Magmatic evolution of the Karmøy Ophiolite Complex,SW Norway: relationships between MORB-IAT-boninitic-calc-alkaline and alkaline magmatism

The polyphasal magmatic evolution of the Caledonian Karmøy Ophiolite Complex includes: (1) formation of an axis sequence from island-arc tholeiitic (IAT) and more MORB-like magmas (493+7/-4 Ma); (2) intrusion of magmas of boninitic affinity (485±2 Ma); (3) intrusion of MORB- and IAT-like magmas; (4) intrusion and extrusion of calc-alkaline magmas (470+9/-5 Ma); (5) intrusion and extrusion of basalts with alkaline trace-element affinity. Repeated intrusion of MORB and IAT-like magmas may be explained by intermittent magmatism involving magma-chamber solidification and remelting of a source characterized by initial _Nd of approximately +6.5. The boninitic rocks may have formed from two LREE-depleted sources: the primary source of the axis-sequence magmas and the residual source left after extraction of these magmas. These sources have been enriched in LREE, Th and Zr from subducted material exhibiting a continental Nd-isotope signature with initial _Nd less than-8. Covariation between _Nd and Th, Zr, Nd, Y and Yb may be explained by metasomatic enrichment of a LREE-depleted mantle source by a LREE-enriched subduction component, followed by partial melting during which the degree of melting of the metasomatized mantle source increased linearly with the amount of subduction component added to the mantle source. The calc-alkaline magmas may have formed by remelting of a highly depleted source, which became enriched in some trace elements derived from the source of the subsequent alkaline magmatism. The geology and geochemistry of the Karmøy Ophiolite Complex suggest growth of an island-arc upon newly-formed oceanic crust, followed by arc-splitting and the development of a new basin.     

Age and Petrogenesis of Two Late Archean Magmatic Suites, Northwestern Superior Province, Canada: Zircon U-Pb and Lu-Hf Isotopic Relations

The paper presents U–Pb ages for zircon, titanite, andmonazite, and Hf isotopic data for zircon, from the rocks oftwo magmatic suites occurring mostly in the Archean Uchi Subprovinceand partly in the neighbouring Berens River and English Riversubprovinces of the northwestern Superior Province, Ontario.These data, together with observations on the morphologies and,where evident, the inheritance of the zircon crystals, constrainthe nature of the sources of the magmas and provide a recordof various crustal processes in their evolution. The older of the two magmatic suites formed at 2744–2740Ma along segments of a common arc system. The suite consistsof (1) several trondhjemitic to granodioritic plutons, with_Hf values of 6•1, intruded into older crust and possiblyformed from magma produced by partial melting of subducted,juvenile oceanic crust; (2) an assemblage of dacitic pyroclasticvolcanic rocks, with _Hf values of 3•2–4•0, associatedwith tholeiitic basalts and probably derived from magma meltedfrom arc mantle; and (3) a bimodal assemblage of tholeiiticbasalts, rhyolites, and porphyries, also with _Hf values of 6•1,associated with a volcanogenic massive sulphide deposit andapparently formed by differentiation of mantle-derived basalticmelts at shallow levels in an extensional back-arc setting. The second magmatic suite, formed between 2702 and 2693 Ma,comprises late orogenic plutons and batholiths of dioritic todominantly granodioritic composition. The characteristics ofthese intrusions are consistent with a process combining meltingof a metasomatized mantle source and subsequent fractional crystallizationof the derived magmas at shallow depths. However, most of thestudied occurrences show evidence of crustal contamination throughvarious combinations of assimilation of lower-crustal material,assimilation of underthrust sedimentary rocks, and contaminationby wall rock materials during the latest stages in the emplacementof the plutons. The involvement of crustal material is indicatedby the presence of zircon xenocrysts and by _Hf values rangingfrom 1•4 to 4•4. Only one intrusion, with an _Hf valueof 5•0 and no xenocrystic zircon, appears to have escapedwidespread contamination, perhaps because the ascent of itsmagma was facilitated by a crustal-scale fracture system.     

Nd and Sr isotopes in the Aleutians: multicomponent parenthood of island-arc magmas

Young volcanic rocks from different sections of the Aleutian Islands-Alaska Peninsula Arc have been measured for ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd and some trace elements. We found the ¹⁴³Nd/¹⁴⁴Nd to be highly restricted in range ( _Nd=6 to 7) and low as compared to midocean ridge ba-salts (MORB). This indicates that the source of the Aleutian Arc magmas is different from MORB and remarkably isotopically homogeneous with respect to Nd. The range reported here for arc rocks is substantially smaller than found by other workers. However, the Sr isotope ratios vary considerably ( _Sr=–24 to –14). Those samples from small volcanic centers north of the main arc (second arc) are characterized by low _Sr. Our data in combination with previous studies suggest that there are slight geochemical differences between discrete sections of the arc. The general uniformity of Nd isotope ratios are thought to be the surface expression of an efficient mixing or homogenization process beneath the arc plate, but which still causes a wide dispersion in Sr isotopic composition.To relate the arc rocks to the broader tectonic setting and to identify possible sources of arc magmas, measurements were done on volcanic and sedimentary rocks from the North Pacific/Bering Sea area. Alkali basalts from the back-arc islands St. George, Nunivak and St. Lawrence and alkali-rich tholeiites from the fore-arc have _Nd=+4 to +9 and are correlated on the _Sr- _Nddiagram parallel to the mantle array but shifted to lower _Sr. These samples are thought to be isotopically representative of the mantle transported to that region. A tholeiitic basalt from the Kamchatka Basin ocean floor (back-arc), however, yielded typical MORB values ( _Nd=10, _Sr=–24). Composite sediment samples were made from DSDP cores in the Aleutian Abyssal Plain, Gulf of Alaska and the Alka Basin which represent mixtures of continentally and arc-derived materials. These composites have intermediate Nd isotopic ( _Nd= –2 and +2) and high Sr isotopic values ( _Sr=+9 and +37). These data show that possible source materials of the Aleutian Arc volcanics are isotopically different from and much more heterogeneous than the arc rocks themselves.On the basis of this study and of literature data, we developed a set of alternative models for volcanic arc magma generation, based on the restricted range in _Nd and the wider range in _Sr for arc rocks. Different isotopic and trace element characteristics found in different arcs or arc sections are explained by varying mixing proportions or concentrations in source materials. The basic observations require rather strict mixing ratios to obtain constant _Nd. The preferred model is one where the melting of subducted oceanic crust is controlled by the amount of trapped sediment with the melting restricted to the upper part of the altered basaltic layer. Homogenization within the upper part of the oceanic crust is brought about by hydrothermal circulation attending dewatering of the slab during subduction and possibly some oxygen exchange of the magmas on ascent.Division Contribution Number 3849 (411)     

An experimental study of the effects of melt composition on plagioclase-melt equilibria at 5 and 10 kbar: implications for the origin of magmatic high-An plagioclase

An experimental investigation of plagioclase crystallization in broadly basaltic/andesitic melts of variable Ca# (Ca/(Ca+Na)*100) and Al# (Al/(Al+Si)*100) values and H₂O contents has been carried out at high pressures (5 and 10 kbar) in a solid media piston-cylinder apparatus. The H₂O contents of glasses coexisting with liquidus or near-liquidus plagioclases in each experiment were determined via an FTIR spectroscopic technique. This study has shown that melt Ca# and Al#, H₂O content and crystallization pressure all control the composition of liquidus plagioclase. Increasing melt Ca# and Al# increase An content of plagioclase, whereas the effect of increasing pressure is the opposite. However, the importance of the role played by each of these factors during crystallization of natural magmas varies. Melt Ca# has the strongest control on plagioclase An content, but melt Al# also exerts a significant control. H₂O content can notably increase the An content of plagioclase, up to 10 mol % for H₂O-undersaturated melts, and 20 mol % for H₂O-saturated melts. Exceptionally calcic plagioclases (up to An₁₀₀) in some primitive subduction-related boninitic and related rocks cannot be attributed to the presence of the demonstrated amounts of H₂O (up to 3 wt %). Rather, they must be due to the involvement of extremely refractory (CaO/Na₂O>18) magmas in the petrogenesis of these rocks. Despite the refractory nature of some primitive MORB glasses, none are in equilibrium with the most calcic plagioclase (An₉₄) found in MORB. These plagioclases were likely produced from more refractory melts with CaO/Na₂O = 12–15, or from melts with exceptionally high Al₂O₃(>18%). Magmas of appropriate compositions to crystallize these most calcic plagioclases are sometimes found as melt inclusions in near liquidus phenocrysts from these rocks, but are not known among wholerock or glass compositions. The fact that such melts are not erupted as discrete magma batches indicates that they are effectively mixed and homogenized with volumetrically dominant, less refractory magmas. The high H₂O contents (∼ 6 wt%) in some high-Al basaltic arc magmas may be responsible for the existence of plagioclases up to An₉₅ in arc lavas. However, an alternative possibility is that petrogenesis involving melts with abnormally high CaO/Na₂O values (> 8) may account for the presence of highly anorthitic plagioclases in these rocks. Received: 31 August 1993 / Accepted: 20 May 1994     

Intra-oceanic production of continental crust in a Th-depleted ca. 3.0 Ga arc complex,western Superior Province,Canada

The English Lake magmatic complex in the western Superior Province of Canada represents a fragment of early (3.0 Ga) continental crust exposed in oblique cross section through tonalitic upper levels and subjacent quartz diorite, diorite and gabbro, which are cut by late gabbro, anorthosite and hornblendite dykes. Massive, foliated and gneissic units of tonalitic to gabbroic composition, crystallized over a 10 to 18 m.y. period, bear common geochemical attributes, including negative Th, U and Nb anomalies, and only slight LREE and LILE enrichment on NMORB-normalized trace-element profiles. Epsilon Nd values (+0.1 to +1.7) and ¹⁸O (+6.7 to +8.0 ) do not co-vary with silica or other crustal contamination indices. High Mg#'s and Ni contents suggest derivation from, or interaction with mantle, and large positive anomalies for Ba, Sr and Pb, as well as high U/Th, suggest metasomatism by hydrous fluids. Trace-element profiles resemble those of primitive intra-oceanic island arc magmas except for the negative Th-U anomaly, which precludes the involvement of either oceanic (sedimentary or basaltic) or continental crust in the petrogenesis of English Lake magmas. In order to account for the unusual geochemical character of the suite, we postulate that water-rich fluids derived from subducted, sea-floor-altered serpentinite provided the flux for melting a depleted mantle wedge. Contemporaneous, proximal high Th/Nb tonalites suggest that the zone of serpentinite subduction occurred within a restricted arc segment possibly due to subduction of either: (a) a seamount chain oriented broadly perpendicular to an arc, or (b) a similarly oriented serpentinite-enclosed oceanic fracture zone or fault.Electronic Supplementary Material Supplementary material is available in the online version of this article at .Editorial responsibility: T.L. Grove     

Petrology and geochemistry of Pan-African granitoids, Kab Amiri area, Egypt – implications for tectonomagmatic stages in the Nubian Shield evolution

Summary Three distinctive metaluminous granitic suites have been identified from the Pan-African belt of the Kab Amiri area, Eastern Desert, Egypt. These are: 1) a trondhjemite-tonalite suite, 2) a calc-alkaline granodiorite suite, and 3) an alkali leucogranite suite. The trondhjemite-tonalite and the granodiorite suites resemble I-type granitoids whereas the alkali leucogranites display A-type characteristics. Geochemical attributes and field aspects indicate that three independent magmas, at different tectonic stages of the Pan-African crustal growth, are required to explain the origin of these granitoid suites. Rocks of the trondhjemite-tonalite suite correspond to granites of the arc stage and possess a narrow range of SiO₂ with low K₂O, Sr, Rb, Ba, Nb and Zr. Its composition is consistent with 20–30% partial melting of a primitive low-K tholeiitic source, similar to the early formed tholeiitic metavolcanics of the Egyptian basement. The granodiorite suite belongs to the collision stage and displays higher K₂O, Rb, Ba, and Sr. Its magma was derived by 30–40% partial melting of LILE-enriched mafic island arc crust. The presence of abundant microdiorite enclaves in the trondhjemite-tonalite and the granodiorite suites suggests that mantle-derived mafic magma played an important role in their petrogenesis, acting as a heat source for melting via underplating and/or intrusion. The A-type leucogranites are post-collision highly fractionated granites. They exhibit low Al₂O₃, MgO, CaO, TiO₂, Sr, and Ba and high Rb, Nb, Y. The wide chemical variations within this suite are consistent with its evolution by fractional crystallization of plagioclase, K-feldspar, amphibole, Fe–Ti oxides, and apatite from a mafic magma. The parent magma was originated in the upper mantle due to crustal attenuation associated with extension in the late stage of the Pan-African crustal evolution. Received September 13, 2000; revised version accepted May 4, 2001     

Porphyry Cu (–Mo–Au) deposits related to melting of thickened mafic lower crust: Examples from the eastern Tethyan metallogenic domain

Most porphyry Cu deposits in the world occur in magmatic arc settings and are formed in association with calc-alkaline arc magmas related to subduction of oceanic lithosphere. This contribution reviews a number of significant porphyry Cu deposits in the eastern Tethyan metallogenic domain. They widely occur in a variety of non-arc settings, varying from post (late)-collisional transpressional and extensional environments to intracontinental extensional environments related to orogenic and anorogenic processes. Their spatial–temporal localization is controlled by strike–slip faults, orogen-transverse normal faults, lineaments and their intersections in these non-arc settings. These deposits are dominated by porphyry Cu–Mo deposits with minor porphyry Cu–Au and epithermal Au deposits, and exhibit a broad similarity with those in magmatic arcs. The associated magmas are generally hydrous, relatively high fO₂, high-K calc-alkaline and shoshonitic, and show geochemical affinity with adakites. They are distinguished from arc magmas and/or oceanic-slab derived adakites, by their occurrence as isolated complexes, high K₂O contents (1.2–8.5%), and much wider range of ε_Nd(t) values(? 10 to + 3) and positive ε_Hf(t) values (+ 4.6 to + 6.9). These potassic magmas are most likely formed by partial melting of thickened juvenile mafic lower-crust or delaminated lower crust, but also involving various amounts of asthenospheric mantle components. Key factors that generate hydrous fertile magmas are most likely crust/mantle interaction processes at the base of thickened lower-crust in non-arc settings, rather than oceanic-slab dehydration (as in arc settings). Breakdown of amphibole in thickened lower crust (e.g., amphibole eclogite and garnet amphibolite) during melting is considered to release fluids into the fertile magmas, leading to an elevated oxidation state and higher H₂O content necessary for development of porphyry Cu–Mo–Au systems. Copper and Au in hydrous magmas are likely derived from mantle-derived components and/or melts, which either previously underplated and infiltrated at the base of the thickened lower crust, or were input into the primitive magmas by melt/mantle interaction. In contrast, Mo and (part of the) S in the fertile magmas are probably supplied by old crust during melting and subsequent ascent.     

Fluid flow in subduction zones: evidence from Nd- and Sr-isotope variations in metabasalts of the Franciscan complex,California

Greenstone, blueschist and eclogite metabasaltic blocks from the Franciscan complex of California preserve extensive petrographic and chemical evidence for interaction with hydrous fluids at high-P, low-T metamorphic conditions. The Nd and Sr isotope variations within and among the blocks constrain the origin of the basaltic protoliths, the nature of the fluid metasomatism that occurred within the upper levels (15–45 km) of the paleosubduction zonc, and the character and provenance of the rock that generated the hydrous fluids within the paleosubduction zone. Samples with little or no petrographic evidence of retrograde alteration and unaltered garnet separates have _Nd. With increasing degrees of retrograde alteration, Nd isotope compositions are consistently lower, ranging down to _Nd(160)=5. Actinolitic alteration rinds which are present on some blocks have the least radiogenic compositions with _Nd=1.6 to 6.1. While Nd isotope compositions of unaltered blockes are in the range expected for basalt derived from normal depleted mantle, the Sr isotope compositions are more radiogenic ranging from _Sr(160)=–5 to +11. Compositions of unaltered eclogite and blue-schist blocks are consistent with a protolith origin in normal oceanic crust derived from depleted mantle. The Sr isotopy systematics indicate that the protoliths were modified by seawater alteration in an ocean-floor hydrothermal system. Isotopic compositions of samples from parts of blocks that have a retrograde metamorphic overprint show a strong correlation between less radiogenic Nd compositions and the extent of retrograde metamorphism. Maximum Nd isotope ratios of the metasomatizing fluid are provided by analyses of actinolitic rinds, and range from _Nd(160)=1.6 to 6.1. A possible source for fluids of this composition is subducted sediment that was derived from a continental craton. Because rind formation occurred while the basaltic blocks were within an ultramafic matrix, the fluids must have migrated from sediments in the accretionary wedge into an overlying wedge of mantle material imbricated with blocks of oceanic crust. This suggests possibly km-scale movement of fluids that carry an amount of the rare-earth elements sufficient to significantly modify the trace-element budget of subducted basalt.     

Paleoproterozoic (1.90–1.86 Ga) arc volcanism in the Flin Flon Belt,Trans-Hudson Orogen,Canada

Geochemical and isotopic (Nd, Sr) data are reported on Paleoproterozoic (1904–1864 Ma), maficintermediate (<63% SiO₂), arc metavolcanic rocks from the Flin Flon greenstone belt, Manitoba and Saskatchewan. Major element criteria permit subdivision of the rocks into tholeiitic (TH), calc-alkaline (CA), alkaline, and boninitic (BO) magma series. Subaqueously erupted, TH and related CA basalt-basaltic andesite, and rare high-Ca boninites dominated between 1904 Ma and 1890 Ma. The TH rocks are similar to modern island are tholeiites, having low high-field-strength element (HFSE) and rare earth element (REE) abundances, and chondrite-normalized light REE depletion to slight enrichment. The boninites have even lower HFSE and REE abundances (1–2X chondrites). Along with their extreme ratios of refractory incompatible elements (e.g., high Al/Ti, Ti/Zr, low Ti/V, Zr/Y), these features indicate that the arc mantle source was strongly depleted, probably residual after MORB or back-arc basin basalt extraction. Elevated Th/Yb, Ba/La, La/Nb values, and the spread in Nd isotopic compositions (initial _Nd=–0.4 to +4.8) suggest recycling of small amounts (0–8%) of Archean and possibly older Proterozoic crust via sediment subduction and, locally, intracrustal contamination. Calcalkaline andesite-rhyolite and rare shoshonite and trachyandesite, erupted between 1890 Ma and 1864 Ma, are more strongly light REE enriched and have comparatively higher HFSE abundances, and higher Zr/Y and Nb/Y values. The rocks have strong arc trace element signatures (e.g., high Th/Nb, La/Nb), and initial _Nd values (+2.3 to +4.6) indicate that depleted mantle contributions to the magmas continued to be dominant. The geochemistry and geology of these younger volcanic rocks suggest a mature island arc setting in which the arc lithosphere was thicker than in the previous period, and a more fertile sub-arc mantle source was tapped. The pre-1890 Ma volcanism occurred in one or more separate arcs, probably characterized by rapid subduction of oceanic lithosphere, relatively thin, tholeiitic arc crust, and extensive backarc basin formation. In contrast, post-1890 Ma volcanism is dominantly calc-alkaline to (rarely) alkaline, and is interpreted to reflect crustal thickening due to longterm growth of arc edifice(s) and tectonic thickening associated with intraoceanic arc-arc (>1870 Ma) collision and subsequent intra-arc deformation.     

Nd isotopes demonstrate the role of contamination in the formation of coexisting quartz and nepheline syenites at the Abu Khruq Complex,Egypt

The petrogenesis of Abu Khruq, an 89 Ma alkaline ring complex of eastern Egypt which is composed of alkali gabbros and both silica over- and undersaturated syenites, has been investigated. Major and trace element relationships and Nd and Sr isotope data are consistent with formation of the gabbros from an alkaline mafic magma that experienced extensive fractionation, and all syenites from a felsic derivative of this melt. The parental magma had an ⁸⁷Sr/⁸⁶Sr of 0.7030 and an ¹⁴³Nd/¹⁴⁴Nd of 0.512750 (_Nd = +4.4) indicating derivation from a depeleted mantle source. The initial ¹⁴³Nd/¹⁴⁴Nd ratios are: 0.512721 to 0.512748 for the gabbros, 0.512739 to 0.512750 for the alkali syenites and trachytes, 0.512717 to 0.512755 for the nepheline syenites, and, 0.512706 to 0.512732 for the quartz syenites. In contrast, analyzed Precambrian granites from eastern Egypt have generally lower ¹⁴³Nd/¹⁴⁴Nd ratios (ranging from 0.51247 to 0.51261 or _Nd = -0.8 to 1.7, for 89Ma); their Nd model ages range from 775 to 935 Ma and suggest there was no significant input of pre-Pan-African crust in their formation. Among Abu Khruq rocks, ¹⁴³Nd/¹⁴⁴Nd ratios indicate that the quartz syenites formed by open-system, crustal contamination processes whereas the nepheline syenites experienced little or no contamination. Modeling shows that contamination occurred at various stages, affecting both mafic and more evolved compositions with input of about 20% crustal Nd for the most contaminated samples. The degree of contamination is related to the silica saturation of the quartz syenites. Simplified modeling of magma evolution within Petrogeny's Residua System demonstrates the ability of AFC processes to cause a critically undersaturated magma to evolve across the feldspar join and produce oversaturated rocks. The oversaturated syenites at Abu Khruq were produced in this manner whereas the nepheline syenites formed by fractionation without similarly large degrees of contamination. The results have broad implications for the formation of subvolcanic complexes in continental settings beyond the important production of silica oversaturated compositions from crustal interaction. They underscore the importance of crustal interactions in the formation of the various lithologies. Such interactions occur at various stages in the evolution of the magmas and, as such, are not strictly coupled with fractional crystallization. While previous study of Abu Khruq has demonstrated extensive hydrothermal alteration of O and Sr isotopes, the present work shows that the Nd isotope ratios were not significantly affected and thus reflect magmatic signatures. This feature combined with relatively small corrections for initial ratios emphasizes the utility of Nd isotopes for petrogenetic studies.     

Geology and petrology of a deep crustal zone from the Famatinian paleo-arc, Sierras de Valle Fértil and La Huerta, San Juan, Argentina

The ranges of the Sierras Valle Fértil-La Huerta expose natural cross sections through a paleo-arc crust that formed in the Late Cambrian - Early Ordovician Famatinian magmatic arc, northwestern Argentina. Thick mafic sequences of amphibole gabbronorites to orthopyroxene-amphibole-biotite diorites form the lower levels of the exposed paleo-arc section. This mafic unit includes lens-shaped bodies of olivine-bearing cumulate rocks and tabular-shaped sill/dike intrusions of fine-grained chilled amphibole gabbro. The mafic magmas were emplaced into regional metasedimentary sequences at lower crustal levels, corresponding to pressure from 5 to 7 kbar. Gabbronorites likely representing the parental magmas that fluxed into the exposed paleo-arc crust differ from primitive magmatic arc rocks in having somewhat lower Mg-number (ca. 0.60) and compatible (Cr and Ni) trace element contents, and slightly higher Al₂O₃ contents. This difference is taken to indicate that a pyroxene-rich olivine-bearing assemblage with a bulk high Mg/Fe ratio and low Al₂O₃ content crystallized from mantle-derived melts before mafic magmas reached the crustal levels currently exhumed. However, some gabbronorites have incompatible trace element signatures typical of primitive mafic arc magmatism. Igneous rocks to some extent more evolved than those of the mafic unit make up a tonalite-dominated intermediate unit. The intermediate unit consists of a heterogeneous suite that ranges from orthopyroxene-bearing amphibole-rich diorites to biotite-rich amphibole-poor tonalites. Within the intermediate unit, chilled mafic rocks are found as a network of dikes, whereas metasedimentary migmatites appear interlayered as m-wide septa and km-long strips. The tonalite-dominated intermediate unit passes into a granodiorite batholith through a transitional zone that is up to 2-km wide. The boundary zone separating the tonalite-dominated and granodiorite-dominated units is characterized by mingling of tonalitic and leucogranitic magmas, which together appear multiply-intruded by mafic sill/dike bodies. Within the tonalite- and granodiorite-dominated units, the less evolved mafic rocks occur as: (1) bodies tens of meters long, (2) chilled dikes and sills, and (3) microgranular inclusions (enclaves), supporting the inference that mafic magmatism was the main source for generating a vast volume of intermediate and silicic igneous rocks. Mass balance calculations and trace element systematics are combined to demonstrate that tonalites and granodiorites formed by concurrent closed-system fractional crystallization and open-system incorporation of paragneissic migmatites and/or anatectic leucogranites into the evolving igneous sequence. This study argues that the sequence of igneous rocks from Valle Fértil-La Huerta was formed as the result of complementary petrogenetic processes that operated concurrently at different levels of the Famatinian arc crust.     

Neodymium and strontium isotopic characteristics of New Zealand granitoids and related rocks

Initial ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of Phanerozoic granitoids and related intrusions of the New Zealand block display a mixing-type array indicative of the involvement in their sources of old continental crustal material, most likely of Proterozoic age. _Sr(T) values range from –4 to +273 (⁸⁷Sr/⁸⁶Sr=0.7041–0.7233), while _Nd(T) ranges from +2.7 to –11.0. Preexisting metasedimentary rocks have generally higher _Sr and lower _Nd (ranging to present-day values of +646 and –15.0, respectively), and, particularly for the Mesozoic intrusives, are isotopically appropriate mixing end-members. The widespread, early Paleozoic Greenland Group graywackes, which are derived from Proterozoic sources, are modeled as the source of the crustal end-member mixing with mantle-derived mafic magmas to produce the intrusive rocks. Four different types of models are applied to the isotopic and trace-element (Rb, Sr, Ba, REE) data: simple mixing; mixing with a partial melt of the metasedimentary rock, with or without isotopic equilibrium; and assimilation-fractional crystallization. Based on these models, some constraints may be applied on petrogenesis (e.g., the lack of high Rb concentrations points to the presence of biotite, and HREE depletion points to the presence of garnet); however, the models fail to adequately explain all the data. The New Zealand granitoids show similarities in isotopic character not only to rocks from offshore islands on the New Zealand block, but also to similar-aged granitoids in adjacent regions of Antarctica and Australia. This points to similarities in crustal character between continental blocks formerly proximal in Gondwanaland. We note an overall increase in _Nd and decrease in _Sr in felsic magmas from the Paleozoic to the Mesozoic to the Cenozoic in New Zealand, indicative of a decrease over time in the level of influence of recycled continental crust in subduction-related magmatism.Division Contribution No. 4538 (582)     

Origin and evolution of post-collisional magmatism: Coeval Neoproterozoic calc-alkaline and alkaline suites of the Sinai Peninsula

Two Late Neoproterozoic post-collisional igneous suites, calc-alkaline (CA) and alkaline–peralkaline (Alk), widely occur in the northernmost part of the Arabian–Nubian Shield. In Sinai (Egypt) and southern Israel they occupy up to 80% of the exposed basement. Recently published U–Pb zircon geochronology indicates a prolonged and partially overlapping CA and Alk magmatism at 635–590 Ma and 608–580 Ma, respectively. Nevertheless in each particular locality CA granitoids always preceded Alk plutons. CA and Alk igneous rocks have distinct chemical compositions, but felsic and mafic rocks in general and granitoids from the two suites in particular cannot be distinguished by their Nd, Sr and O isotope ratios. Both suites are characterized by positive ε_Nd(T) values, from + 1.5 to + 6.0 (150 samples, 28 of them are new analyses), but predominance of juvenile crust in the region prevents unambiguous petrogenetic interpretation of the isotope data. Comparison of geochemical traits of felsic and mafic rocks in each suite suggests a significant contribution of mantle-derived components to the silicic magmas. Model calculation shows that the alkaline granite magma could have been produced by partial (~ 20%) melting of rocks corresponding to K-rich basalts. Material balance further suggests that granodiorite and quartz monzonite magmas of the CA suite could form by mixing of the granite and gabbro end-members at proportions of 85/15. In the Alk suite, alkali feldspar and peralkaline granites have evolved mainly by fractional crystallization of feldspars and a small amount of mafic minerals from a parental syenogranite melt. Thus the protracted, 20 m.y. long, contemporaneous CA and Alk magmatism in the northern ANS requires concurrent tapping of two distinct mantle sources. Coeval emplacement of CA and Alk intrusive suites was described in a number of regions throughout the world.     

Nd,Sr, and O isotopic variations in metaluminous ash-flow tuffs and related volcanic rocks at the Timber Mountain/Oasis Valley Caldera,Complex, SW Nevada: implications for the origin and evolution of large-volume silicic magma bodies

Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16–9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km³), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initial _Nd values (1 _Nd unit), higher⁸⁷Sr/⁸⁶Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similar _Nd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higher _Nd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from low _Nd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20–40% (by mass) wall-rock into magmas that were injected into the upper crust. The low _Nd magmas most likely formed via the incorporation of low ¹⁸O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higher ¹⁸O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13–14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have low _Nd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70–80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0–10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing basification of a lower crustal magma source by repeated injection of mantle-derived mafic magmas.     

Trace element geochemistry of the rhyolitic volcanic rocks,Central North Island,New Zealand. Phenocryst data

Data are presented for K, Ba, Sr, Rb, Li, Ga, Mg, Mn, and Fe for twelve rhyolitic plagioclases (An₂₈-An₄₆), one dacitic (An₅₃), and three andesitic plagioclases (An₆₈-An₈₁). Additional data are presented for Ga, Gr, V, Ni, Co, Sc, Y, La, Sr, and Ba for two augites, nine hypersthenes, and five hornblendes separated from the same rocks. Distribution factors have been calculated, using these data, and previously published results for coexisting groundmass compositions (=liquids).The plagioclases show a positive correlation between, and a progressive increase in K and Ba (range 0.09–0.58% and 61–610 p.p.m. respectively) with increasing Ab-content. Sr (range 465–880 p.p.m.) shows a well defined maximum between An₄₀-An₅₅. The plagioclases have extremely high K/Rb ratios (mostly > 1,000).This volcanic series is characterised by relatively Mg-rich pyroxenes and hornblendes. The augites contain higher Sc, Cr, Y, Sr, and Y relative to their coexisting hypersthenes, while the hornblendes exhibit higher Sc, V, Ba, Sr, Y, and La relative to coexisting hypersthenes. Very marked differences in concentrations of these elements exist between the rhyolitic and andesitic ferromagnesian phenocrysts. There is also evidence of a systematic distribution of Sc, V, Cr, Y, Co, and Ni between coexisting hypersthenes and hornblendes, and between these minerals and their coexisting whole rock and groundmass compositions.The data are discussed from a petrological viewpoint, as they are interpreted to indicate that the phenocrysts crystallised in the magmas in which they are found, and are not xenocrystic. No evidence of hybridisation or contamination, subsequent to the onset of crystallisation, is found.     

Diverse mantle and crustal components in lavas of the NW Cerros del Rio volcanic field,Rio Grande Rift,New Mexico

Products of Pliocene (2–4 Ma) mafic to intermediate volcanism in the northwestern Cerros del Rio, a dominantly mafic volcanic field in the Española Basin of the Rio Grande Rift (RGR), range from 49% to 63% SiO₂ and exhibit diversity in silica saturation, trace-element patterns, and isotopic compositions. Tholeiites, which are largely confined to west of the Rio Grande, have trace-element abundances that resemble those of oceanic basalts, but with mild depletions in Nb and Ta, and high ⁸⁷Sr/⁸⁶Sr, low ¹⁴³Nd/¹⁴⁴Nd, and high δ¹⁸O compared to typical OIB. They are regarded as asthenospherically-derived magmas contaminated with continental crust. Alkali basalts and hawaiites erupted from vents east of the Rio Grande are geochemically distinct, having generally higher overall incompatible-element abundances, but with pronounced depletions in K, Rb, Nb and Ta with respect to Th and LREE. Spatially-associated benmoreites, mugearites and latites (collectively termed “evolved” lavas) have similar trace-element characteristics to the mafic mildly-alkaline compositions, but are typically not as depleted in K. Hawaiites and evolved lavas exhibit a good negative correlation of ¹⁴³Nd/¹⁴⁴Nd with SiO₂, due to interaction with lower continental crust. The most silicic “evolved” lavas carry the highest proportions of crustal material, and consequently have higher K/Th than the related hawaiites. Several (mostly mafic) lavas contain abundant crustally-derived resorbed quartz xenocrysts in O-isotope disequilibrium with the host magma. The δ¹⁸O values of xenocrystic quartz range over 4‰, indicating a variety of quartz-bearing crustal contaminants beneath the Española Basin. The hawaiites, with their unusual combination of trace-element enrichments and depletions, cannot be generated by any process of fractionation or crustal contamination superposed on a common mantle source type (oceanic or arc-source). It is a regional mantle source type, inasmuch as it was also present beneath NW Colorado during the mid-late Cenozoic. We argue that the hawaiite source must have originally existed as arc-source mantle enriched in LILE, generated during Mesozoic to early Cenozoic subduction at the western margin of North America. This arc-source mantle lost K, Rb and Ba, but not Th or LREE, prior to magmagenesis. Selective element loss may have occurred during lithospheric thinning and uprise of hydrated phlogopitebearing peridotite-possibly as a thermal boundary layer between lithosphere and asthenosphere — to shallow mantle depths, with consequent conversion of phlogopite to amphibole (an inferior host for K, Rb and Ba). We suggest that this occurred during the early extensional phase of the northern RGR. Further extension was accompanied by partial melting and release of magma from this source and the underlying asthenosphere, which by the Pliocene was of oceanic type. The hawaiite source mantle is the product of a long history of subduction succeeded by lithospheric extension of the formerly overriding plate. Similar chemical signatures may have developed in the mantle beneath other regions with comparable histories.     

Petrogenesis of post-collisional Middle Eocene volcanism in the Eastern Pontides (NE,Turkey): Insights from geochemistry,whole-rock Sr-Nd-Pb isotopes,zircon U-Pb and 40Ar-39Ar geochronology

The debate about whether Eocene magmatism is considered to be post-collisional or subduction-related or not still continues. Here we offer new ⁴⁰Ar-³⁹Ar ad U-Pb zircon geochronology, mineral chemistry, bulk rock and Sr-Nd-Pb isotope geochemistry data obtained from the southern dike (SD) suite, in comparison with the northern dike (ND) suite, from the Eastern Pontides. The geochronological data indicate that the SD suite erupted between 45.89 and 45.10 Ma corresponding to the Lutetian (Middle Eocene). The magmas of the ND suite are characterised by slightly more alkaline affinity compared to the SD suite. The trace and rare earth element (REEs) content of the SD suite is characterised by large ion lithophile element (LILEs; Sr, K₂O, Ba, Rb) enrichment and depletion of Nb, Ta, and TiO₂ elements to different degree with high Th/Yb ratios, which indicate that the magmas forming the SD and ND suites were derived from lithospheric mantle sources enriched by mostly slab-derived fluids in the spinel stability field. The Sr, Nd and Pb radiogenic isotope ratios of the dikes support the view that the magma for the hydrous group (H-SD) was derived from a relatively more enriched mantle source than the other SD and ND suites. The ND suite and the anhydrous group (A-SD) display similar geochemical features characterised by moderate light earth element (LREE)/heavy rare earth element (HREE) ratios, while the H-SD group has respectively lower LREE/HREE ratios indicating higher melting degree. Detailed considerations of the alkalinity, enrichment and partial melting degree for the source of the studied volcanic rocks indicate that the magmas of the northern dike suite are characterised by slightly more alkaline affinity, whereas the magmas throughout the southern dike suite show increments in the enrichment rate and melting degree. In light of the obtained data and comparative interpretations, the geodynamic evolution and differences in petrogenetic character of the Lutetian magmas from both the northern and southern parts of the Eastern Pontides may be explained by different degrees of melting of a net veined mantle source initially metasomatized by mostly subduction fluids during asthenospheric upwelling due to fragmented asymmetric delamination in a post-collisional extensional tectonic environment.     

Petrogenesis of Early Cretaceous bimodal volcanic rocks in the Fanchang Basin,SE China: an energy-constrained assimilation–fractional crystallization model

Volcanic rocks in the Middle–Lower Yangtze River Valley (MLYRV) constitute a bimodal magmatic suite, with a significant compositional gap (between 50% and 63% SiO₂) between the mafic and felsic members. The suite is characterized by a relatively wide spectrum of rock types, including basalts, trachytes, and rhyolites. The basaltic rocks have low-to-moderate SiO₂ contents of 46.00–50.01%, whereas the trachytes and rhyolites possess SiO₂ contents in the range of 63.08–77.61%. Rocks of the bimodal suite show moderate enrichment of LILEs, negative Nb, Ta, and Ti anomalies, and are significantly enriched in LREEs. The basalts were most likely generated by parental mafic magmas derived from enriched lithospheric mantle with minor assimilation of crustal materials involving coeval crystal fractionation during magma evolution. The results of energy-constrained assimilation and fractional crystallization simulations demonstrate that the felsic magma was produced by the mixing of 5–20% lower crustal anatectic melts with an evolved mafic magma (～48% SiO₂) and accompanied by extensive clinopyroxene, plagioclase, biotite, and Fe–Ti oxide fractionation. Our model for the genesis of felsic rocks in bimodal suites is different from the traditional models of crustal melting and fractional crystallization or assimilation–fractional crystallization of basaltic liquids.     

Sr-Nd isotopic constraints on the petrogenesis of the Central Bohemian Pluton,Czech Republic

The Sr-Nd isotopic data for selected granitoids of the Central Bohemian Pluton show a broad negative correlation with the total range of (⁸⁷Sr/⁸⁶Sr)₃₃₀ = 0.7051–0.7129 and _Nd ³³⁰ = +0.2 to –8.9. The older intrusions have more depleted Sr-Nd compositions and calc-alkaline geochemistry (Sázava suite), whereas the younger intrusions shift towards K-rich calc-alkaline (Blatná suite) and shoshonitic rocks (íany and ertovo bemeno suites) with more evolved isotopic signatures. The distribution of the data is interpreted as reflecting a diversity of sources and processes, rather than a single progressive crustal contamination trend. The Sázava suite could have originated by partial melting of metabasites, or of a mantle source with an isotopic composition close to bulk earth, or by hybridization of crustally-derived tonalitic and mantle-derived magmas. Variation within the Blatná suite is modelled by mixing between a moderately enriched [(⁸⁷Sr/⁸⁶Sr)₃₃₀ 0.708, _Nd ³³⁰ –3] mantle component with either an isotopically evolved metasedimentary component, or with more evolved magmas of the suite. The íany suite was most probably produced by partial melting of peraluminous lithologies, possibly of the adjacent Moldanubian unit. The ertovo bemeno suite evolved from strongly enriched mantle-derived magmas [(⁸⁷Sr/⁸⁶Sr)₃₃₀0.7128, _Nd ³³⁰ –7], either through closed-system fractional crystallization or interaction with magma corresponding to leucogranites of the Central Bohemian Pluton.     

Chemical and mineralogical evidence of the occurrence of mantle metasomatism by carbonate-rich melts in an oceanic environment (Santiago Island, Cape Verde)

Lavas from Santiago Island attest to a complex magmatic history, in which heterogeneous mantle source(s) and the interactions of advecting magmas with thick metasomatised oceanic lithosphere played an important role in the observed isotopic and trace element signatures. Young (<3.3 Ma) primitive lavas from Santiago Island are characterised by pronounced negative K anomalies and trace element systematics indicating that during partial melting D_K>D_Ce. These features suggest equilibration with an oceanic lithospheric mantle containing K-rich hydrous mineral assemblages, consistent with the occurrence of amphibole + phlogopite in associated metasomatised lherzolite xenoliths, where orthopyroxene is partially replaced by newly formed olivine + (CO₂ + spinel + carbonate inclusion-rich) clinopyroxene. Metasomatism induced a decrease in $ a ^{{{\text{melt}}}}_{{{\text{SiO}}_{{\text{2}}} }} $ and Ti/Eu ratios, as well as an increase in fO ₂ , Ca/Sc and Sr/Sm in the Santiago magmas, suggesting a carbonatitic composition for the metasomatic agent. Santiago primitive lavas are highly enriched in incompatible elements and show a moderate range in isotopic compositions (⁸⁷Sr/⁸⁶Sr?=?0.70318–0.70391, ¹⁴³Nd/¹⁴⁴Nd?=?0.51261–0.51287, ¹⁷⁶Hf/¹⁷⁷Hf?=?0.28284–0.28297). Elemental and isotopic signatures suggest the involvement of HIMU and EM1-type mantle end-members, in agreement with the overall isotopic characteristics of the southern Cape Verde Islands. The overall geochemical characteristics of lavas from Santiago Island allow us to consider the EM1-like end-member as resulting from the involvement of subcontinental lithospheric mantle in the genesis of magmas on Santiago.