Rock Rb Research Articles

Zircon U–Pb geochronology of the Konggar granitoid and migmatite: Constraints on the Oligo-Miocene tectono-thermal evolution of the Xianshuihe fault zone, East Tibet

Abstract The Konggar massif, about 120 km long and 13 – 18 km wide, developed along the eastern segment of the Xianshuihe fault zone in East Tibet. It consists of two rock units: the Konggar granitic pluton and an elongate migmatite zone about 70 km long and 1 – 3 km wide. A metamorphic event was well recorded by the growth rims of zircons in the migmatite, their SHRIMP U–Pb dating of two rock samples taken from leucosome and melanosome yields respectively the ages of ca. 31.75 Ma and ca. 26.9 Ma. The SHRIMP U–Pb dating of zircons from two rock samples of the granitic pluton yields respectively the crystalline ages of 17.35 Ma and 14.4 Ma. These new data, together with the previously zircon U–Pb ages, the mica Ar–Ar dating ages and whole rock Rb–Sr age of the granitoid, allows to constraining the tectono-thermal evolution history of the Xianshuihe fault zone. It is inferred that this fault zone suffered along its eastern segment from high temperature metamorphism and migmatization during the Oligocene (in 32 – 27 Ma), which was followed by magma intrusion during the Miocene (in 18 – 12 Ma); sinistral shearing began to occur at about 10 Ma and continues to present-day.

Single crystal U–Pb zircon age and Sr–Nd isotopic composition of impactites from the Bosumtwi impact structure, Ghana: Comparison with country rocks and Ivory Coast tektites

The 1.07Myr old Bosumtwi impact structure (Ghana), excavated in 2.1–2.2Gyr old supracrustal rocks of the Birimian Supergroup, was drilled in 2004. Here, we present single crystal U–Pb zircon ages from a suevite and two meta-graywacke samples recovered from the central uplift (drill core LB-08A), which yield an upper Concordia intercept age of ca. 2145±82Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana. Whole rock Rb–Sr and Sm–Nd isotope data of six suevites (five from inside the crater and one from outside the northern crater rim), three meta-graywacke, and two phyllite samples from core LB-08A are also presented, providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the Bosumtwi crater rocks. Our Rb–Sr and Sm–Nd data show also that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our new isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites (i.e., distal impactites), which is thought to consist of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than the suevites (i.e., proximal impactites). When plotted in a Rb/Sr isochron diagram, the sample data points (n=29, including literature data) scatter along a regression line, whose slope corresponds to an age of 1846±160Ma, with an initial Sr isotope ratio of 0.703±0.002. However, due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material, this age, which is in close agreement with a possible “metamorphic age” of ∼1.8–1.9Ga tentatively derived from our U–Pb dating of zircons, is difficult to consider as a reliable metamorphic age. It may perhaps reflect a common ancient source whose Rb–Sr isotope systematics has not basically been reset on the whole rock scale during the Bosumtwi impact event, or even reflect another unknown geologic event.

Geochronology, petrogenesis and tectonic implications of granites from the Fuxin area, Western Liaoning, NE China

New geochemical and Sr–Nd isotopic data from the Ping'andi granitic pluton in the Fuxin area, which straddles the North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB) border, are presented here to discuss the geochronology, petrogenesis and tectonic implications. Petrographically, the Ping'andi granites contain monzogranites to minor granodiorites. They are high-K calc-alkaline series, silica-rich (SiO 2 = 66–78 wt.%), peraluminous (A/CNK = 0.98–1.10), with enrichment in LREE (La n/Yb n = 6.51–8.53), moderately negative Eu anomalies (Eu/Eu* = 0.33–0.90), with enrichment in LILE (Rb and K) and with depletion in HFSE (Nb, Ti and P). They also have slightly negative and positive ε Nd(t) (− 1.1–0.4), low initial I Sr (0.70587–0.70620) and young depleted-mantle model ages ( T DM2 = 911–1027 Ma). The emplacement of the Ping'andi granites took place at 150 Ma as revealed by whole rock Rb–Sr isochron age. The petrography, geochemistry and isotopic compositions indicate that they are I-type and were the result of partial melting of mixed sources composed of a juvenile component and a pre-existing crust, coupled with plagioclase-dominated fractionation, and that there was a significant crustal growth in the Phanerozoic in the northern margin of the NCC. The formation of the Ping'andi granites was related to the tectonic transformation from the convergent tectonic regime of Paleo-Asian Ocean to that of the Paleo-Pacific Ocean, which also induced the huge NNE-trending Jurassic granite belt and Mo–Cu metallogenic belt in eastern China.

Experimental study of polybaric REE partitioning between olivine, pyroxene and melt of the Yamato 980459 composition: Insights into the petrogenesis of depleted shergottites

A synthetic composition representing the Yamato 980459 martian basalt (shergottite) has been used to carry out phase relation, and rare earth element (REE) olivine and pyroxene partitioning experiments. Yamato 980459 is a sample of primitive basalt derived from a reduced end-member among martian mantle sources. Experiments carried out between 1–2 GPa and 1350–1650 °C simulate the estimated pressure–temperature conditions of basaltic melt generation in the martian mantle. Olivine-melt and orthopyroxene-melt partition coefficients for La, Nd, Sm, Eu, Gd and Yb ( D REE values) were determined by LA-ICPMS, and are similar to the published values for terrestrial basaltic systems. We have not detected significant variation in D-values with pressure over the range investigated, and by comparison with previous studies carried out at lower pressure. We apply the experimentally obtained olivine-melt and orthopyroxene-melt D REE values to fractional crystallization and partial melting models to develop a three-stage geochemical model for the evolution of martian meteorites. In our model we propose two ancient (∼4.535 Ga) sources: the Nakhlite Source, located in the shallow mantle, and the Deep Mantle Source, located close to the martian core-mantle boundary. These two sources evolved distinctly on the ε 143Nd evolution curve due to their different Sm/Nd ratios. By partially melting the Nakhlite Source at ∼1.3 Ga, we are able to produce a slightly depleted residue (Nakhlite Residue). The Nakhlite Residue is left undisturbed until ∼500 Ma, at which point the depleted Deep Mantle Source is brought up by a plume mechanism carrying with it high heat flow, melts and isotopic signatures of the deep mantle (e.g., ε 182W, ε 142Nd, etc.). The plume-derived Deep Mantle Source combines with the Nakhlite Residue producing a mixture that becomes a mantle source (herein referred to as “the Y98 source”) for Yamato 980459 and the other depleted shergottites with the characteristic range of Sm/Nd ratios of these meteorites. The same hot plume provides a heat source for the formation of enriched and intermediate shergottites. Our model reproduces the REE patterns of nakhlites and depleted shergottites and can explain high ε 143Nd in depleted shergottites. Furthermore, the model results can be used to interpret whole rock Rb–Sr and Sm–Nd ages of shergottites.

Petrogenesis of the Nesryin gabbroic intrusion in SW Sinai, Egypt: new contributions from mineralogy, geochemistry, Nd and Sr isotopes

The Wadi Nesryin gabbroic intrusion is part of the Neoproterozoic Pan-African basement cropping out in southern Western Sinai of Egypt. The intrusion comprises hornblende gabbro, pyroxene–hornblende gabbro, diorite and appinitic varieties. It exhibits chilled margins against the older rocks represented by fine-grained gabbro and dolerite and belongs to what is known throughout Egypt as the “younger gabbro suite”. Mineralogy, mineral chemistry and whole rock geochemistry indicate that these rocks were derived from tholeiitic magmas with minor calc-alkaline affinity. They have chemical signatures of subduction related arc rocks formed at an active convergent plate margin. They were formed by 15–30% of partial melting of a garnet lherzolite and to a minor extent of spinel-garnet lherzolite sources, modified by fluids related to a subducting slab. Pressure estimates using the amphibole geobarometer indicate that the gabbroic rocks crystallized at pressures between 2.8 and 5.6 kbar (average = 4.3 kbar). Diorites record lower formation pressures between 1.8 and 3.7 kbar (average = 3.0 kbar). The temperature estimates calculated by several geothermometers yielded crystallization temperatures ranging from 674°C to 961°C, with an average of about 817°C. The whole rock Rb–Sr isochron age of the Nesryin gabbroic intrusion is 617 ± 19 Ma with initial 87Sr/86Sr = 0.70322 ± 0.00004. This age indicates that the mafic–ultramafic plutons in the Pan-African belt in southern Sinai belong to the Egyptian younger gabbros and not to the older metagabbro–diorite complexes or ophiolitic suites. The rocks have low 87Sr/86Sr initial ratios ranging from 0.703141 to 0.703338 and negative ɛ Sr ranging from −6.34 to −9.14. The initial 143Nd/144Nd ratios range from 0.511944 to 0.512145 with positive and high ɛ Nd values (1.93 to 5.86) reflecting a mantle contribution in their petrogenesis.

A deformed alkaline igneous rock–carbonatite complex from the Western Sierras Pampeanas, Argentina: Evidence for late Neoproterozoic opening of the Clymene Ocean?

A deformed ca. 570 Ma syenite–carbonatite body is reported from a Grenville-age (1.0–1.2 Ga) terrane in the Sierra de Maz, one of the Western Sierras Pampeanas of Argentina. This is the first recognition of such a rock assemblage in the basement of the Central Andes. The two main lithologies are coarse-grained syenite (often nepheline-bearing) and enclave-rich fine-grained foliated biotite–calcite carbonatite. Samples of carbonatite and syenite yield an imprecise whole rock Rb–Sr isochron age of 582 ± 60 Ma (MSWD = 1.8; Sri = 0.7029); SHRIMP U–Pb spot analysis of syenite zircons shows a total range of 206Pb– 238U ages between 433 and 612 Ma, with a prominent peak at 560–580 Ma defined by homogeneous zircon areas. Textural interpretation of the zircon data, combined with the constraint of the Rb–Sr data suggest that the carbonatite complex formed at ca. 570 Ma. Further disturbance of the U–Pb system took place at 525 ± 7 Ma (Pampean orogeny) and at ca. 430–440 Ma (Famatinian orogeny) and it is concluded that the Western Sierras Pampeanas basement was joined to Gondwana during both events. Highly unradiogenic 87Sr/ 86Sr values in calcites (0.70275–0.70305) provide a close estimate for the initial Sr isotope composition of the carbonatite magma. Sm–Nd data yield ɛNd 570 values of +3.3 to +4.8. The complex was probably formed during early opening of the Clymene Ocean from depleted mantle with a component from Meso/Neo-proterozoic lower continental crust.

Geological and geochemical characteristics of the Sawaya'erdun gold deposit, southwestern Chinese Tianshan

The Sawaya'erdun gold deposit in the western segment of the South Tianshan, Xinjiang, China, is a large, low-grade orogenic gold deposit. An abundance of index fossils in the ore-hosting strata, including brachiopoda, corals and Schwagerina, show that the fossil age of the country rocks ranges from Late Carboniferous to Early Permian. Whole rock Rb–Sr and Sm–Nd isochron ages, ranging from 292.43 ± 0.38 to 304.7 ± 11.6 Ma, are concordant with a Late Carboniferous age. Gold mineralization is strictly controlled by a fault zone. Ore minerals are dominated by sulfides, with gold inhomogeneously distributed within them; gangue minerals are quartz, siderite or calcite. Data suggest that the ore-forming fluid was derived predominantly from an active meteoric groundwater system. Ore-forming temperatures are estimated to have been within the range 110 to 220 °C. New 40Ar/ 39Ar isotope age determinations of auriferous quartz are in excellent agreement with Rb–Sr ages obtained from fluid inclusions in quartz. They suggest gold mineralization at 206 to 213 Ma (Late Indosinian). Such an age is consistent with the age of formation given for other gold deposits in the region, such as Bulong and Dashankou. Combined with K–Ar geochronological data and observed geological features, it is concluded that the Indosinian metallogentic epoch represents a previously unrecognized, major period of gold metallogeny in the southwestern Chinese Tianshan. In some ways, the Sawaya'erdun gold deposit can be compared with the Muruntau gold deposit in Uzbekistan with respect to geological setting, host lithology, mineralization style, mineral assemblages, geochemical association and metallogenic processes involved. Despite this, the Sawaya'erdun gold deposit possesses a number of characteristics typical of only low-temperature metallogenesis.

Late Neoproterozoic Dokhan Volcanics, North Eastern Desert, Egypt: Geochemistry and petrogenesis

Origin and tectonic setting of the late Neoproterozoic Dokhan Volcanics (ca. 610–560 Ma by whole rock Rb–Sr and ca. 600–590 Ma by SHRIMP U–Pb zircon) in the Egyptian Eastern Desert is debated. Debate concerns the tectonic setting they formed in, during transition between convergent to extensional, during or after collision of E and W Gondwana ∼600 Ma. In order to solve this problem, we studied the geology and analyzed major and trace elements of lavas from Wadi Um Sidra and Um Asmer in the northern Eastern Desert. These range from medium- to high-K basalt to rhyolite. They comprise voluminous medium- to high-K calc-alkaline lavas, subordinate adakitic lavas, and minor alkali basalt. Adakitic lavas contain very low Y (<18 ppm) and heavy REE (HREE) (Yb ≤ 1 ppm) contents, with high Sr (>767 ppm) and Sr/Y (>40). They also have high Cr and Ni and low Nb, Rb, and Zr contents compared to coexisting calc-alkaline lavas. The adakitic lavas exhibit a fractionated rare earth element (REE) patterns with HREE depletion (La N/Lu N = 11.1–14.4), while calc-alkaline rocks have less fractionated REE patterns (La N/Lu N = 6.3–9.4). REE patterns of the alkali basalt are also similar to that of adakitic lavas. These alkali basalt, calc-alkaline, and adakitic lavas all show enrichment in large ion lithophile elements (LILEs) and show negative Nb–Ta anomalies with high Th/Zr, which suggest involvement of subducted slab melts in their petrogenesis. Our proposal is subduction of hot oceanic ridge before collision and later melting of the hot oceanic slab beneath Dokhan even after collision event.

The geochemistry and age of ophiolitic strata of the Xinglongshan Group: Implications for the amalgamation of the Central Qilian belt

The Xinglongshan Group comprises a metabasalt and metasedimentary rock (quartz–mica schist) assemblage representative of low-grade metamorphic basement strata in the Central Qilian belt. Metabasaltic compositions include MORB-like tholeiites and alkaline basalts of axial seamount or ocean island origin. Depending on assumed origin, the latter yield Sm–Nd model ages between 824 and 490 Ma. Whole rock Rb–Sr, and Sm–Nd systematics of acid leached metasedimentary rock samples define a Late Cambrian isochron of 513 ±17 Ma with initial an 87Sr/ 86Sr ratio of 0.70930 ±0.00050 which is interpreted to mark the timing of basin closure. High abundances of trace elements (Cr, Ni, Sc) associated with ferromagnesian minerals, relative to elements (Th, Zr) associated with resistant minerals, precludes derivation of the sediment assemblage solely from gneisses of the Central Qilian belt. Rather, the Nd–Sr isotope systematics of the metasedimentary rock samples indicate mixing between a mafic or arc component and an ancient felsic crustal composition. Th–Sc abundances suggest the felsic component was not the supracruastal sequences of the North China block. Isotopic and geochemical data thus suggest low-grade metamorphic basement strata in the Central Qilian belt are of likely NeoProterozoic age, assembly of the belt took place during the early stages of the Caledonian orogenic cycle before closure of ocean basins in the North Qilian belt, and that the Central Qilian belt likely bears an allochthonous relationship to North China.

Late Neoproterozoic strongly peraluminous leucogranites, South Eastern Desert, Egypt ? petrogenesis and geodynamic significance

Late Neoproterozoic garnet-bearing leucogranites are developed locally along thrust faults in the South Eastern Desert, Egypt. This work presents field observations, whole rock major and trace element abundances, Rb–Sr isotope data and mineral chemistry for three occurrences in the Sikait-Nugrus area. Field observations show that the leucogranites cut the faults and their contact with the country rocks is sharp with no indication of contact metamorphism. They were intruded into a low-grade metamorphosed ophiolitic melange and a high-grade metamorphosed metasedimentary succession of biotite schist composition. Numerous biotite schist enclaves, having irregular and diffuse contacts, are recorded within the leucogranites. Whole rock Rb–Sr ages of the leucogranites from two different localities are 610±20 and 594±12 Ma respectively; they are interpreted as emplacement ages. The leucogranites contain more than 70% SiO2, and they are strongly peraluminous (A/CNK>1.1) with low TiO2, Fe2O3*, MgO, CaO, Ba, Sr, LREE, Eu/Eu* and Sr/Ba and high Rb, Rb/Zr, Rb/Sr and Rb/Ba. These geochemical parameters and the low initial 87Sr/86Sr ratios (∼0.703) indicate crustal derivation by dehydration partial melting from a juvenile protolith similar to the exposed biotite-rich metasediments. Models for the tectonic setting of these leucogranites suggest their emplacement during an extensional tectonic stage that follows continental collision. It is proposed that crustal heating, caused by decompression along shear zones, is responsible for the production of these granitic melts. The results support previous hypotheses and further document a regional late Neoproterozoic extensional tectonic event, which is probably related to the initial break-up of Gondwana.

New 207Pb– 206Pb and 40Ar– 39Ar ages from SW Montana, USA: constraints on the Proterozoic and Archæan tectonic and depositional history of the Wyoming Province

This paper addresses the Precambrian depositional and thermotectonic history of a prominent suite of metasupracrustal rocks exposed within Laramide uplifts of the northern Wyoming province, southwestern Montana. This suite consists of highly metamorphosed carbonates, pelites, sandstones and banded iron formations. New isotopic data supporting 430 40Ar– 39Ar biotite ages and four 207Pb– 206Pb step-leach garnet ages, not only constrain the timing of Early Proterozoic thermotectonism to between 1820 and 1740 Ma, but also challenge the conventional view that the supracrustal rocks were deposited and metamorphosed during the Late Archæan (pre-2750 Ma). No mineral-isotopic evidence has been found for a ≈2750 Ma thermotectonic event which, based upon a published but highly equivocal whole rock Rb–Sr errorchron, is widely assumed to have affected the metasupracrustal rocks. Therefore, this metamorphic age and a corresponding pre-2750 Ma age of deposition are discounted pending further data. Instead, it is proposed that the supracrustal rocks were derived from gneisses now exposed in the Beartooth Mountains to the east and deposited between the ≈2800 Ma metamorphism age of these gneisses and the 1820 Ma age of the oldest garnets in the Ruby Range. An older cryptic thermotectonic event is also potentially recognised beginning at ≈2470 Ma, based upon the age of monazite-dominated garnet determined in one sample of paragneiss. The nature of this cryptic event is obscured by the pervasive ≈1820–1785 Ma overprint, but its timing coincides with global superplume activity previously associated with the incipient break-up of supercontinent Kenorland and may be related in some way. We propose that the metasupracrustal sequence was deposited in late Archæan to early Proterozoic time, between ≈2750 and ≈2470 Ma and then underwent two episodes of Early Proterozoic thermotectonism. The first metamorphic event is the ≈2470 Ma cryptic event. The rocks were then metamorphosed for the second time at ≈1820 Ma producing the main NE-trending fabric and was followed by ≈1780–1740 Ma post-tectonic cooling through ≈300 °C. Most likely, this event reflects the timing of Wyoming–Medicine Hat continental collision along the Great Falls tectonic zone during the assembly of supercontinent Laurentia. This proposal may constrain the role of the Archæan Wyoming province during Early Proterozoic supercontinent break-up (Kenorland) and reassembly (Laurentia) in what is now central North America.

Precise U–Pb mineral ages, Rb–Sr and Sm–Nd systematics for the Great Dyke, Zimbabwe—constraints on late Archean events in the Zimbabwe craton and Limpopo belt

U–Pb dating of zircon and rutile from bronzitites of the P1 pyroxenite layer of the Great Dyke precisely constrains the crystallization age of this part of the intrusion to 2575.4±0.7 Ma. Whole rock Rb–Sr and Sm–Nd data of various rock types sampled along the entire length of the Great Dyke record inhomogeneous initial isotope ratios, and also later (<1.5 Ga) disturbances of the Rb–Sr and Sm–Nd isotope systems. The 2575.4±0.7 Ma emplacement age of the Great Dyke is ≈120 Ma older than assumed until recently, and calls for new interpretations of the crustal development of the Zimbabwe craton. Close temporal links between the intrusion of the Great Dyke and the emplacement of late Archean granitoids (Chilimanzi and Razi suites) of the Zimbabwe craton are indicated by the new precise age data. The Chilimanzi and Razi suites of granitoids form at least two sub-suites, which can be described as pre- and post-Great Dyke in age. The Great Dyke age now falls within the range of ages for tectonic events in the Limpopo belt including granitoid magmatism, metamorphism, and thrusting of the Northern Marginal Zone over the Zimbabwe craton. A west-to-east diachroneity in both thrusting and crustal stabilization is suggested by the observations that the Great Dyke cuts across the thrust in the west, but syn-tectonic granitoids that are younger than the Great Dyke are deformed by the thrusting in the east. Intrusion of the Great Dyke cannot be linked to collision of the Zimbabwe and Kaapvaal cratons. The overlap in ages of intrusion of the Great Dyke and late Archean events in the Zimbabwe craton shows that Archean crust was cratonized shortly after large-scale melting and granite intrusion.

Zircon U–Pb sensitive high mass‐resolution ion microprobe dating of granitoids in the Ryoke metamorphic belt, Kinki District, Southwest Japan

Abstract Zircon U–Pb sensitive high mass‐resolution ion microprobe dating was carried out on three types of granitic rock (gneissose biotite granodiorite, biotite granite and two‐mica granite) from the Cretaceous Ryoke belt of the Kinki district, Southwest Japan. The results give the ages of granitic magmatism in the Shigi‐san area of between 87 and 78 Ma and suggest extensive melting of the Cretaceous Ryoke granitic crust to form the two‐mica granite, probably at ca 80 Ma. Discrimination into older and younger granites based on development of gneissosity does not appear to represent the sequence of magma generation, although there is some scope in the interpretation of the zircon U–Pb data that would allow all three granites to form at 83 Ma. Compilation of chemical Th‐U‐total Pb isochron dating method ages, whole rock Rb–Sr isotope ages and U–Pb isotope ages indicates that most Ryoke plutonism occurred from ca 70 Ma to ca 100 Ma. Younger (85 Ma–70 Ma) plutonism with the formation of two‐mica granite occurred only in the eastern sector of the Ryoke belt, including the Kinki District.

Geochemistry of Precambrian mafic magmatic rocks of the Western Himalaya, India: petrogenetic and tectonic implications

Precambrian sequences of the Higher Himalayan Crystallines (Vaikrita Group) and the Lesser Himalaya (Chail, Jutogh and Jaunsar Groups), in the Garhwal and Himachal regions of the Western Himalaya, include abundant metamorphosed mafic lavas and dykes (amphibolites). A gabbroic body within the Chail Group has been dated at 1907±91 Ma (initial 87 Sr/ 86Sr ratio of 0.7022±0.0008) by the whole rock Rb–Sr method. This age is consistent with several age data (1800–2000 Ma: whole rock Rb–Sr method), assigned to the granitoids and gneisses of this region. These amphibolites and gabbros exhibit low-Ti tholeiite characteristics. The Lesser Himalayan samples are enriched in light rare earth elements (LREE) and large ion lithophile element (LILE), with distinct negative Nb, Sr, P and Ti anomalies. Conversely, the Vaikrita Group samples are characterized by less enriched LREE–LILE and absence of the above anomalies but have distinct positive Sr anomalies. The chemical characteristics of the Lesser Himalayan samples are remarkably similar to the basal Aravalli volcanics of the NW Indian shield, inferred to reflect significant components from enriched mantle sources and crustal contamination. Samples of the Vaikrita Group appear to have been influenced by an asthenospheric mantle and are less contaminated as indicated by the absence of negative Nb, P and Ti anomalies. These distinct geochemical characteristics are used to demarcate the Main Central Thrust along the Vaikrita Thrust. The Chail and Jutogh Groups are affiliated with the Lesser Himalaya and the Chail Group gabbro body may represent portion of one among the numerous magma chambers that fed a large Precambrian magmatic province in the south of the Main Central Thrust. The similar emplacement ages (1800–2000 Ma) for the mafic and felsic magmatic rocks, suggest that this region experienced a major episode of crustal generation and evolution in a rift environment.

Magma source and evolution of Late Neoproterozoic granitoids in the Gabal El-Urf area, Eastern Desert, Egypt: geochemical and Sr–Nd isotopic constraints

Granitoids in the Gabal El-Urf area in Eastern Egypt consist of a monzogranite pluton, belonging to the Younger Granite province, emplaced in granodioritic rocks. Whole rock Rb–Sr dating indicate ages of 650±95 Ma and 600±11 Ma for the granodiorites and monzogranites, respectively. The granodiorites (65–70% SiO2) are calc-alkaline and metaluminous with low Rb/Sr, Th and Nb contents, moderate enrichment in the LILE (K2O, Rb, and Ba) and display most of the chemical and field characteristics of syn-to late-tectonic I-type granitoids described elsewhere in the Arabian–Nubian Shield. The monzogranites (72–77% SiO2) are metaluminous to mildly peraluminous, highly fractionated and depleted in Al2O3, MgO, CaO, TiO2, Sr and Ba with corresponding enrichment in Rb, Nb, Zr, and Y. They can be correlated with the undeformed post-orogenic granites in the Arabian–Nubian Shield that chemically resemble A-type granites emplaced in extensional settings. The mineralogical and chemical variations within the granodiorites and monzogranites are consistent with their evolution by fractional crystallization. The granodiorites have a low initial 87Sr/86Sr ratio (0.7024) and high ∈Nd values (+6.9–+7.3) and are significantly different from those (initial 87Sr/86Sr ratio=0.7029, ∈Nd values=+5.2–+5.8) of the monzogranites. These data suggest a predominant mantle derivation for both granite types and demonstrate that they originated from different source materials.The granodiorite melt was most probably generated through vapour-saturated partial melting of an early Neoproterozoic depleted mafic lower-crust reservoir due to crustal thickening associated with orogenic compression and/or arc magma underplating. The mineralogical and geochemical data of the A-type monzogranites are consistent with their derivation as a residual granitic liquid from a LILE-enriched mafic magma through crystal-liquid fractionation of plagioclase, amphibole, Fe–Ti oxides and apatite. The parental mafic magma was originated in the upper mantle due to crustal thinning associated with extension in the late stage of the Neoproterozoic crustal evolution of north-eastern Egypt.

The Thermal History of the Huangmeijian Granite Intrusion in Anhiii and Its Relation to Mineralization: Isotopic Evidence

Abstract Whole—rock Rb—Sr, zircon U—Pb and hornblende, biotite and K—feldspar K—Ar ages are used to reconstruct the cooling history of the Huangmeijian intrusion in the Anqing—Lujiang quartz—syenite belt in Anhui. Oxygen isotope geothermometry of mineral pairs demonstrates that diffusion is a dominant factor controlling the closure of isotopic systems. Assuming the cooling of the intrusion is synchronous with a dicrease in local geothermal gradients, an emplacement depth of about 8 km and the magma crystallization temperature of 800 ± 50°C are estimated. The Huangmeijian intrusion experienced a rapid cooling process and uplifted after its emplacement and crystallization at 133 Ma B.P. with a cooling rate of 34.5°C / Ma and an uplifting rate of 0.35 mm/ a. The intrusion was rising until it rested at a depth of 3 km at a temperature of 300 ± 50°C about 14 Ma later. Then the intrusion was in slow cooling and uplifting with a cooling rate of 4.4°C / Ma and an uplifting rate of 0.04 mm/ a.U—Pb dating of pitchblende is done for the hydrothermal uranium deposit formed in the contact zone of the Huangmeijian intrusion. The result shows that the mineralization age is close to the closing time of the K—Ar system in biotite. The fluid inclusion thermometry indicates that the mineralization temperature is in agreement with the closure temperature of the biotite K—Ar system. This suggests a close relationship between the slow cooling of the intrusion and the hydrothermal uranium mineralization process.

Geochronology and isotope geochemistry of the Xiqiu spilite- keratophyre in Zhejiang Province, China

Spilite- keratophyre is a key member of the Shuangxiwu Group volcanic series in Zhejiang Province. Presented in this paper are the reliable Sm- Nd internal isochron age (1012±28 Ma, ЄNd(T)=4.4±0.1) and whole- rock Rb- Sr isochron age (972±40 Ma,ISr=0.70327±8) obtained for the first time by the authors. From the available Nd, Sr and O isotope data in conjunction with the major and trace elements data it is suggested that the Xiqiu spilite- keratophy re is the product of island- arc volcanism during the late Middle Proterozoic.

Rb-Sr and U-Pb dating of the Daguzhai and Luobuli granitic massifs in South China

Rb-Sr and U-Pb isotopic studies of the two contrasting granite types of the Daguzhai and Luobuli massifs in South China provide new constraints on the interpretation of isotopic age data for plutonic igneous rocks. A Rb-Sr internal isochron age of 146±7Ma for the Luobuli adamellite is interpreted to represent the age of magma crystallization, whereas the whole rock Rb- Sr isochron yields an older apparent age of 161±10Ma which is regarded as resulting from contamination processes affecting the petrogenesis of this adamellite. In the Daguzhai granite the marked scatter of whole- rock Rb-Sr data in isochron diagram is ascribed to the open system behavior of Rb during postmagmatic autometasomatism. Uniformity of initial87Sr /86Sr ratio in this granite is indicated in a plot of87Sr versus86Sr. The autometasomatism has also affected zircon U-Pb system, resulting in a spread of data along the concordia curve between 165 and 125Ma. This spread is regarded as indicating the duration of the autometasomatism.

Petrology of the Lower Tertiary Clarno Formation in north central Oregon: The importance of magma mixing

Predominantly Eocene rocks of the Clarno Formation in north central Oregon consist of lava flows, volcaniclastic rocks, and tuffaceous sedimentary rocks lying between Cretaceous shallow marine/deltaic sedimentary rocks and younger volcanic rocks. Whole rock Rb and Sr isotopic data yield a poor isochron with an age of 47±5 Ma. Clarno rocks are broadly calc‐alkaline, range from basalt to rhyolite, and show complex textural and compositional properties. Plagioclase and clinopyroxene phenocrysts are present in most lavas, with olivine in mafic rocks and orthopyroxene, quartz, and subordinate biotite and hornblende in intermediate rocks. Spongy (sieve‐textured) plagioclase and glass‐bearing plagioclase are both common. Many quartz grains are resorbed, and some pyroxene grains are spongy. Some samples contain quartz in which plagioclase inclusions have undergone partial melting along the plagioclase‐quartz boundaries; plagioclase inclusions in these rocks are more calcic (An54–66) than the cores of their companion xenocrysts (An41), suggesting digestion of earlier mafic rocks. Mafic cumulates have high MgO, Cr, and Ni values. Some oxide‐ and element‐SiO2 diagrams show linear trends, but scatter from the trends is large. Plots of SiO2 versus Al2O3, Sr, Y, and possibly other components show inflections in trends in the range of 55–60% SiO2. The abundance of disequilibrium textures shows that the dominant process in the evolution of the Clarno suite was mixing of magmas with liquid and (partly) solid sources, probably including earlier crystallized Clarno magmatic rocks. Cumulate textures and compositions, however, indicate that Clarno magmas were modified by crystal fractionation, which also may have contributed to the scatter from linear trends on composition diagrams. Initial 87Sr/86Sr ratios (corrected for a model age of 50 Ma) range from 0.70350 to 0.70397, consistent with mantle source(s) or with contamination by young mafic material but not with interaction with old upper continental crust.

Caractérisation pétrologique et géochimique du magmatisme ubendien du secteur de Pepa-Lubumba, sur le plateau des Marungu (Nord-Est du Shaba, Zaire). Signification géodynamique dans l'évolution de la chaîne ubendienne

Volcanic and plutonic rocks of Pepa-Lubumba area in the Marungu plateau (Zaire) are mainly represented by intermediate and acid members which contain low abundances in TiO 2 (0.3 – 1.3 wt%). All these rocks exhibit a high-K calc-alkaline affinity and belong to a calc-alkaline continental-margin series. The large proportion of frequently zoned (normal and reverse zoning) plagioclase, the presence of hydrous phases (tschermakitic and Mg-hornblende, Mg-biotite), the early crystallization of FeTi oxides indicate crystallization under high water pressure conditions, consistent with a calc-alkaline affinity. High abundances in K, Rb, Sr and Ba; lack of correlation between Rb and Ba, low values of Rb/Zr (0.5 – 0.9) in granitoids, high contents of LREE and HFSE (Th/Ta : 6 – 14), are characteristic of “Andinotype” magmas. A whole rock RbSr isochron gives an emplacement age for granitoids of 1861 ± 28 Ma (MSWD : 0.82), Sr initial ratio of 0.7026 and epsilon Sr i of + 4.7. PbPb isotopes systematics for the same rocks give the following ratio ranges : 206Pb/ 204Pb : 17.26 – 20.10; 207Pb/ 204Pb 15.52 – 15.85, 208Pb/ 204Pb : 37.2 – 41.80. The low Sr initial ratio indicates an upper mantle source; while the PBPb isotope ratios suggest that the mantellic liquids have been contaminated by crustal material en route to the upper levels. In the regional geologic context, this magmatism forms a part of an important plutovolcanic Ubendian (Early Proterozoic) complex which crops out in Western Tanzania, North-Eastern Zambia and Marungu plateau (Zaire) and contains two magmatic cycles which define a spatial and temporal zonation suggesting a geodynamic evolution model for the Ubendian belt comprising subduction-obduction-collision processes. This latter episode has been relayed by a relaxation phase marked by emplacement of a third tholeiitic basic plutonic cycle.