High Light Rare Earth Research Articles

Melting and Multi-stage Metasomatism in the Mantle Wedge beneath a Frontal Arc Inferred from Highly Depleted Peridotite Xenoliths from the Avacha Volcano, Southern Kamchatka

Peridotite xenoliths from the Avacha volcano, Kamchatka, derived from the mantle beneath the volcanic front of the Kamchatka arc, are mainly highly depleted clinopyroxene-poor harzburgites with highly forsteritic olivine (Fo 90^92) and high Cr-number spinel (0 5^0 7). The Avacha peridotites have experienced metasomatism to various extents, with the formation of metasomatic orthopyroxene replacing primary olivine, by infiltration of SiO2-rich fluids. The metasomatic orthopyroxenes can be subdivided into two textural types; (1) radially aggregated prismatic grains (opx II-1); (2) stout grains associated with interstitial glass and metasomatic minerals (opx II-2).The Avacha peridotites exhibit light rare earth element (LREE) enrichment relative to heavy REE (HREE), even in primary lithologies with52 vol.% of metasomatic orthopyroxene. However, the metasomatism has not altered the chemical characteristics of the Avacha peridotites significantly, and the Fo contents of olivine do not increase with an increase in the Cr-number of spinel. The clinopyroxene in the most depleted primary peridotite, with the highest spinel Cr-number, has the highest CeN (where subscript N indicates normalized to primitive mantle) and relatively lowYbN contents.We conclude that the Avacha peridotites are residues of high-degree melting induced by infiltration of an aqueous fluid with relatively high Fe/Mg and LREE/HREE ratios. In situ analysis of olivine grain boundaries by laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) showed significant concentrations of some trace elements (Ce and Ba), even on glassfree boundaries; such grain boundaries can be a repository for largeion lithophile elements (LILE). Orthopyroxene and Ca-amphibole contribute to the HREE and middle REE (MREE) budget, respectively, of the bulk peridotites.Three types of mantle metasomatism are recorded in the Avacha peridotites. The agent that formed HREE-poor opx II-1 was a slab-derived aqueous fluid rich in SiO2. It was a successor of the fluid influx involved in the initial partial melting event. On the other hand, the metasomatic minerals in highly metasomatized peridotites have high LILE (Th, U and Sr) and LREE contents relative to HREE, indicating involvement of a hydrous melt of adakitic affinity derived from the subducted slab. Glasses associated with opx II-2 are similar in their trace-element characteristics to the host Avacha volcanic rocks, indicating the likelihood that the metasomatic agent was a forerunner of the recent Avacha arc magmatism.

Podiform chromitites in the lherzolite‐dominant mantle section of the Isabela ophiolite, the Philippines

Abstract The Isabela ophiolite, the Philippines, is characterized by a lherzolite‐dominant mantle section, which was probably formed beneath a slow‐spreading mid‐ocean ridge. Several podiform chromitites occur in the mantle section and grade into harzburgite to lherzolite. The chromitites show massive, nodular, layered and disseminated textures. Clinopyroxene (±orthopyroxene/amphibole) inclusions within chromian spinel (chromite hereafter) are commonly found in the massive‐type chromitites. Large chromitites are found in relatively depleted harzburgite hosts having high‐Cr♯ (Cr/(Cr + Al) atomic ratio = ∼0.5) chromite. Light rare earth element (LREE) contents of clinopyroxenes in harzburgites near the chromitites are higher than those in lherzolite with low‐Cr♯ chromite, whereas heavy REE (HREE) contents of clinopyroxenes are lower in harzburgite than in lherzolite. The harzburgite near the chromitites is not a residual peridotite after simple melt extraction from lherzolite but is formed by open‐system melting (partial melting associated with influx of primitive basaltic melt of deeper origin). Clinopyroxene inclusions within chromite in chromitites exhibit convex‐shaped REE patterns with low HREE and high LREE (+Sr) abundances compared to the host peridotites. The chromitites were formed from a hybridized melt enriched with Cr, Si and incompatible elements (Na, LREE, Sr and H2O). The melt was produced by mixing of secondary melts after melt–rock interaction and the primitive basaltic melts in large melt conduits, probably coupled with a zone‐refining effect. The Cr♯ of chromites in the chromitites ranges from 0.65 to 0.75 and is similar to those of arc‐related magmas. The upper mantle section of the Isabela ophiolite was initially formed beneath a slow‐spreading mid‐ocean ridge, later introduced by arc‐related magmatisms in response to a switch in tectonic setting during its obduction at a convergent margin.

Petrogenesis of a Late Jurassic Peraluminous Volcanic Complex and its High-Mg, Potassic, Quenched Enclaves at Xiangshan, Southeast China

A late Mesozoic belt of volcanic–intrusive complexes occurs in SE China. Volcanic activity at Xiangshan in the NW of the belt took place mainly in the Late Jurassic (158–135Ma). The volcanic rocks from the Xiangshan volcanic complex include rhyolitic crystal tuffs, welded tuffs, rhyolite lavas, porphyritic lavas, and associated subvolcanic rocks. Mineral assemblages in these magmatic rocks include K-feldspar, plagioclase, quartz, Fe-rich biotite and minor amphibole, orthopyroxene and almandine. Mineral geothermometry indicates a high crystallization temperature (>850 C) for the Xiangshan magmas. The volcanic rocks are generally peraluminous; SiO2 contents are between 65 4% and 76 8% and the samples have high alkalis, rare earth elements (REE), high field strength elements and Ga contents and high Ga/Al ratios, but are depleted in Ba, Sr and transition metals. Trace element geochemistry and Sr–Nd–O isotope systematics imply that the Xiangshan magmas were probably derived from partial melting of Middle Proterozoic metamorphic lower-crustal rocks that had been dehydrated during an earlier thermal event. These features suggest an A-type affinity. Quenched mafic enclaves, hosted by the subvolcanic rocks, consist mainly of alkali feldspar, plagioclase, clinopyroxene, phlogopite and amphibole. Geothermometry calculations indicate that the primary magmas that chilled to form the quenched enclaves had anomalously high temperatures (>1200 C). The quenched enclaves have boninitic affinities; for example, intermediate SiO2 contents, high MgO and low TiO2 contents, high Mg-numbers and high concentrations of Sc, Ni, Co and V. However, they also have shoshonitic characteristics, e.g. enrichment in alkalis, high K2O contents with high K2O/Na2O ratios, high light REE and large ion lithophile element contents, low initial eNd values ( 4 2) and high initial Sr/Sr ratios (0 7081). We suggest a phlogopite-bearing spinel harzburgitic lithospheric mantle source for these high-Mg potassic magmas. Underplating of such anomalously high-temperature magmas could have induced granulite-facies lower-crustal rocks to partially melt and generate the Xiangshan A-type volcanic suite. A back-arc extensional setting, related to subduction of the Palaeo-Pacific plate, is favoured to explain the petrogenesis of the Xiangshan volcanic complex and quenched enclaves.

Geochemistry of Cenozoic basalts in the Fukuoka district (northern Kyushu, Japan): implications for asthenosphere and lithospheric mantle interaction

Fukuoka volcanic field in northern Kyushu (Japan) is comprised of scattered small, monogenetic volcanoes with ages ranging from 1.1 to 4.4 Ma. A set of samples from the area, together with some from nearby localities, was collected and analyzed for major and trace element abundances and Sr, Nd and Pb isotope compositions. The basalts, unlike lavas from other nearby centers in northern Kyushu, show the highest FeO*, TiO 2 and lowest SiO 2 characteristics, which are interpreted to reflect high melting temperature and pressure; whereas high Sr, Sm and high-field-strength elements (HFSE) such as Zr and Nb, high light rare earth element (LREE), relatively low Ba, Rb, and broadly oceanic island basalt (OIB)-like primitive mantle normalized incompatible trace element patterns are interpreted to reflect source characteristics. In addition to lead isotopic compositions that are the most radiogenic yet analyzed from northern Kyushu and the Sea of Japan, strontium and neodymium isotopic compositions of Fukuoka lavas free from crustal contamination are among the highest and lowest, respectively (average, 0.7052 and 0.5126), in the region. The samples show the signature of enriched mantle type 2 (EM2), differing from most of the other Dupal anomaly bearing lavas reported from the Sea of Japan and elsewhere in northern Kyushu. The EM2-like characteristics and relatively low concentrations of large ionic lithophile elements (LILE), low LILE/LREE and LREE/high-field-strength elements (HFSE), and mid-ocean ridge basalt (MORB)-like Rb/Sr and Nb/Zr ratios in the Fukuoka lavas are explained by melts from an asthenospheric source that experienced previous melt extraction. Because the chemical characteristics of Fukuoka basalts are strictly, geographically localized, we suggest that, while the mantle beneath most of northern Kyushu is very much similar to that of the Sea of Japan, represented by a spectrum of depleted MORB-EM1 (Dupal-like) hybrids, the Fukuoka EM2-rich component may have been added from shallower levels.

Petrogenesis and evolution of Mt. Vulture alkaline volcanism (Southern Italy)

The Late Pleistocene Mt. Vulture strato-volcano developed at the intersection of NE-SW and NW-SE lithospheric fault systems, on the easternmost border of the Apennine compressional front overthrust onto the Apulian foreland. The initial phase of the volcanic activity is represented by pyroclastic deposits, including lava blocks, and subordinate eccentric domes, mostly phonolitic in composition. The later stages of activity formed the bulk of the strato-volcano (pyroclastic products and subordinate lavas), mostly tephritic in composition, with minor intercalations of basanite, mela-foidite and melilitite lavas and dikes. Variations in rock and mineral composition suggest that the volumetrically predominant basanite-tephrite (foidite)-phonotephrite-phonolite series can be accounted for by fractional crystallization processes starting from basanitic parental magmas, in agreement with the remarkably constant 87Sr/86Sr isotopes (0.70586–0.70581). Mass-balance calculations indicate that the variably differentiated magmas may have been produced by removal of wehrlite, clinopyroxenite and syenite cumulates, some of which are occasionally found as cognate xenoliths in the volcanics. Fractionation processes probably developed in multiple-zoned magma chambers, at depths of 3–5 km, corresponding to the tectonic discontinuity between the allochthonous Apennine formations and the underlying Apulian platform. Highly differentiated phonolitic magmas capping the magma chambers and their conduits thus appear to have fed the initial volcanic activity, whereas dominantly tephritic products were erupted in later stages. The least evolved mafic magmas, namely basanites, mela-foidites and melilitites, are characterized by diverse Na/K ratios and critical SiO2-undersaturation, which indicate their derivation as independent melts generated from distinct, heterogeneously enriched mantle sources and by variable partial melting degrees. Primitive mantle-normalized incompatible element patterns of Vulture mafic lavas invariably share analogies with both orogenic subduction-related magmas (high Low Field Strength Elements/High Field Strength Elements ratios, K, Rb and Th contents and marked Ti and Nb negative anomalies) and alkaline lavas from within-plate and rift settings (high Light Rare Earth Elements, P, Zr, Nb and Na). These geochemical features may be accounted for by magma generation from deep lithospheric mantle sources, enriched in Na-alkali silicate/carbonatite anorogenic components, subsequently affected by orogenic subduction-related K-metasomatism, analogous to that which modified magma sources of the Roman Magmatic Province along the internal Apennine Chain.

Constraints on timing of peak and retrograde metamorphism in the Dabie Shan Ultrahigh-Pressure Metamorphic Belt, east-central China, using U–Th–Pb dating of zircon and monazite

The Dabie Shan Ultrahigh-Pressure Metamorphic (UHPM) Belt occupies the suture between the Yangtze and Sino-Korean blocks in east-central China. The timing of UHPM in the Dabie belt is controversial, and most recent data come from dating of zircons. Monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones, and monazite growth has been documented to occur during prograde, peak, and retrograde metamorphism. Zircons and monazites from UHP mafic rocks from Maowu and a UHP jadeite quartzite from Shuanghe were imaged in thin sections and in mineral separates and dated using a high-resolution ion microprobe. Maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite. Maowu eclogites and garnet pyroxenites contain zircons with mean 206Pb/ 238U age of ∼230±4 Ma, and monazites from a clinopyroxenite have 208Pb/ 232Th ages of ∼209±4 Ma. Multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages (i.e., they do not correspond to the cessation of diffusional Pb loss as rocks cooled below the closure temperature during exhumation). Shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921±22 Ma and lower intercept age of 236±32 Ma, interpreted to represent the ages of the source rocks and of peak metamorphism, respectively. Zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1.5 GPa. Their pooled age of 238±3 Ma agrees well with the lower intercept defined by cores, suggesting that Pb loss from cores and growth of new rims occurred during UHPM at ∼235–240 Ma. Monazite records multiple events during retrograde metamorphism. Maowu clinopyroxenite and Shuanghe jadeite quartzite experienced monazite growth at ∼209 Ma, interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration. Only the jadeite quartzite records growth at 223±1 Ma. Estimated exhumation rates for Maowu are 7–8 km/Ma.

Coexisting monazite and allanite in peraluminous granitoids of the Tribeč Mountains, Western Carpathians

Monazite, a typical light rare-earth element (LREE) mineral of S-type granitoids in the Western Carpathians, was found in the peraluminous biotite granodiorite-tonalite in the Tribec Mountains commonly containing polymineralic inclusions. These inclusions are dominated by anhedral allanite, although allanite also occurs rarely as discrete grains not enclosed by monazite. The monazite studied here is relatively homogeneous and characterized by high Th contents with proportions of huttonite (ThSiO4) and brabantite [CaTh(PO4)2] up to 14.6 and 9.3%, respectively. The discrete allanite grains are highly aluminous with a composition consistent with the peraluminous type of host rock. However, allanite included in monazite is extremely variable in LREE, Al, Fe, and Mg contents. This variation is interpreted to result from entrapment of allanite (+ melt) in monazite before local equilibrium was attained. The change from allanite to monazite as the stable LREE-rich phase is related to an overall decrease in Ca concentration caused by the onset of plagioclase crystallization. The early precipitation of allanite was possible because of the high LREE concentrations in the melt. The crystallization temperature of allanite must have been higher than monazite saturation (>856–845 °C and 798–790 °C for two analyzed samples). The Zr saturation temperature based on zircon solubility and REE thermometry based on monazite solubility reflect an increase in temperature from the edge to the center of the pluton, which coincides with an increase in the huttonite content in monazite. The primary LREE assemblage is accompanied by small grains of late huttonite(?) replacing monazite and brabantite replacing allanite.

Infilling history of a Neoproterozoic intracratonic basin: Nd isotope provenance studies of the Uinta Mountain Group, Western United States

Neodymium isotope and selected trace element concentration data were obtained from 27 samples of shales and arkosic sandstones comprising the Neoproterozoic Uinta Mountain Group (UMG) in north-eastern Utah and northwestern Colorado to determine the provenance of siliciclastic detritus deposited in this region and to reconstruct aspects of the Proterozoic geologic history of the southern margin of the Archean Wyoming Province. In the east lobe of the Uinta Mountains, coarse-grained components of the Jesse Ewing Canyon Formation, the oldest recognized formation of the UMG, have measured ϵ Nd values of −23 to −28 and were most probably derived from Archean basement of the Wyoming Province to the north. Fine-grained sediment in the southern portions of the Jesse Ewing Canyon Formation and shales and quartz arenites from overlying undifferentiated UMG all contain at least a component of detritus derived from Proterozoic crust. In the west lobe, the oldest members of the UMG (the Red Castle Lake, Gilbert Peak and Moosehorn Lake Formations) and arkosic arenite comprising the northern portions of the overlying Mount Watson Formation were also derived exclusively from Archean sources. However, as in the east lobe, the more mature sandstones that dominate the southern sections of the Mount Watson Formation have isotopic compositions that require a significant Proterozoic-crust derived detrital component. Our data support a model in which sedimentary rocks with an Archean provenance represent the preserved remnants of alluvial fans marking the northern margin of the basin in which the UMG was deposited. A source for these sediments within the Archean Wyoming Province is consistent with the high light rare-earth element (LREE) and Th abundances of finegrained, low ϵ Nd, sedimentary rocks, given that Late Archean granitic rocks within the Wyoming province have similar trace element characteristics. Sediments with Proterozoic components in both the east and west lobes were apparently deposited from a fluvial system that transported sediment along the basin axis. The Proterozoic detrital component in these sediments originated either through direct erosion of the Colorado Province in the Neoproterozoic, or through erosion of ‘mixed’ Archean and Proterozoic crust located to the east of the UMG basin that originally formed during the Paleoproterozoic accretion of the Colorado Province to the southern margin of the Wyoming craton.

Petrogenesis and regional tectonic significance of Late Devonian mafic intrusions in the Meguma Zone, Nova Scotia

Late Devonian (377–368 Ma, 40Ar/39Ar; 376 Ma, U–Pb) mafic intrusions in the Meguma Zone crop out as dykes, plugs, and synplutonic bodies of gabbro, diorite, or lamprophyre. All of the intrusions have similar lithologie characteristics and hydrous ferromagnesian mineral assemblages, and they appear to represent a genetically related series of mafic bodies with similar petrogenetic histories in the crust of the Meguma Zone. The intrusions show wide chemical variation of SiO2 (45.7–65.7 wt.%), Al2O3 (8.9–26.5 wt.%), MgO (2.8–26.5 wt.%), CaO (1.2–11.2 wt.%), and K2O (0.1–4.4 wt.%), and they have calc-alkaline, high-K calc-alkaline, and shoshonitic characteristics. Large-ion lithophile elements (LILE) are present at variable but high concentrations (e.g., Ba = 62–1920 ppm, Sr = 176–2567 ppm) relative to most high field strength element (HFSE) abundances (e.g., Y = 10–37 ppm, Zr = 8–421 ppm), and light rare-earth elements (LREE) have much higher concentrations than heavy rare-earth elements (HREE) (La/Lu = 24–330). Initial Sr isotopic ratios (0.7044–0.7079) and εNd values (−4.36 to 3.69) are highly variable. Scatter on major oxide variation diagrams probably results from the fractionation of all the major modal phases in the intrusions (olivine, augite, hornblende, and (or) plagioclase), and the cumulate characteristics of some bodies support this suggestion. Nevertheless, parallel patterns for the intrusions on mid-ocean ridge basalt (MORB) normalized spider diagrams support the notion of similar mafic parent melts, and Sr–Nd isotopic data identify contamination by continental crust in only one of the intrusions. The most primitive picrite contains approximately basaltic HFSE in conjunction with HREE at 5–11CN, perhaps suggesting that the magmas emanated from depleted peridotite or pyroxenite, but high alkalies, LILE (<60MN), and LREE (10–100CN), and elevated initial Sr ratios in all of the intrusions, also require the existence of an enriched source component. Troughs in the spider diagrams at Ta, Nb, and Ti, and Sr–Nd isotopic values comparable with modern island-arc basalts, suggest that fluids derived from subducted ocean lithosphère metasomatized the mantle. Tectono-magmatic discriminators imply a continental margin arc environment rather than an island arc, and the intrusions record either Early Devonian subduction of Iapetus Ocean lithosphère beneath the Avalon terrane, Middle Devonian subduction of Theic Ocean lithosphère beneath the Meguma terrane, or an inherited subduction signature formed during a much older event.

Origin of Archean lamprophyre dykes, Superior Province, Canada: rare earth element and Nd?Sr isotopic evidence

Hornblende- and clinopyroxene-phyric lamprophyre dykes exposed in the Roaring River Complex, Superior Province are alkaline, nepheline-normative, basaltic compositions (>50 wt% SiO2), that range from primitive to fractionated [Mg/(Mg + total Fe)=0.66–0.40; Ni=200–35 ppm], and which have high abundances of light rare earth elements (REE) [(Ce/Yb)n=16–26, Cen=60–300; n = chondrite normalized], Sr (870–1,800 ppm), P2O5 (0.4–1.3 wt%), and Ba (150–900 ppm). Crystal fractionation of the lamprophyres produced coeval gabbro and clinopyroxenite cumulate bodies. A whole-rock Sm−Nd isochron for lamprophyres and gabbro-pyroxenite yields a crystallization age of 2,667±51 Ma Ma (I=0.50929±0.0004; ɛNd = + 2.3 α 0.7). Whole-rock Sr isotope data are scattered, but suggest an initial 87Sr/86Sr ratio of ≈ 0.7012, similar to bulk Earth. The elevated levels of light REEs and Sr in the lamprophyres were not due to crustal contamination or mixing with contemporaneous monzodioritic magmas, but a result of partial melting of a mantle source which was enriched in these and other large-ion-lithophile elements (LILEs) shortly before melting. The lamprophyres were contemporaneous with mantle-derived, high-Mg, LILE-enriched monzodiorite to granodiorite of the Archean “sanukitoid” suite. Both suites have concave-downward light REE profiles, suggesting that depleted mantle was common to their source regions, but the higher light REE abundances, higher Ba/La ratios, and lower ɛNd values (+1.3±0.3) of the parental monzodiorites suggest a more enriched source. The lamprophyres and high-Mg monzodiorites were derived from a mineralogically and compositionally heterogeneous, LILE-enriched mantle lithosphere that may have been part of a mantle wedge above a subducting plate in an arc environment.

Geochemical stratigraphy of the Huronian continental volcanics at Thessalon, Ontario: contributions of two-stage crustal fusion

The 1500 m thick sequence of Huronian continental volcanics at Thessalon, Ontario is subdivided into 4 volcanic cycles, each of which includes abundant early mafic end-members, central intermediate flows, and late rhyolite units. Major and trace element concentrations are dominated by extensive gabbroic fractionation trends that ultimately produced two types of felsic flows: (1) rhyolites with high light rare earth element (LREE) and relatively low large-ion lithophile element (LILE) concentrations (high-LREE, low-LILE rhyolites), and (2) following late separation of REE-rich accessory phases, rhyolites depleted in LREE (low-LREE, high-LILE rhyolites). Mafic end-members of individual volcanic cycle are progressively less siliceous and less enriched in LILE and LREE with height in the stratigraphic section. Ti/Zr ratios gradually rise from 35 in early mafic flows to stabilize at about 85 in late units, while average SiO2 contents decrease from 56 to about 50% and Mg# rises from about 48 to 52. ɛ-Nd values are consistently negative, indicating variable degrees of pre-fractionation crustal contamination of the end-member magmas during their uprise through the crust. Mixing models are consistent with up to 50% contamination by crustal material of tonalitic hornblende-gneiss composition. A progressive increase in ɛ-Nd, from about-5.0 to-0.5 upward in the volcanic succession, reflects a decreasing degree of crustal contamination due to development of insulating layers along margins of the feeder system. Detailed stratigraphic variations suggest that successive magmas batches were intercepted by a progressively fractionating, periodically replenished magma source, giving rise to open-system magmatism. Despite the prevalence of crustal assimilation in the Huronian lavas, (La/Sr)N ratios are too low in least contaminated end-members to be explained by contamination of tholeiitic magmas. The late basalts resemble instead modern island are basalts, and it is suggested that the subcontinental mantle source was enriched by subduction-related processes during crustal formation. Within individual volcanic cycles gabbroic fractionation trends systematically deviate from calculated factors toward compositions characteristic of hornblende-gneiss. Such relations suggest that further crustal contamination of the magmas occurred simultaneous with crystal fractionation. probably within undulating sills at upper crustal levels. Quantitative analysis suggests assimilation/fractional crystallization (A/FC) ratios of about 0.45. As a result of extensive two-stage contamination, rhyolites from the initial volcanic cycle incorporate a total of over 60% of crust.

Petrology of the Rainy Lake area, Minnesota, USA-implications for petrotectonic setting of the archean southern Wabigoon subprovince of the Canadian Shield

The Rainy Lake area in northern Minnesota and southwestern, Ontario is a Late Archean (2.7 Ga) granite-greenstone belt within the Wabigoon subprovince of the Canadian Shield. In Minnesota the rocks include mafic and felsic volcanic rocks, volcaniclastic, chemical sedimentary rocks, and graywacke that are intrucded by coeval gabbro, tonalite, and granodiorite. New data presented here focus on the geochemistry and petrology of the Minnesota part of the Rainy Lake area. Igneous rocks in the area are bimodal. The mafic rocks are made up of three distinct suites: (1) low-TiO2 tholeiite and gabbro that have slightly evolved Mg-numbers (63–49) and relatively flat rare-earth element (REE) patterns that range from 20–8 x chondrites (Ce/YbN=0.8–1.5); (2) high-TiO2 tholeiite with evolved Mg-numbers (46–29) and high total REE abundances that range from 70–40 x chondrites (Ce/YbN=1.8–3.3), and (3) calc-alkaline basaltic andesite and geochemically similar monzodiorite and lamprophyre with primitive Mg-numbers (79–63), enriched light rare-earth elements (LREE) and depleted heavy rare-earth elements (HREE). These three suites are not related by partial melting of a similar source or by fractional crystallization of a common parental magma; they resulted from melting of heterogeneous Archean mantle. The felsic rocks are made up of two distinct suites: (1)low-Al2O3 tholeiitic rhyolite, and (2) high-Al2O3 calc-alkaline dacite and rhyolite and consanguineous tonalite. The tholeiitic felsic rocks are high in Y, Zr, Nb, and total REE that are unfractionated and have pronounced negative Eu anomalies. The calcalkaline felsic rocks are depleted in Y, Zr, and Nb, and the REE that are highly fractionated with high LREE and depleted HREE, and display moderate negative Eu anomalies. Both suites of felsic rocks were generated by partial melting of crustal material. The most reasonable modern analog for the paleotectonic setting is an immature island arc. The bimodal volcanic rocks are intercalated with sedimentary rocks and have been intruded by pre- and syntectonic granitoid rocks. However, the geochemistry of the mafic rocks does not correlate fully with that of mafic rocks in modern are evvironments. The low-TiO2 tholeiite is similar to both N-type mid-ocean-ridge basalt (MORB) and low-K tholeiite from immature marginal basins. The calc-alkaline basaltic andesite is like that of low-K calc-alkaline mafic volcanic rocks from oceanic volcanic arcs; however, the high-TiO2 tholeiite is most similar to modern E-type MORB, which occurs in oceanic rifts. The conundrum may be explained by: (1) rifting of a pre-existing immature arc system to produce the bimodal volcanic rocks and high-TiO2 tholeiite; (2) variable enrichment of a previously depleted Archean mantle, to produce both the low- and high-TiO2 tholeiite and the calc-alkaline basaltic andesite, and/or (3) enrichment of the parental rocks of the high-TiO2 tholeiite by crustal contamination.

Granite-Molybdenite System of Ambalavayal, Kerala: Part I. Geochemistry and Petrogenesis of the Granite

Abstract The 595 Ma old granite pluton punctures Precambrian low-alumina trondhjemites and occurs spatially related to the Moyar and Calicut fault-lineament. Mineralogically, it is a quartz-alkali-feldspar granite. Molybdenite occurs as disseminated flakes in the granite and associated pegmatites and as flaky aggregates in quartz veins. K: Na : Ca relationship suggests an adamellitic composition, whereas SiO2 vs. Log10K2O/MgO proportions confirm its alkaline nature. In Harker variation diagrams, the oxide weight percentages show overall magmatic trends. Among trace elements, the notable features are low large ion lithophile element contents, moderate to high transition element levels and extremely high light rare earth element values. High SiO2, K2O and Na2O values, with low abundance of Ba, Rb and Sr and enrichment of Nb and Zr are typical features comparable to alkali granites elsewhere. The Rb, Nb and Y characteristics denote a within-plate affinity. Petrochemical features indicate derivation of the granite from an alkali-enriched partial melt through quartz and alkali feldspar fractionation. The extremely high K/Rb values imply a K-enriched, Rb-depleted deep crustal source. The enrichment of LREE and HFS elements indicate an active role of CO2-rich volatiles in the petrogenetic regime.