³⁸U ratio of 1.17. A single Neoproteozoic metamorphic date value was obtained from the rim of a zircon crystal of Paleoproterozoic core. The age of 635 ± 6 Ma (²⁰⁷Pb/²⁰⁶Pb age), with Th/U ratio < 0.1 and 1% discordance, is interpreted as compatible with adjacent SBSB magmatism. The three associations are interpreted to represent the record of successive magmatic pulses that mark the evolution of a Paleoproterozoic continental magmatic arc. In the study area, these magmatic arc associations represent relict areas partly reworked and relatively well-preserved from Neoproterozoic tectono-magmatic post-collisional events during the construction of the Southern Brazilian Shear Belt.     

Early Mesoproterozoic metamorphism in the Barossa Complex,South Australia: links with the eastern margin of Proterozoic Australia

LA-ICP-MS U–Pb geochronological data from metamorphic monazite in granulite-facies metapelites in the Barossa Complex, southern Australia, yield ages in the range 1580–1550 Ma. Metapelitic rocks from the Myponga and Houghton Inliers contain early biotite–sillimanite-bearing assemblages that underwent partial melting to produce peak metamorphic garnet–sillimanite-bearing anatectic assemblages. Phase equilibrium modelling suggests a clockwise P–T evolution with peak temperatures between 800 and 870°C and peak pressures of 8–9 kbar, followed by decompression to pressures of ～6 kbar. In combination with existing age data, the monazite U–Pb ages indicate that the early Mesoproterozoic evolution of the Barossa Complex is contemporaneous with other high geothermal gradient metamorphic terranes in eastern Proterozoic Australia. The areal extent of early Mesoproterozoic metamorphism in eastern Australia suggests that any proposed continental reconstructions involving eastern Proterozoic Australia should share a similar tectonothermal history.     

Petrogenesis of Ordovician magmatic rocks in the southern Chiapas Massif Complex: relations with the early Palaeozoic magmatic belts of northwestern Gondwana

The recent discovery of Early Ordovician S-type granites in the southwest of the Chiapas Massif Complex adds a new perspective to the Palaeozoic history of the Maya block, inasmuch as no rocks of such age had previously been reported in this region. New geologic mapping west of Motozintla, Chiapas, revealed pelitic to psammitic metasedimentary successions (Jocote Unit) intruded by granitoids and metabasites. The Jocote Unit is unconformably underlain by the newly defined Candelaria Unit, which comprises deformed calc-silicate rocks and interlayered folded amphibolites. The Candelaria Unit is the oldest rock succession so far recognized in the southern Maya block. We used laser-ablation multicollector inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating to determine the ages of the rock, yielding Early Ordovician (ca. 470 Ma) and Late Ordovician (ca. 450 Ma) ages.

Major and trace element geochemistry, as well as Nd and Sr isotope data, suggest that folded amphibolites of the Candelaria Unit are mantle-derived and are probably related to rifting. The Early Ordovician bimodal magmatism of the Jocote Unit is more strongly differentiated; it reflects crustal contamination and volcanic-arc chemical signatures. A granitic stock (Motozintla pluton) intruded the area in the Late Ordovician. Its geochemical composition indicates less crustal contamination and a mixed signature between volcanic-arc and within-plate settings. Magmatic rocks analogous in age and chemical character crop out in the Rabinal and the Altos Cuchumatanes areas of Guatemala, suggesting the existence of a semi-continuous Ordovician magmatic belt from Chiapas to central Guatemala. Similar but somewhat younger granites also occur in the Maya Mountains of Belize, suggesting that magmatism migrated in the Silurian from the Chiapas–Guatemala belt towards the Maya Mountains.     

Petrology, geochemistry and tectonic settings of the mafic dikes and sills associated with the evolution of the Proterozoic Cuddapah Basin of south India

In this article we summarize the petrological, geochemical and tectonic processes involved in the evolution of the Proterozoic intracratonic Cuddapah basin. We use new and available ages of Cuddapah igneous rocks, together with field, stratigraphic, geophysical and other criteria, to arrive at a plausible model for the timing of these processes during basin evolution. We present petrological and geochronological evidence of dike emplacement along preferred lineament directions around the basin in response to stresses, which may have been responsible for the evolution of the basin itself. Basaltic dike intrusion started on the south Indian shield around 2400 Ma and continued throughout the Cuddapah basin evolution and sedimentation. A deep mantle perturbation, currently manifested by a lopolithic cupola-like intrusion under the southwestern part of the basin, may have occurred at the onset of basin evolution and played an important role in its development. Paleomagnetic, gravity and geochronological evidence indicates that it was a constant thermal source responsible for dike and sill emplacement between 1500 and 1200 Ma both inside and out-side the basin. Lineament reactivation in the NW-SE and NE-SW directions, in response to the mantle perturbation, intensified between 1400 and 1200 Ma, leading to the emplacement of several cross cutting dikes. Fe-Mg partition coefficients of olivine and augite and Ca-Na partition coefficient of plagioclase, calculated from the composition of these minerals and bulk composition of their host rocks, indicate that the dikes outside the Cuddapah basin are cumulates. The contemporary dikes may be related by fractional crystallization as indicated by a positive correlation between their plagioclase Ca# (atomic Ca/[Ca+Na]) and augite Mg# (atomic Mg/[Mg+Fe]). A few NW-SE and NE-SW cross cutting dikes of the period between 1400 and 1200 Ma, preserve petrographic evidence of episodic magmatic intrusive activity along preferred directions. Petrological reasoning indicates that a magmatic liquid reacted with a set of cross cutting dikes, intruding into one that was already solidified and altering the composition of the magma that produced the other dike. The Cuddapah basin tholeiites may be related by fractional crystallization at 5 kb and 1019-1154‡ C, which occurred in the lopolithic cupola near the southwestern margin of the basin. Xenolith bearing picrites, which occur near the periphery of the cupola, originated by the accumulation of xenoliths in the tholeiites. This is indicated by the composition of the olivine in the xenoliths (Fo_78.7-81.9), which are closely similar to calculated olivine compositions (Fo_77.8-78.3) in equilibrium with the tholeiites under the sameP-T conditions. It is inferred that fractionation in the cupola resulted in crystals settling on its walls. Hence, the xenolith-bearing sills occur at the periphery of the lopolithic body. The tholeiites both inside and outside the basin are enriched in incompatible elements compared to mid oceanic ridge basalts. The Ba, Rb and K contents of the Cuddapah and other Proterozoic Gondwana tholeiites indicate that a widespread metasomatic enrichment of the mantle source may have occurred between R∼2.9 and R∼2.7Ga. There may be local heterogeneity in the source of the Cuddapah tholeiites as indicated by different Ba/Rb, Ti/Zr, Ti/Y, Zr/Nb and Y/Nb in samples inside and outside the basin. Large-scale differences such as the low P₂O₅-TiO₂ and high P₂O₅-TiO₂ basaltic domains of the Jurassic Gondwana basalts, however, did not exist during the Proterozoic time period under consideration. Although we are beginning to understand the tectono-magmatic processes involved in the evolution of the Cuddapah basin, much work remains to be done to obtain a complete picture. Future research in the Cuddapah basin should focus on obtaining accurate ages of the igneous rocks associated with the evolution of the basin.     

Triassic ore-bearing and barren porphyries in the Zhongdian Arc of SW China: implications for the subduction of the Palaeo-Tethys Ocean

The NS-treading Zhongdian Arc located in the southern part of the Yidun Arc is an important region to address the evolution and reconstruction of the Palaeo-Tethys Ocean and related mineralization. In this study, we investigate three barren intrusions in the Zhongdian Arc and present geochemical compositions, zircon U–Pb dating and Hf isotopic compositions. Zircons from the three intrusions yielded U–Pb ages of ~227.5, ~222.5, and ~230 Ma, with highly variable ε_Hf(t) values (?20.5 to 4.3). These quartz monzonite porphyries show typical adakitic affinity, and it is inferred that these intrusions in the Zhongdian Arc, together with those in the northern Yidun Arc, were derived from the partial melting of mantle wedge and contaminated by minor lower crustal components during the westward subduction of the Ganzi-litang Ocean, which probably resulted from the Triassic continental collision between the south China and the north China blocks. In the Yidun Arc, the Triassic ore-bearing intrusions have ε_Hf(t) values that cluster around zero, while the barren intrusions possess negative ε_Hf(t) values, suggesting that the mantle lithospheric components played an important role in the Triassic ore-bearing porphyries.