Times Chondrite Research Articles

Enriched and depleted arc basalts, with Mg-andesites and adakites: A potential paired arc–back-arc of the 2.6 Ga Hutti greenstone terrane, India

The ∼2.6 Ga Hutti greenstone belt is one of several Neoarchean greenstone terranes of the eastern Dharwar Craton. There are prevalent mafic volcanic flows with subordinate felsic volcanic units and siliciclastic sedimentary rocks. All lithologies show variable intensities of submarine hydrothermal alteration, polyphase deformation and greenschist to amphibolite grade metamorphism, yet pillow, cumulus, and other primary volcanic features are locally preserved. Well exposed interlayered metabasalts, Mg-andesites (MA), and felsic flows outcrop along an 11 km sector in the SE of the terrane. Based on combined petrographic and geochemical characteristics, two tholeiitic basalt populations have been identified within the metabasalts: (1) those with enriched LREE at 20–50 times chondrite, and (2) an depleted LREE population at 12–20 times chondrite. The former has fractionated LREE, where (La/Sm) N = 1.2–1.7, but flat HREE, and negative anomalies at Nb, P, and Ti relative to neighbouring REE. The latter has lower absolute abundances of compatible and incompatible elements, mildly fractionated LREE, smaller anomalies at Nb, P, and Ti, with (Gd/Yb) N = 1.1–1.6. Several samples have the “N-MORB” signature of LREE depletion coupled with positive Nb anomalies. On the Th/Yb vs. Nb/Yb discrimination diagram depleted basalts plot near the MORB field whereas enriched basalts overlap the backarc and arc fields, consistent with a paired arc–back-arc. Mg-andesites feature SiO 2 57–61 wt.%, multielement pattens similar to enriched basalts, coupled with Cr, Co, Ni contents greater than “normal” andesites. Felsic volcanic rocks are characterized by low Y, high (La/Yb) N , and Zr/Sm, but low Nb/Ta, with zero to positive Eu anomalies, thus conforming to most of the compositional criteria of Archean and Phanerozoic adakites. Similar associations of enriched and depleted arc basalts, with adakites, are known from Neoarchean greenstone terranes of the Superior Province. During intraoceanic subduction, slab dehydration-wedge melting generated arc basalts whereas slab melting-wedge hybridization, generated adakites and Mg-andesites.

Post-collisional lamprophyric event in Sierra Norte, Córdoba, Argentina: Mineralogical, geochemical and isotopic characteristics

Over 20 lamprophyre dykes, varying in width between a few centimeters and several meters, have been identified in central Sierra Norte – Eastern Pampean Ranges, Córdoba, Argentina. Their mineralogy and chemistry indicate that they are part of the calc-alkaline lamprophyres clan (CAL). They contain phenocrysts of magnesiohornblende ± augite set in a groundmass of magnesiohornblende, calcic-plagioclase, alkali feldspar, and opaque minerals, which designate them as spessartite-type lamprophyres. Alteration products include chlorite, calcite and iron oxides after mafic phenocrysts, though some are partially replaced by actinolite. Feldspars are replaced by carbonate and clay minerals. The dykes are relatively primitive, and show restricted major element variation (SiO 2 51.1–55.3 wt.%, Al 2O 3 12–16.6 wt.%, total alkalies 1.5–4.7 wt.%), high Mg# (55–77), high Cr contents (27–988 ppm) and moderate to high Ni contents (60–190 ppm). Lamprophyre LILE (e.g. Rb averages 110 ppm, Sr 211–387 ppm, Ba 203–452 ppm) are high relative to HFSE (e.g., Ta 0.2–1.6 ppm, Nb 4–11 ppm, Y 17–21 ppm), and are enriched in LREE (30–70 times chondrite). They are characterized by relatively high 208Pb/ 204Pb (38.8–39.9), 207Pb/ 204Pb (∼15.7), and 206Pb/ 204Pb (18.7–20.1), combined with low (epsilon)ε Nd (−4.69 to −1.52) and a relative moderately high ( 87Sr/ 86Sr) i of 0.7055–0.7074. The Rb–Sr whole rock isochron indicates an Early Ordovician age of 485 ± 25 Ma. The calculated T DM (1.7 Ga) suggests that these rocks appear to have originated from a reservoir that was created during a mantle metasomatism event related to the Pampean orogeny. The Sierra Norte lamprophyres show affinities with a subduction-related magma in an active continental margin. Their geochemical and isotopic features suggest a multicomponent source, composed of enriched mantle material variably contaminated by crustal components. The lamprophyric suite emplacement occurred at the dawning stage of the Pampean orogeny, in a regional post-collisional extensional setting developed in the Sierra Norte-Ambargasta batholith (SNAB) in Early Ordovician times.

Chemical composition and ultrahigh-P metamorphism of garnet peridotites from the Sulu UHP terrane, China: Investigation of major, trace elements and Hf isotopes of minerals

The Zhimafang peridotite body recovered by a pre-pilot borehole (PP1) of the Chinese Continental Scientific Drilling Project occurs in gneiss from the southern Sulu ultrahigh-P (UHP) terrane, east-central China. Detailed petrographic observation of the freshest lherzolites reveals the existence of pre-peak mantle relics, rare enstatite inclusions (Opx1) in garnet, and coarse-grained diopside (Cpx1) in the peridotites. Matrix phases Ol + Grt + Opx + Cpx ± Phl represent the assemblage of the peak-stage. At least > 1.5 GPa pressure increase is recognized from the mantle setting to the peak-stage of subduction-zone UHP metamorphism at ~ 800 °C and 6.8 GPa for the peridotites. All peridotitic minerals have high Mg# (Ol, 91–93, Cpx, 93–94 and Opx, 92–94). Garnets are depleted in LREE, and have increased abundances from Pr through the MREE to a plateau of heavy REE (HREE) at about 10 times chondrite. In contrast, clinopyroxenes display humped REE patterns with high (La/Yb) N values of 27–68. Phlogopite is a major carrier of Ba (5205 ppm) and Th (5.5 ppm). Partition coefficients ( D Cpx/Grt) for REE between clinopyroxene and garnet decrease from LREE (~ 100) to HREE (< 1 but > 0.1), suggesting that clinopyroxene and garnet equilibrated at low- T (< 1100 °C). Zircons from three phlogopite and/or magnesite-bearing peridotites show very low 176Lu/ 177Hf values (between 10 − 3 and 10 − 6 ), and a wide range in 176Hf/ 177Hf ratios (0.28221–0.28250). The zircons with low 176Hf/ 177Hf and relatively high 176Lu/ 177Hf document that metasomatism affected the peridotite protolith. The model ages ( T DM) of zircons mostly cluster around 1.3–1.4 Ga, which probably reflects the metasomatic source. These chemical data combined with previous published bulk-rock major and trace element data indicate that the Zhimafang peridotites probably represent depleted and metasomatized subcontinental lithospheric mantle (SCLM) beneath the Sino-Korean craton. Investigation of the Sulu UHP metamorphic garnet peridotites provides additional insights into the Earth's geochemical and tectonic evolution, and enlarges our understanding of the exhumation of orogenic peridotites.

Lithospheric Origin of Oligocene-Miocene Magmatism in Central Chile: U-Pb Ages and Sr-Pb-Hf Isotope Composition of Minerals

Establishing the petrogenesis of volcanic and plutonic rocks is a key issue in unraveling the evolution of distinct subduction-related tectonic phases occurring along the South American margin. This is particularly true for Cenozoic times when large volumes of magma were produced in the Andean belt. In this study we have focused on Oligo-Miocene magmatism in central Chile at 33°S. Our data include field and petrographic observations, whole-rock major and trace element analyses, U–Pb zircon dating, and Pb, Sr, and Hf isotope analyses of plagioclase, clinopyroxene, and zircon mineral separates. Combined with earlier dating results the new zircon ages define a 28·8–5·2 Ma period of plutonic and volcanic activity that ceased as a consequence of flattening subduction of the Nazca–Farallon plate. Rare earth elements patterns are variable, with up to 92 times chondrite concentrations for light rare earth elements yielding (La/Yb)N between 3·6 and 7·0, and an absence of Eu anomalies. Initial Pb isotope signatures are in the range of 18·358–19·023 for 206Pb/ 204Pb, 15·567–15·700 for 207Pb/ 204Pb and 38·249–39·084 for 208Pb/ 204Pb. Initial 87Sr/ 86Sr are mostly in the range of 0·70369–0·70505, with two more radiogenic values at 0·7066. Initial Hf isotopic compositions of zircons yield exclusively positive εHfi ranging between + 6·9 and + 9·6. The newly determined initial isotope characteristics of the Oligo-Miocene magmas suggest that the mantle source lithologies are different from both those of Pacific mid-ocean ridge basalt and ocean island basalt, plotting in the field of reference values for subcontinental lithospheric mantle, characterized by moderate large ion lithophile element–high field strengh element depletion and high 238U/ 204Pb. A Hf model age of 2 Ga is estimated for the formation of the subcontinental mantle–continental crust assemblage in the region, suggesting that the initial Sr and Pb isotope ratios inferred for the source of the Oligo-Miocene parental magmas are the result of later Rb and U enrichment caused by mantle metasomatism. A time-integrated model Rb/Sr of ≈0·039 and μ ≈ 16 are estimated for the source of the parental magmas, consistent with ratios measured in peridotite xenoliths from continental areas. Evolution from predominant (>90%) basaltic–gabbroic to andesitic–dioritic magmas seems to involve a combination of (1) original trace element differences in the metasomatized subcontinental mantle, (2) different degrees of partial melting and (3) fractional crystallization in the garnet- and spinel-peridotite stability fields. The genesis of more differentiated magmas reaching rhyolitic–granitic compositions most probably also includes additional crystal fractionation at both shallow mantle depths and within the crust, possibly leading to some very minor assimilation of crustal material.

The role of HIMU metasomatic components in the North African lithospheric mantle: petrological evidence from the Gharyan lherzolite xenoliths, NW Libya

Abstract The Neogene–Quaternary alkali-basalt–hawaiite lavas of the Gharyan volcanic field (NW Libya) contain mantle xenoliths. These mostly consist of protogranular spinel lherzolites with superimposed metasomatic textures represented by reaction patches where primary orthopyroxene (opx), clinopyroxene (cpx) and spinel (sp) are the main reacting phases. The secondary parageneses include clinopyroxene (cpx2), olivine (ol2) and feldspar (feld) as reaction rims around opx, spongy-textured clinopyroxene with recrystallized portions (cpx2±feldspar), and brown spinel destabilized in a higher Cr/(Cr+Al) black vermicular aggregate (sp2) generally associated with feldspar microlites. Cpx2 are typically depleted in Na 2 O and Al 2 O 3 relative to cpx; feldspar includes both alkali-feldspar (Or 17–51) and plagioclase (An 23–64). Bulk rocks have flat heavy rare earth element (HREE) patterns (1.2–2.3 times chondrite) and are variably enriched in light REE (LREE; La N /Yb N up to 6.6). The constituent clinopyroxenes are characterized by flat HREE distributions (8–14.5 times chondrite) and variable LREE enrichment with La N /Yb N up to seven, which generally conform to the bulk-rock chemistry. Samples relatively unaffected by metasomatism have clinopyroxene Sr–Nd isotopic composition ( 87 Sr/ 86 Sr down to 0.7023, 143 Nd/ 144 Nd up to 0.5139) that approaches the depleted mantle (DM), suggesting that the lithospheric mantle beneath the area underwent a long-term depletion probably by pre-Palaeozoic extraction of basic melts. The remaining samples approach 87 Sr/ 86 Sr c . 0.7030, 143 Nd/ 144 Nd c . 0.5130, with 206 Pb/ 204 Pb up to 19.66. These data imply that the causative agents of metasomatism were Na-alkali silicate melts with a clear HIMU affinity, in accordance with the isotopic signature of the host lavas ( 87 Sr/ 86 Sr=0.7032, 143 Nd/ 144 Nd=0.5130, 206 Pb/ 204 Pb=19.60). This prevalent HIMU geochemical signature is comparable with that recorded in Cenozoic alkaline basic lavas and associated mantle xenoliths from other occurrences of the northern–central African lithosphere, suggesting a common regional sub-lithospheric component. The relatively low 3 He/ 4 He of the Gharyan xenoliths (5.3–6.5 R a ) indicates that this component originates within the upper mantle and is unrelated to the deep-seated mantle plume source of the Ethiopian–Yemen plateau basalts. Therefore, the Cenozoic volcanic districts of the Saharan belt could be related to smaller-scale shallow mantle upwellings (also referred to as ‘hot fingers’) triggered by intraplate reactivation of regional tectonic lineaments within the Pan-African cratonic basement, as a foreland reaction of the African–Europe collisional system.

Geochemical Characteristics of Late Mesozoic Dikes, Jiaodong Peninsula, North China Craton: Petrogenesis and Geodynamic Setting

Widespread Cretaceous dikes in the Jiaodong Peninsula of eastern China are genetically related to gold deposits. Bulk-rock major element analyses show that they are composed of medium-K, subalkaline to shoshonitic rocks characterized by wide ranges of SiO2 (48.98-71.38%) and MgO (0.63-10.02%), and extremely high contents of Al2O3 (14.32-16.06%). They are enriched in large-ion lithophile elements (LILE, e.g., K, Rb, Sr, Ba) and light rare-earth elements (LREE), but depleted in high-field-strength elements (HFSE, e.g., Ta, Nb, Zr, Hf), and display consistent primitive mantle- and chondrite-normalized multi-element patterns on spidergrams, different from Cenozoic basalts in the Peninsula. These data suggest a magma origin by partial melting of an ancient enriched lithospheric mantle. High Th/Yb and Pb/Ce ratios and weak negative Ce anomalies reflect involvement of recycled sediments in the mantle reservoir. Strong fractionation in REE, with LREE >100 times chondrite and HREE <10 times chondrite, indicates the presence of garnet in the mantle source. Nb* values of 1.49-2.38 (>1), K* values of 0.08-0.14 (<1), and similar Zr/Hf (32.15-40.65), U/Th (0.10-0.33), and Hf/Sm (0.45-0.67) ratios in all the dikes, along with published chemical and Sr-Nd isotopic compositions of lamprophyres (Cheng et al., 1998; Sun et al., 2000; Yang et al., 2004) preclude crustal contamination during magma ascent and binary mixing between mantle and crustal magmas. The geochemical features of these dikes can be interpreted as the result of interaction of old lithospheric mantle with melt and/or fluid generated from subducted lower/middle crust of the Yangtze craton and/or Izanagi oceanic crust. The source for the Late Mesozoic dikes was completely reconstructed, enriched lithospheric mantle containing disseminated amphibole-and phlogopite-bearing pyroxenite veins surrounded by refractory peridotite wall rock. The enriched source was modified by subsequent partial melting due to sinistral transtensional strike-slip movement of the Tan-Lu fault that led to widespread dike intrusion. Trace element geochemistry and regional geology indicate that these dikes formed in a back-arc extensional environment.

Geochemical systematics of 2.7 Ga Kinojevis Group (Abitibi), and Manitouwadge and Winston Lake (Wawa) Fe-rich basalt–rhyolite associations: Backarc rift oceanic crust?

We present new geochemical data for Neoarchean Kinojevis and Manitouwadge–Winston Lake volcanic rocks from the formerly contiguous Abitibi and Wawa subprovinces. The 2702–2701 Ma Kinojevis Group (KG), Abitibi greenstone belt, is dominated by Fe- to Mg-rich basalt flows (Mg-number = 30–56) having a tholeiitic fractionation trend, and prominent magnetic striping. REE patterns are near-flat at 4 to 15 times chondrite. Normalized La/Sm and Gd/Yb ratios span 0.70 to 1.17 and 0.96 to 1.34 respectively, the latter indicative of a variable involvement of garnet in the residue. Basalts do not plot on mixing lines with average Archean upper crustal composition, and accordingly were likely erupted in an oceanic setting. Tholeiitic high-Si rhyolites also feature near-flat REE patterns, with pronounced negative Eu anomalies, consistent with extreme fractionation products of basaltic melts. Niobium contents and Zr/Nb ratios (1.7–6.4 ppm; 10–24) of basalts are mostly intermediate between respective values for Archean primitive oceanic arc tholeiites (PAT), and ocean plateau tholeiitic basalts (OPB: 2–9 ppm; 12–20) associated with komatiites, likely representing ocean plateaus erupted from mantle plumes. Similarly, Nb/Th (4–8, versus 8 for primitive mantle) ratios in KG basalts are intermediate between ranges for Archean PAT (2–6) and OPB (8–20). Compositionally, these basalts are commensurate with recent backarc basalts (BAB) that are intermediate between mid-ocean ridge basalts (MORB) and PAT: they are interpreted as oceanic crust in a backarc rift, from mildly hydrous melting of depleted upper mantle asthenosphere. Kinojevis basalts plot mostly on the MORB–ocean island basalt (OIB) array in log Nb/Y versus Zr/Y coordinates, whereas structurally overlying Blake River basalts cross that array from depleted plume asthenosphere to adakite “end-members”. Consequently, their asthenosphere sources and geodynamic settings may have differed. Tholeiitic basalts and FIII-type rhyolites in the 2722–2680 Ma Manitouwadge and Winston greenstone belts, Wawa subprovince, are compositionally akin to their Kinojevis counterparts. Basalts and rhyolites in both belts are spatially and temporarily associated with volcanogenic massive sulphide deposits (VMS), indicative of the potential of the KG for similar deposits. The KG is interpreted as intra-oceanic backarc crust. Similarly, the Wawa volcanic and metallogenic associations are interpreted to have been deposited in a backarc setting, to the north-northwest of the Schreiber-Hemlo subduction–accretion complex.

Tectonic significance of Late Triassic post-collisional lamprophyre dykes from the Qinling Mountains (China)

The Qinling–Dabie orogen in central China is one of the major orogenic belts in East Asia. In the eastern Dabie–Sulu region, mafic lamprophyres show the enriched signatures of old sub-continental lithospheric mantle. However, little is known about the mafic igneous rocks and their lithospheric mantle sources in the western Qinling Range. New 40Ar–39Ar age dating, major- and trace-element data, and isotopic analyses of Qinling lamprophyres reveal their differences from the Dabie Sulu lamprophyres. Biotite 40Ar–39Ar dating yielded a plateau age of 219±2 Ma, identical to the ages of rapakivi-textured granitoids in the area. The association of lamprophyre dykes and rapakivi-textured granitoids indicates that the Qinling region was a post-collisional setting at c. 220 Ma. The Qinling lamprophyres are calc-alkaline, and rich in large ion lithophile elements (e.g. Ba, K), but depleted in Nb, Ta and Ti. They show highly fractionated REE patterns with LaN&gt;100 and HREE &lt;10 times chondrite abundances. εNd (219 Ma) values range from −0.5 to −3.3 and initial Sr isotope values from 0.7036 to 0.7058. These features suggest generation of the lamprophyre by partial melting of a metasomatized, garnet peridotite mantle source. The Qinling lamprophyres are distinct from the Dabie–Sulu lamprophyres in emplacement age (c. 135 Ma for Dabie–Sulu) and isotopic composition, suggesting that the nature of the lithospheric mantle and geodynamic evolution of the Qinling region contrasts with that of the Dabie–Sulu region.

Allanite and chevkinite in A-type granites and syenites of the Graciosa Province, southern Brazil

Allanite and chevkinite–perrierite are important accessory phases in the granitic and syenitic A-type rocks of the Graciosa Province, southern Brazil. Allanite–(Ce) is the main REE phase in the granites of the aluminous association – which includes metaluminous to weakly peraluminous granites – while chevkinite–(Ce) is the typical REE phase in the metaluminous syenites and alkali-feldspar syenites, and the peralkaline alkali-feldspar granites of the alkaline association. Allanite shows complex zoning patterns but is always rich in the ferriallanite molecule and in TiO 2, a compositional trait that distinguishes allanite in A-type aluminous associations from allanite in calcic-, alkali-calcic, and calc-alkaline granitoids. Normalized REE patterns show strong fractionation of heavy over light REE, with La N /Nd N ratios between 3 and 5, and La N /Y N ratios between 200 and 400. Primary allanite is variably affected by post-magmatic transformations due to the interaction with a fluid-phase and metamictization caused by radioactive decay. Chevkinite–(Ce) is always primary and is only present in the alkaline association. The early crystallization of chevkinite indicates that the parental magmas were probably close to REE-saturation. The REE + Y abundance in chevkinite (43–49 wt.%) is almost twice that observed in primary allanite; yet, the normalized patterns are similar in terms of fractionation between light and heavy REE, as indicated by similar La N /Nd N ratios in chevkinite and allanite. La N /Y N ratios are on average lower in chevkinite (90–300). Chevkinite compositions were altered by interaction with post–magmatic fluids, which led to marked increase in TiO 2, and a reduction in REE and Fe T . Post-magmatic allanite is present in both the alkaline and aluminous associations, and corresponds to ferriallanite–(Ce) in the alkaline association, and allanite–(Y) in the aluminous association. Compared to primary allanite, post-magmatic allanite–(Ce) shows smaller REE fractionation (2 < La N /Nd N < 3), while allanite–(Y) shows relative enrichment in the middle REE, with Sm values close to 10 5 times chondrite. The mineralogy of the rocks indicates crystallization conditions close to the titanite–magnetite–quartz–amphibole–ilmenite (TMQAI) buffer for the aluminous association, and close to the fayalite–quartz–magnetite (FQM) buffer for the alkaline association, and temperatures between 800 and 900 °C (liquidus) and ca. 750 °C (solidus). The experimentally determined stability fields for chevkinite and allanite are in agreement with the textural evidence for early chevkinite crystallization and late allanite crystallization, even though allanite and chevkinite only rarely coexist in the alkaline association. Wherever allanite and chevkinite coexist, they reveal reaction textures, such that it is possible that only allanite or chevkinite is stable at any given condition. The distribution of allanite and chevkinite in the alkaline association suggests that the composition of the magmas is an important control on the stability fields of these minerals, a fact that is supported by the presence of primary allanite in rocks formed by processes of mixing and mingling of magmas. In this sense, the importance of composition on the stability of allanite and chevkinite may be larger than previously suggested.

Geochemistry of an ultramafic-rodingite rock association in the Paleoproterozoic Dixcove greenstone belt, southwestern Ghana

Rodingite occurs in ultramafic rocks within the Paleoproterozoic (Birimian) Dixcove greenstone belt in southwestern Ghana. U–Pb analyses of zircons from granitoids intrusive into the greenstone belt constrain the age of the rodingite-ultramafic association to be older than 2159 Ma. The ultramafic complex consists of variably serpentinized dunite and harzburgite overlain by gabbroic rocks, which together show petrographic and geochemical characteristics consistent with their formation by fractional crystallization involving olivine and plagioclase cumulates. Major and trace element concentrations and patterns in the ultramafic–mafic cumulate rocks and associated plagiogranite are similar to rocks in ophiolitic suites. The rodingites, which occur as irregular pods and lenses, and as veins and blocks in the serpentinized zones, are characterized by high Al 2O 3 and CaO contents, which together with petrographic evidence indicate their formation from plagioclase-rich protoliths. The peridotites are highly depleted in REE and display flat, chondrite-normalized REE patterns with variable, but mostly small, positive Eu anomalies whereas the rodingites, which are also highly depleted, with overall REE contents from 0.04 to 1.2 times chondrite values, display distinct large positive Eu anomalies. It is proposed that the rodingite-bearing Birimian ultramafic complex and associated pillowed greenstone belt basalts may represent fragments of a Paleoproterozoic oceanic lithosphere.

Rare earth element systematics of acidic geothermal waters from the Taupo Volcanic Zone, New Zealand

Concentrations of rare earth elements (REE) in acidic thermal waters from the Taupo Volcanic Zone, New Zealand have been determined. Acidity in waters from Rotokawa results from oxidation of H 2S upon encounter with oxygen-rich meteoric waters. The Ruapehu Crater lake is acidic due to input of magmatic volatiles such as HCl and SO 2. REE concentrations in both types of water are high, ranging from 10 − 3 to nearly 1 times chondrite. Chondrite-normalized patterns for heavy REE in waters from Rotokawa parallel those of their host rocks, suggesting little fractionation upon water–rock interaction. However, the light REE are depleted relative to their host rock, resulting in a “gull-wing” pattern. The hyperacidic Ruapehu Crater lake water generally does not have a “gull-wing” pattern. The patterns for these waters were invariant for several years prior to the September 1995 eruption, but changed substantially afterwards, generally decreasing in slope with time.

Late Archaean mantle metasomatism below eastern Indian craton: Evidence from trace elements, REE geochemistry and Sr—Nd—O isotope systematics of ultramafic dykes

Trace, rare earth elements (REE), Rb-Sr, Sm-Nd and O isotope studies have been carried out on ultramafic (harzburgite and lherzolite) dykes belonging to the newer dolerite dyke swarms of eastern Indian craton. The dyke swarms were earlier considered to be the youngest mafic magmatic activity in this region having ages not older than middle to late Proterozoic. The study indicates that the ultramafic members of these swarms are in fact of late Archaean age (Rb-Sr isochron age 2613 ± 177 Ma, Sri ∼ 0.702 ± 0.004) which attests that out of all the cratonic blocks of India, eastern Indian craton experienced earliest stabilization event. Primitive mantle normalized trace element plots of these dykes display enrichment in large ion lithophile elements (LILE), pronounced Ba, Nb and Sr depletions but very high concentrations of Cr and Ni. Chondrite normalised REE plots exhibit light REE (LREE) enrichment with nearly flat heavy REE (HREE; (ΣHREE)N ∼ 2–3 times chondrite, (Gd/Yb)N ∼ 1). The εNd(t) values vary from +1.23 to -3.27 whereas δ18O values vary from +3.16‰ to +5.29‰ (average +3.97‰±0.75‰) which is lighter than the average mantle value. Isotopic, trace and REE data together indicate that during 2.6 Ga the nearly primitive mantle below the eastern Indian Craton was metasomatised by the fluid (± silicate melt) coming out from the subducting early crust resulting in LILE and LREE enriched, Nb depleted, variable εNd, low Sri(0.702) and low δ18O bearing EMI type mantle. Magmatic blobs of this metasomatised mantle were subsequently emplaced in deeper levels of the granitic crust which possibly originated due to the same thermal pulse.

Origin of early Cretaceous calc-alkaline lamprophyres from the Sulu orogen in eastern China: implications for enrichment processes beneath continental collisional belt

K–Ar dating, major- and trace-element and Sr–Nd isotopic analyses were carried out for early Cretaceous (122–127 Ma) lamprophyres from the Sulu orogen in eastern China. The results show strong fractionation in rare-earth elements with (LREE) >100 times chondrite, but HREE <10 times chondrite, indicating the presence of residual garnet in the melting source. These rocks are characterized by significant LILE and LREE enrichment but Nb and Ta depletion with moderate Zr/Hf (39.8–50.8 with regard to 36 for primitive mantle) and Nb/Ta (17.8–23.0, compared with 17.5 for primitive mantle) fractionations, probably as a consequence of carbonate- and rutile-rich melt metasomatism induced by dehydration and/or melting of subducted continental slab at mantle depths. Age-correlated Sr–Nd isotope ratios show moderate ranges of 87Sr/ 86Sr(i) from 0.70787 to 0.70934 and −17.2 to −11.6 of ε Nd( t). The lamprophyres from the Sulu orogen were derived from decompression melting of such a metasomatized lithospheric mantle that was mainly composed of phlogophite garnet peridotites and experienced crystal fractionation of a mineral assemblage of olivine+clinopyroxene±plagioclase en route to the surface. Such geochemical and isotopic signatures are also prevalent in the contemporaneous basaltic lavas in the Dabie–Sulu belt, suggesting predominant enrichment processes by carbonate- and rutile-rich metasomatic assemblage beneath the continental collisional belt.

Tracing Lithosphere Evolution through the Analysis of Heterogeneous G9-G10 Garnets in Peridotite Xenoliths, II: REE Chemistry

Following previous publication of major---minor element data, this paper presents rare earth element (REE) data for heterogeneous (chemically zoned) garnets belonging to the peridotite suite of mantle xenoliths from the Jagersfontein kimberlite pipe, South Africa. The rim compositions of the garnets in the highest temperature---pressure (deepest) deformed peridotites show a typical megacryst-like pattern, of very low light REE (LREE) increasing through the middle REE (MREE) to a plateau of heavy REE (HREE) at c. 20 times chondrite; these compositions would be in equilibrium with small-volume melts of the mid-ocean ridge basalt (MORB) source (asthenosphere). With decreasing depth the garnet rims show increasing LREE and decreasing HREE, eventually resulting in humped relative abundance patterns. A set of compositions is calculated for melts that would be in equilibrium with the garnet rims at different depths. These show decreasing relative abundance of each REE from La to Lu, and the La/Lu ratio of the melts increases with decreasing depth of formation. Modelling of the effects of crystal fractionation shows that this process could largely generate the sequence of garnet rim and melt compositions found with decreasing depth, including the humped REE patterns in high-level garnets. Considering the behaviour of major---minor elements as well as REE, a process of percolative fractional crystallization is advocated in which megacryst source melts percolate upwards through peridotites and undergo fractionation in conjunction with exchange with the peridotite minerals. The initial megacryst melt probably includes melt of lithospheric origin as well as melt from the MORB source, and it is suggested that the process of percolative fractional crystallization may form a variety of metasomatic and kimberlitic melts from initial megacryst melts. Repeated metasomatism of the lower lithosphere by such differentiating melts is suggested by consideration of garnet core compositions. Such metasomatism would progressively convert harzburgites to lherzolites by increasing their CaO content, and this may account for the fact that the Cr-rich diamond---garnet harzburgite paragenesis is commonly preserved only where it has been encapsulated in diamonds.

Geochemistry of the Coastal Sonora batholith, Northwestern Mexico

New major- and trace-element and radiogenic Sr and Nd data from granitic rocks help to characterize the geochemical composition of the Coastal Sonora batholith, which represents the westernmost portion of the Laramide Magmatic arc (~80–40 Ma) in Mexico. Compared with more inland-located regions of the arc in central and eastern Sonora (Inner Granites), the plutons within the coastal batholith have higher MgO, FeO, and CaO and lower K2O contents, and tend to be more depleted in Nb, U, and to a lesser extent Rb, but relatively enriched in Sr. These rocks display flatter slopes of rare-earth elements, with La abundances around 80 to 200 times chondrite values and smaller but well-developed negative Eu anomalies. Initial 87Sr/86Sr ratios and εNd values from a few studied samples of the Coastal Sonora batholith range from 0.70586 to 0.70679 and from –2.3 to –4.7, respectively. These isotopic signatures allow recognition of a more mantle-related magma source for this batholith as compared with the Inner Granites, which have higher Sr and lower Nd ratios. In a more regional context, the compositions of the Coastal Sonora batholith rocks resemble those of the eastern portion of the Peninsular Ranges batholith to the northwest. Based on our observations, we propose that partial melting of a crust thickened by overlapping periods of Jurassic through Cretaceous arc magmatism along the southwestern edge of the North American continent may account for the geochemical and isotopic characteristics displayed by the Coastal Sonora batholith granitic rocks.

Rare-earth elements in geothermal waters from Oregon, Nevada, and California

The concentrations of rare-earth elements (REE) were determined in thermal waters from hot springs in the Oregon Cascades and southeastern Oregon, and from wells in the Beowawe (NV), Dixie Valley (NV) and Heber (CA) geothermal fields. The waters are all near-neutral to slightly alkaline, and dominated by sodium chloride or sodium bicarbonate. Concentrations of REE range from <10−6 to approximately 10−3 times chondrite. In general, filtered aliquots of the fluids contain substantially less REE (sometimes by an order of magnitude or more) than corresponding unfiltered aliquots, suggesting a considerable particulate contribution to the total REE. Concentrations of REE in the waters from Beowawe and Dixie Valley are generally quite low, possibly owing to loss of REE on boiling. Most of the waters exhibit LREE-enriched, chondrite-normalized REE patterns, with slight or non-existent Eu anomalies. The main exceptions are the waters from Heber, which exhibit a concave-upward pattern with a prominent, positive Eu anomaly (“Mexican hat” pattern). The behavior of REE in waters from continental geothermal systems is a useful indicator of water–rock interaction and holds promise as a potential tool for exploration.

Geochemistry of the metamorphosed Ordovician Taconian Magmatic Arc, Bronson Hill anticlinorium, western New England

The Bronson Hill anticlinorium (BHA) in western New England is a north- to northeast-trending belt of gneiss domes containing a metamorphosed stratified sequence of Ordovician, Silurian, and Devonian sedimentary and volcanic rocks. These overlie mostly late Ordovician intrusive igneous rocks. The intrusive rocks, exposed in the dome centers, are dominated by felsic gneisses of tonalitic to granitic compositions. They are the exposed parts of a composite batholith that developed along the axis of the Taconian volcanic arc. The Fourmile Gneiss of the Pelham dome, and the Monson Gneiss of the Monson dome, covered in most detail here, are calc-alkaline, dominantly tonalitic to granodioritic gneisses having 61–78% SiO 2. These are divided into two geochemical groups. The most abundant rocks have low Sr (∼50–200 ppm), high Y (∼10–50 ppm), low Al 2O 3, high FeO t, commonly negative Eu anomalies, and have flat to somewhat concave upward MREE and HREE patterns. These rocks were probably derived from melting of plagioclase–pyroxene–amphibole granulites in the deep crust. Rocks of the subordinate high-Sr group are similar in composition to adakites and have high Sr (∼300–600 ppm), low Y (∼1–13 ppm), higher Al 2O 3, lower FeO t, commonly no Eu anomalies, and are strongly depleted in HREE. These rocks were probably derived from a pyroxene–amphibole–garnet ± plagioclase source at higher pressure. Amphibolites are low in abundance in the Fourmile and Monson Gneisses. Most are a low-Nb type (<6 ppm Nb), compositionally similar to typical calc-alkaline island arc basalts. These were probably derived by melting of spinel lherzolite in the mantle wedge under the arc. Gabbroic anorthosite and ultramafic rocks also occur in the Monson dome and appear to be cumulates from magmas similar to the low-Nb amphibolites. A high-Nb (6–12 ppm) amphibolite group was identified and is unique to the Monson Gneiss. These rocks are transitional between calc-alkaline and alkaline basalts, are strongly LREE-enriched (La is 46–440 times chondrite), are enriched in strongly incompatible elements, and two of seven samples have normative nepheline. A broader survey of available analytical data from BHA felsic gneisses shows that gneiss compositions vary from dominantly tonalitic and granodioritic in the southern BHA to dominantly granitic from the Croydon and Mascoma domes (west-central New Hampshire) northward. This variation suggests that the southern part of the arc exposed in the BHA was based on mafic crust, whereas the northern part was based on intermediate to felsic crust. Examination of radiometric dates of Taconian igneous rocks, and biostratigraphic constraints in the Taconic allochthons and in the Taconian foreland, are consistent with a model in which the BHA gneisses are the youngest igneous components of the Taconian arc proper. Collision of this composite arc with Laurentia reached a conclusion in the latest Caradoc or Ashgill of the late Ordovician or in the very earliest Silurian. Younger plutons continuing into the Silurian (e.g., Highlandcroft in part) may be related to magmas generated during post-collision delamination or detachment of the subducted Iapetus oceanic slab.

Trace element and Nd–Sr isotopic composition of ultramafic lamprophyres from the East Antarctic Beaver Lake area

The trace element and Nd–Sr isotopic compositions of Cretaceous (110–117 Ma) ultramafic lamprophyres from the Beaver Lake area in East Antarctica, which are developed as sill, dyke and plug intrusions, have been investigated. Rare earth elements of lamprophyres are strongly fractionated, with LREE >100 times chondrite, whereas HREE are <10 times chondrite, presumably indicating the presence of residual garnet in the source region. A characteristic feature of the Beaver Lake rocks is low concentrations of Zr and Hf (Zr, 50–250 ppm; Hf, 1.0–5.0 ppm) and to a lesser extent Nb (17–90 ppm) which cause strong negative anomalies in the normalized trace element patterns. Eruption age-corrected Nd isotope values vary within narrow ranges ( εNd (t)=+2.1 to +4.3), whereas Sr isotope values show more variation ( 87Sr/ 86Sr (t)=0.704336–0.706431), similar to the isotopic compositions of other ultramafic lamprophyres. The genesis of the ultramafic lamprophyres is explained as a result of the gradual widening of the Lambert-Amery rift during the Phanerozoic. An increase in the geothermal gradient from cold continental conditions beneath the Archean block to the west may have resulted in grazing of the peridotite solidus at depths greater than 110 km. The initial carbonate-rich melts migrated upwards and froze as carbonate-bearing veins in the overlying lithosphere. Later upward and outward migration of the asthenosphere beneath the rift caused remelting of the veined region, resulting in melts of ultramafic lamprophyre composition with a mixed geochemical abundance and isotope signature from carbonate-rich veins and depleted peridotite wall-rock. The CO 2-rich component of the lamprophyres is derived principally from the vein assemblage, from which phlogopite gives rise to the variable Sr isotopes, and baddeleyite to the differing Zr and Hf abundances. The difference between the lamprophyre facies of the Beaver Lake intrusions can be explained by minor amounts of olivine and Cr-spinel fractionation.

Depletion Events, Nature of Metasomatizing Agent and Timing of Enrichment Processes in Lithospheric Mantle Xenoliths from the Veneto Volcanic Province

analogues of the modelled metasomatizing agent/s occur in the Mantle xenoliths included in the alkaline basic lavas from the South Alpine domain as late Cretaceous lamprophyric dykes, as Paleogene Veneto Volcanic Province (VVP) consist of predominant well as VVP alkaline basic lavas. An estimate of timing of the spinel lherzolites (21–6% clinopyroxene) and minor spinel harzenrichment processes was achieved by a REE inward diffusion burgites (4–2% clinopyroxene), mainly protogranular textured. Most model in primary and spongy clinopyroxenes, assuming LREEof the xenoliths show superimposed textural evidence of metasomatic enriched secondary clinopyroxenes as the boundary surface comprocesses, consisting of reaction patches and spongy clinopyroxenes, position. Model calculations imply a complete chemical revariably associated with glass and secondary olivine, clinopyroxene, homogenization of a 1 mm crystal in a time span of 4·8–16 kyr. spinel and feldspar. Whole-rock and mineral major and trace element Consequently, the observed REE zoning of clinopyroxenes, together data indicate a complex history of depletion and enrichment processes with the presence of glassy patches, indicates that the most recent undergone by the continental lithospheric mantle beneath a withinmetasomatic processes occurred shortly before the entrainment of plate region. Protogranular-textured clinopyroxenes from lherzolites mantle xenoliths by the host lavas. show heavy rare earth element (HREE) contents >10 times chondrite and strong light REE (LREE) depletion [(La/Yb)N = 0·002–0·025)], whereas those in harzburgites vary from slightly LREE depleted to LREE enriched [(La/Yb)N = 0·64–4·00],

Evidence for a Deep-Mantle Origin of a NaPX-EN Inclusion in Diamond

A diamond mineral inclusion (NaPx-En) with a garnet structure and unusual composition (16 mol% Na2MgSi5O12—84% Mg4Si4O12) was reported from a Chinese diamond and demonstrated to be from the Earth's transition zone (Wang and Sueno, 1996; Gasparik and Hutchison, 2000). Concentrations of LREEs in the inclusion increase gradually with the atomic number; the MREE and HREE are about 7-10 times chondrite. Trace-element partitionings between NaPx-En garnet and CO2-rich melt were experimentally determined at 1900°C and 23 GPa with a split-sphere anvil apparatus (USSA-2000). Partitioning coefficients increase regularly from 0.002 for La to 0.8 for Yb. The calculated concentrations of REEs and Ti, Sr, Y, and Zr in hypothetical melt coexisting with the NaPx-En inclusion in the source region are consistent with those typical of natural carbonatite melts. We propose that the NaPx-En inclusion formed originally in the lower mantle as MgSiO3 perovskite. It was later converted to garnet by reaction with a carbonatite melt upon mantle diapir movement into the transition zone. It was only then encapsulated as a diamond inclusion.