Arc-related Basalts Research Articles

Early Neoproterozoic magmatism in the Central Tianshan Block, China: Implications for its tectono-magmatic evolution and relationship to Rodinia

As an important ancient continental constituent of the Central Asian Orogenic Belt (CAOB), the Central Tianshan Block (CTB) is indispensable to improving our understanding of the early evolution and architectural framework of this orogenic belt. Here, we present new zircon U–Pb ages and Hf isotopes, as well as bulk-rock geochemistry, for amphibolite, gneissic granite, and augen granite from several localities in the CTB. Zircon U–Pb ages of ten samples demonstrate that the protolith of the amphibolite, gneissic granite, and augen granite formed in the early Neoproterozoic (from 905.9 ± 4.8 Ma to 940.0 ± 4.5 Ma). Geochemically, the early Neoproterozoic amphibolite is depleted in Nb, Ta, P, and Ti, but enriched in light REE (LREE) and large ion lithophile elements (LILE), similar to arc-related basalt. The gneissic granites from the Shardland area and the Alatage area, and gneissic granites and augen granites from the Xingxingxia area are enriched in SiO2, Na2O + K2O, LREE, and LILE, and depleted in heavy REE (HREE) and high field strength elements (HFSE), exhibiting an I-type granite affinity. Zircons from the amphibolite in the Shardland area exhibit positive εHf(t) values of +1.7 to +4.9 and TDM1 model ages of 1.27–1.39 Ga, indicating that the protolith crystallized from a mantle-derived magma. Zircons from the gneissic granites in the Shardland area, the Alatage area and the Xingxingxia area have εHf(t) values that range from –5.9 to +7.3 and TDM2 model ages of 1.33–2.17 Ga, indicating that the granites may have been derived from partial melting of ancient continental crust that contained compositionally variable juvenile material. These new results, combined with previous studies, indicate that early Neoproterozoic magmatic rocks are well-developed throughout the CTB and formed along a subduction-related continental arc. The CTB and other microcontinents of the southwestern CAOB may have been located proximal to the northern margin of the Tarim Craton during the early Neoproterozoic, thereby representing the active continental margin of the circum-Rodinia subduction–accretion system.

Meso- to Neoproterozoic terrane accretion: Insights from juvenile mafic magmatism from the Votuverava Group and Embu Complex, southern Ribeira Belt, Brazil

Terranes of the Neoproterozoic Ribeira Belt record processes related to West Gondwana assembly. However, the evolutionary history of the belt in the preceding Rodinia supercontinent cycle and its role in the transition to West Gondwana assembly is poorly known. New geochemical and geochronological data of mafic magmatism from Meso- to early Neoproterozoic rock units (1500–900 Ma) integrated with published data provides new constraints to unravel the tectonic setting of the Ribeira Belt. Geochemical and isotopic data from the Votuverava Group and Embu Complex from the southern Ribeira Belt record three distinct periods of juvenile magmatism. Mafic rocks from the Votuverava Group define the oldest magmatism as Calymmian (1490–1475 Ma) and Ectasian (1300–1260 Ma). Both age groups display Th/Yb ratios greater than N-MORB, low Ta/Yb and Nb/La ratios, low Nb and Zr concentrations, negative Nb anomaly, whole-rock ɛNd(T) from +2.25 to +4.01, an Nd two-stage TDM model age spanning 1629–1471 Ma, and zircon εHfT mainly between +5 and +10, characteristics of juvenile mantle source arc-related magmatism. The third event occurred during early Tonian (900 Ma) and is recorded in the mafic rocks from the Embu Complex. Geochemical data indicate that these mafic rocks belong to the tholeiitic series with low LREE enrichment, negative Nb and Ta anomalies, low Nb and Zr contents and low Nb/La and Zr/Y ratios, consistent with arc-related basalts. Whole-rock Sm–Nd isotopic data yielded ɛNd(T) of +0.72 and Nd two-stage TDM model age of 1921 Ma, with zircon Hf isotopes indicating juvenile mantle sources to the mafic magmatism with ɛHfT between +4.5 and +17.2. Thus, mafic rocks from the southern Ribeira Belt record multiple periods of accretion and convergence settings associated with assembly of Rodinia and Gondwana.

Oxidation of Archean upper mantle caused by crustal recycling

The redox evolution of Archean upper mantle impacted mantle melting and the nature of chemical equilibrium between mantle, ocean and atmosphere of the early Earth. Yet, the origin of these variations in redox remain controversial. Here we show that a global compilation of ∼3.8-2.5 Ga basalts can be subdivided into group B-1, showing modern mid-ocean ridge basalt-like features ((Nb/La)PM ≥ 0.75), and B-2, which are similar to contemporary island arc-related basalts ((Nb/La)PM < 0.75). Our V-Ti redox proxy indicates a more reducing upper mantle, and the results of both ambient and modified mantle obtained from B-1 and B-2 samples, respectively, exhibit a ∼1.0 log unit increase in their temporal evolution for most cratons. Increases in mantle oxygen fugacity are coincident with the changes in basalt Th/Nb ratios and Nd isotope ratios, indicating that crustal recycling played a crucial role, and this likely occurred either via plate subduction or lithospheric drips.

Three episodes of Precambrian mafic magmatism in the southern Central Tianshan Block (NW China): Insight into an evolving geodynamic model

Precambrian microcontinents are an essential component in the tectonic evolution and architecture of the Central Asian Orogenic Belt (CAOB). However, the early crustal evolution and origin of Precambrian microcontinents remain poorly constrained due to the scarcity of early Precambrian rocks. The Chinese Central Tianshan Block (CTB) constitutes a microcontinent in the western CAOB with Precambrian basement rocks along its southern margin. The basement rocks, comprising amphibolite-facies supracrustal assemblages of possible volcano-sedimentary origin, are exposed in the Wuwamen Complex, which occurs as tectonic slivers in the late Paleozoic Wuwamen ophiolitic mélange. Here we present in situ zircon U–Pb and Lu–Hf isotopic data and the whole-rock geochemistry of three suites of (meta-)mafic rocks from the Wuwamen Complex and an associated Neoproterozoic gabbro body. The zircon dating revealed that the Wuwamen Complex contains two suites of meta-mafic rocks with protolith formation at ca. 2.55 Ga and 2.21 Ga followed by an amphibolite-facies metamorphic overprint at ca. 1.83 Ga. In addition, an individual gabbro body adjacent to the Wuwamen Complex yielded an emplacement age of ca. 0.69 Ga, reflecting a mid-Neoproterozoic mafic magmatic event in the CTB. Furthermore, zircon U–Pb dating of a crosscutting diorite dyke and a gneissic diorite sourced from a composite batholith in the CTB yielded weighted mean intrusive ages of ca. 320 Ma and 454 Ma, respectively. In conjunction with the field observation, the emplacement ages suggest that the Wuwamen Complex represents a part of the continental arc basement of the CTB. The three suites of (meta-)mafic rocks show geochemical characteristics similar to typical arc-related basalts. Along with the evolved zircon Hf isotopic compositions, it is suggested that these rocks originated from partial melting of a metasomatized sub-continental lithospheric mantle during different events. In conjunction with previous studies, the here presented new data further suggest that the CTB and other microcontinents in the western CAOB record similar Precambrian magmatic and metamorphic episodes, which are comparable to those of the northern Tarim during Proterozoic times. The here outlined new model indicates that the microcontinents of the western CAOB and the Tarim Craton may have shared a common geodynamic evolution.

Discovery of Latest Cretaceous OIB-type alkaline gabbros in the Eastern Pontides Orogenic Belt, NE Turkey: Evidence for tectonic emplacement of seamounts

The Meso-Cenozoic geodynamic evolution of the Eastern Pontides Orogenic Belt, a mountain chain extending parallel to the southeastern margin of the Black Sea, has been controversial for the last forty years. Here we present data for a newly discovered alkaline gabbro body and its surrounding basaltic rocks in the northern part of the Eastern Pontides Orogenic Belt. We also provide a comprehensive assessment of the Late Mesozoic-Cenozoic geodynamic evolution of the Eastern Mediterranean region. The gabbroic body is bounded by reverse faults along its northern and southern borders and is surrounded by vesicular, pillow-fragment breccias and pillow basalts. Mineral compositions suggest that crystallization of the gabbros began at about 1170 °C, and the lowest preserved crystallization T is near 1000 °C. Estimated pressure at the beginning of crystallization is 5.7–7.4 kb. The 40Ar/39Ar dating of kaersutite and plagioclase and UPb dating of titanite indicated that the Hayrat gabbro crystallized at ~67 Ma (Late Maastrichtian). Whole rock major-trace-rare earth element and Sr-Nd-Pb isotope data indicate that the gabbros and basalts have different origins. The gabbros are alkaline and exhibit the geochemical features of OIB, whereas the basalts are tholeiitic and reveal depletions of HFSE that are similar to those of arc rocks. The gabbros are strongly fractionated, and derive from an enriched, lithospheric mantle source, with partial melting occurring in a garnet-stable environment. The basalts are less fractionated, and probably derive from a shallower source in which spinel peridotite was the predominant lithology. Considering all new and old geological, geochemical, geochronological and geophysical data from the Black Sea Basin and the Eastern Pontides-Lesser Caucasus-Alborz Orogenic Belt, we suggest that the alkaline Hayrat gabbro formed in an oceanic intraplate setting, and was accreted to the forearc region of the Eastern Pontides Orogenic Belt during southward subduction of Paleotethyan lithosphere. It was later tectonically juxtaposed with subaqueously erupted, arc-related basalts.

Mid-Neoproterozoic (ca. 830-800 Ma) metamorphicP-Tpaths link Tarim to the circum-Rodinia subduction-accretion system

Long-lived exterior accretionary orogeny shapes tectonothermal evolution of the peripheral building blocks of supercontinents and leads to considerable crustal growth. However, such accretionary orogeny has only been locally recognized for the Rodinia supercontinent. Here a suite of newly discovered mid-Neoproterozoic high-grade metamorphic rocks in the northern Tarim Craton, NW China, are used to test the exterior accretion hypothesis for Rodinia. These rocks occur as dark-colored mafic and calc-silicate boudins in impure marbles and mica schists. Geochemical data suggest a protolith of arc-related basalts metasomatized by Ca-rich fluids. Mineral assemblages, phase diagram modeling, and mineral compositions for a garnet pyroxenite and a garnet clinopyroxene gneiss reveal upper amphibolite to high-pressure granulite facies peak metamorphism (660–700°C, 11–12 kbar) following a counterclockwise P-T path, which is characterized by prograde burial and heating, followed by near-isothermal burial and retrograde exhumation and cooling. This P-T path is interpreted to have recorded crustal thickening of an earlier magmatic arc transformed to a fore arc by subduction erosion and subsequent burial along bent isotherms near the subduction channel. All studied samples record ca. 830–800 Ma metamorphic zircon U-Pb ages, which probably date the early exhumation and cooling according to Ti-in-zircon temperatures, zircon rare earth element patterns, and Hf isotopes. This is the first mid-Neoproterozoic P-T-t path in Tarim, and it provides metamorphic evidence for a mid-Neoproterozoic advancing-type accretionary orogeny, which is coeval with the initial breakup events of Rodinia and thus links Tarim to the circum-Rodinia accretion system, supporting the peripheral subduction model.

Late Mesozoic molybdenum mineralization on Hainan Island, South China: Geochemistry, geochronology and geodynamic setting

Hainan Island has become an important molybdenum (Mo) producer in East China, with the proven and measured metal Mo reserves of >0.29Mt and >0.69Mt, respectively. The known Mo-related deposits and occurrence which genetically might be attributed to porphyry- and hydrothermal vein types are hosted predominantly within the Cretaceous granitoids. These granitoids belong to high-K calc-alkaline, metaluminous to weakly peraluminous I-type series, and have geochemical affinities to arc-related basalts as defined by SiO2 (57.40–77.34wt.%) and K2O (2.45–6.28wt.%) contents, A/CNK ratios (mostly 0.86–1.10), depleted K, Ba, Nb, Sr, P and Ti, and enriched Rb, Th, Pb, Zr and La, and LREE-enriched REE patterns (La/YbN=7.1–66.3) with pronounced negative Eu anomalies to no Eu anomalies (Eu/Eu*=0.35–1.08). The highly variable geochemical compositions might be linked to magmatic processes. The LA-ICP-MS U–Pb dating on zircon have revealed three stages of magma emplacement for the Cretaceous Mo-hosting granitoids during ca. 113–108Ma, ca. 100–94Ma and ca. 90–70Ma, respectively. The initial 87Sr/86Sr ratios (from 0.70567 to 0.71208) and ɛNd(t) values (from −3.9 to −6.9) suggest that the host granitoids were sourced from an enriched subcontinental lithospheric mantle that had been metasomatized by fluids and/or melts released from ancient, subducted oceanic crust under an extension-induced setting. This setting is most favorable to variable degree of magma mixing between mantle-derived and crust-derived melts.With the second event as most important and the third event likely recording the youngest Mo mineralization in East China, the Re–Os dating of molybdenite from two types of Mo deposits on Hainan Island has identified three Mo mineralizing events occurring at ca. 112Ma, ca. 106–95Ma and ca. 89–72Ma. They are consistent with the episodes of the Cretaceous magmatism. The Re concentrations of molybdenites and δ34S values of sulfides (1.36–5.73‰ and average 4.3‰) commonly indicate that the metal Mo is of hybrid origin between mantle and crust but with variable degree of hybridization. Given that the subduction of the Paleo-Pacific plate beneath the eastern Asian margin dominated the Late Mesozoic tectonism of South China, and that Hainan Island has been a part of the Cathaysia Block since Late Permian, the widespread Cretaceous extension and associated Mo mineralizing event(s) on Hainan Island might be a response to episodic rollback of the subducted slab, which led to the underplating of mantle-derived basaltic melts and associated deep crustal melting. Moreover, this tectono-magmatic scenario likely resulted in the migratory pluses of Mo metallogenesis and the change of metal Mo sources from northeast to southwest Hainan Island.

Discrimination of protolithic versus metamorphic zircon ages in eclogites: Constraints from the Erzgebirge metamorphic core complex (Germany)

Protolithic ages of meta-basites are crucial for tectonic reconstructions as they constrain the lifetime of oceans. However, in high-grade metamorphic regions most isotope systems record metamorphic ages due to the metamorphic overprint. The mineral zircon is the most suitable geochronometer for the determination of protolithic ages in such rocks due to its high closure temperature. Zircon from eclogites from the Eastern Erzgebirge metamorphic complex yielded both protolithic (~540Ma; 5 samples) and metamorphic (~340Ma; 4 samples) 207Pb/206Pb ages. Detailed investigations of zircon grains within thin rock sections show that the metamorphic ages are related to new zircon growth caused by water infiltration that triggered decomposition of omphacite into an amphibole–plagioclase symplectite. Rutile and apatite frequently grew together with metamorphic zircons indicating local mobilization of Zr, Nb, LREE and P. Eclogites with omphacite breakdown are very dark in colour. In difference, lighter coloured eclogites contain K-feldspar and quartz indicating minor melt infiltration that caused breakdown of garnet and transformation of amphibole into biotite. This supports additional new growth of zircon grains. In some samples idiomorphic zircon grains with a lot of micro-inclusions indicate a coupled dissolution–reprecipitation process. Additional rounded-resorbed and highly fractured zircon grains together with internal zircon textures imply multiple melt infiltration triggering multiple phases of zircon growth. Therefore, scattering zircon ages may reflect multiple growth stages or inheritance rather than prolonged periods of metamorphic growth. In contrast, protolithic zircon ages were found in eclogites where no water or melt infiltration occurred. This emphasizes the importance of water and/or melt infiltration for the new growth of zircon grains in eclogites as was already shown for granulites in various studies. In the Eastern Erzgebirge, most eclogites and amphibolites were derived from MORB-type or arc-related basalts while the plagioclase-rich amphibolites formed mainly from crustal sources. The protolith ages of meta-basites from the Eastern Erzgebirge (~540Ma) document a bimodal magmatic activity in a Cadomian back-arc basin.

Platinum-alloy and sulfur saturation in an arc-related basalt to rhyolite suite: Evidence from the Pual Ridge lavas, the Eastern Manus Basin

We have measured the platinum group element (PGE) and Re concentrations of arc-type lavas from the Pual Ridge and the surrounding area in the Eastern Manus Basin. These magmas followed an Fe-enrichment trend to produce a wide range of compositions with MgO varying between 8 and 0.1wt.%. We found distinct differences in the PGE geochemistry of the high (>3wt.% MgO) and the low-Mg lavas (<3wt.% MgO). During the early stages of fractionation, the concentrations of Pd, Cu, Au and Re increase whereas Ir, Ru, Rh and Pt decrease. Co-variations of Ir, Ru and Rh with Pt in the high-Mg lavas suggest that the depletion of these elements is due to Pt-rich alloy saturation. This is consistent with the high Pt contents in the high-Mg lavas, which is close to the solubility of Pt in the basaltic melt at similar conditions. In contrast, the concentrations of all PGE and Re drop rapidly in the low-Mg lavas (except for Ru and Ir), with the PGE concentrations falling at a rate that is appreciably faster than Cu, which we attribute to sulfide saturation. As a consequence, there is a marked decline in Pd/Cu in the low-Mg lavas and we suggest that this ratio is the best indicator of sulfide saturation in an evolving magmatic system.A feature of the data is that duplicate analyses of the same sample often do not agree within error. We attribute this scatter to the nugget effect, with nuggets of a Pt-rich alloy in the high-Mg lavas and sulfide blebs in the low-Mg lavas. The PGE concentrations of phenocryst-bearing high-Mg lavas are higher than in the associated glassy lavas, and scatter on MgO variation diagrams is significantly reduced if only glassy lavas are considered, which indicates that the micron scale Pt-rich alloy grains are intimately associated with the phenocrysts. Our results provide strong evidence that Pt-rich alloys can crystallize from a basaltic-andesitic magma, along with the silicate minerals, and fractionate Pt from Pd during magmatic differentiation. As a consequence, Pd/Pt increases during Pt alloy fractionation and this ratio can be used to identify Pt-rich metal saturation. The Pual Ridge alloys are Pt-rich because the primary magmas are Pt-enriched and Ir-depleted, which is typical of arc-related magmas.

Role of crustal melting in petrogenesis of the Cretaceous Water Island Formation (Virgin Islands, northeast Antilles Island arc)

The latest Aptian to earliest Albian (~115 Ma) Water Island Fm in the Virgin Islands contains some of the oldest known arc-related strata in the Greater Antilles Island Arc. Hence, the unit is of considerable significance in tectonic reconstructions of initial subduction parameters along the long-lived destructive plate margin separating the North American and Caribbean Plates. Exposed Water Island strata are bimodal, consisting predominantly of altered dacite and rhyolite (originally called keratophyre; 65-85% SiO2) and subordinate degraded (spilite; 46-57% SiO2). TiO2 content of Water Island basalt averages approximately 0.5%, resembling borderline intermediate-Ti boninite basalts, consistent with low incompatible element abundances and low normalized light rare earth elements (LREE) with respect to Sm. Trace element patterns of the felsic suite, characterized by pronounced negative normalized anomalies for high field-strength elements (HFSE), low Sr/Y, and low absolute rare earth element (REE) abundances, and relatively flat normalized REE patterns, have analogues in plagiorhyolite suites from bimodal Cenozoic arcs, including the western Aleutians, Izu-Bonin, the Kermadecs, and South Sandwich. Relatively low incompatible element concentrations in plagiorhyolites and contrasting normalized incompatible trace element patterns in basalts preclude an origin of Water Island plagiorhyolite through MORB-type fractional crystallization. Compositions are consistent instead with melting models involving partial fusion of amphibole-bearing gabbro at low pressures (within the stability range of plagioclase) in response to introduction of heat and aqueous flux by arc-related basalt melts and associated hydrothermal fluids during transmission to the surface. Truncation of the basalt fractional crystallization trend at SiO2 = 57% indicates evolved island arc basalt (IAB) crystal fractionates were gradually displaced from crustal magma conduits by more buoyant plagiorhyolite melt, and trapped in underplated, sub-crustal magma chambers. Basalts have low (Ce/Ce*)N (average ˜ 0.78), indicating the presence of significant pelagic sediment (0.5 to 1.5% Atlantic Cretaceous pelagic sediment, AKPS). One subunit of relatively high-HFSE plagiorhyolite has (Ce/Ce*)N near-expected values, but another with low-HFSE has slightly lower than expected (Ce/Ce*)N, consistent with a small sediment component. Absence of intermediate andesite from the Water Island Fm is inconsistent, however, with basaltrhyolite magma mixing processes. Consequently, incorporation of sediment by low-HFSE plagiorhyolite is inferred to have resulted from re-melting of arc-related gabbro.

Evidence for subduction at 3.8 Ga: Geochemistry of arc-like metabasalts from the southern edge of the Isua Supracrustal Belt

We present new major and trace element data for rare examples of preserved pillow basalts from a locality of little studied > 3800 Ma rocks at the southwestern edge of the Isua Supracrustal Belt (ISB), West Greenland. The 20 samples have 47.3–59.0 wt.% SiO 2, 4.9–12.2 wt.% MgO, 6.5–11.2 wt.% CaO, 60–340 ppm Ni and 63–1094 ppm Cr, consistent with pre-metamorphic basaltic compositions. Nb/La (n) and Ti/Gd (n) are lower than primitive mantle compositions (0.1–0.6 and 0.3–0.8, respectively), with these ratios interpreted to reflect the composition of their source. Correlations of Nb/La with La/Sm, Ti/Gd and Zr/Sm and between Zr/Nb and Nb/Th are comparable to those of modern subduction related magmas, whereby slab melts enriched the overlying mantle wedge and HFSE were retained by Ti-rich phases. Trends in Ba/La vs. Ba/Nb are comparable to modern day arc related basalts, which reflect a sediment influence and may indicate that, at least early in the evolution of Eoarchean subduction zones, the subducted Eoarchean oceanic crust retained a pelagic sediment cover, with this cover influencing the chemistry of the slab-derived fluid. Three of the ISB pillow lavas show extreme enrichments in LREE (La/Sm ( n) = 1.9–2.6), large depletions in Nb, Ta, Zr and Hf relative to REE that are similar to rare high-K arc basalts from the Clark volcano of the presently active Tongan–Kermadec–New Zealand arc (Gamble, J.A., Christie, R.H.K., Wright, I.C., Wysoczanski, R.J., 1997. Primitive K-rich magmas from Clark volcano, southern Kermadec arc: a paradox in the K–depth relationship. The Canadian Mineralogist, 35, 275–290). The compositions of these highly enriched basalts indicate that the mantle source region of the Isua basalts was occasionally overprinted by small-volume sediment dominated melts rather than hydrous fluids. The compositional affinities of these 3.8 Ga pillow basalts with modern island-arc basalts provide strong evidence for the role of slab fluids and melts in basalt genesis in the Eoarchean, further demonstrating arc-like signatures in the oldest rock sequences. The compositions of Archean mafic rocks including samples from the ISB that are interpreted to have a petrogenesis similar to that of modern island-arc basalts, are compared with Archean ‘non-arc’ rocks (i.e. plume-related komatiites, komatiitic basalts and associated tholeiitic basalts). The two groups are distinguished by differences in La/Sm, Gd/Yb, Nb/La, Ti/Gd, Ba/La, Ba/Nb and Al 2O 3/TiO 2. Thus clear compositional distinctions between different basaltic types, attributed to arc and non-arc origins, are a feature of the rock record for at least the last 3.8 Ga.

Eclogitic and Ultrahigh-Pressure Crustal Garnets and Their Relationship to Phanerozoic Subduction Diamonds, Bingara Area, New England Fold Belt, Eastern Australia

At Bingara-Copeton in the Phanerozoic New England fold belt, New South Wales, Australia, about two million diamonds were mined from Tertiary alluvial deposits that are more than 1,500 km distant from the nearest craton. The diamonds contain unique eclogitic inclusions, some of which gave Phanerozoic age dates and are unlike diamonds from ancient cratons. Few heavy minerals accompany the diamonds. An exploration program of the modern drainage system and soils in the Bingara district was undertaken to search for a hard-rock diamond source. Eight prospects were identified by regional magnetic anomalies, and 118 samples were processed for heavy minerals. Five diamonds were recovered; two rounded macrodiamonds and three small diamonds that retain some delicate features. More importantly, abundant high pressure- (HP) and ultrahigh-pressure (UHP) metamorphic minerals also were recovered, particularly garnet. These were not derived from the low metamorphic grade country rock in the district. Using the major element classification scheme of Schulze (2003) we separate Bingara garnets into “crustal” and “mantle-derived” groups and the following subgroups: eclogite (Group I diamond and Group II lower grade), peridotite (lherzolite), and Cr-poor megacrysts. Major element chemistry and rare earth element analysis of selected garnets identifies their deeply subducted protoliths, including midoceanic ridge basalt (MORB) and picrite, and arc-related basalt. We also define here a new subgroup of distinctive ultrahigh-pressure crustal (UHP-crustal) garnet from the crustal group above with almandine-spessartine compositions and enrichment in Na2O and heavy rare earth elements. These compare with garnets from exhumed UHP schist and/or orthogneiss from Dabie Shan, central China, which formed from deeply subducted leucocratic continental crustal material, and also with rare garnet inclusions in diamonds from Sloan diatremes in Colorado and Wyoming and the Finsch kimberlite pipe, South Africa. Decompression microstructures, such as crystallographic exsolutions of rutile, apatite, or ilmenite in some eclogitic and Fe-Mn–enriched Bingara garnets, confirm their partial exhumation from mantle depth. Garnet types and proportions vary significantly by prospect in the Bingara area, reflecting variations in assemblages sampled at depth by several local igneous sources. Garnets from Group I eclogite and one rounded (resorbed) white diamond (0.265 carat) were recovered near a composite basanite body dated at 181.5 ± 0.4 Ma. UHP-crustal garnets were recovered near the three small diamonds. A conceptual model is presented for subduction formation of Bingara-Copeton diamonds, eclogitic, and UHP-crustal garnets in Carboniferous and Triassic slabs, and their two-stage delivery to the surface, first by partial exhumation, then by capture in local shallow-sourced basaltic magmas without deep-sourced kimberlitic or lamproitic volcanism. This two-stage delivery mechanism explains how diamonds may be sourced within buried UHP terranes, whereas in exposed UHP terranes macrocrystals of diamond are normally extremely rare, variously abundant but completely graphitized, or apparently never formed. This work has calibrated an exploration technique for locating sources of unusual subduction-formed diamonds in collisional tectonic settings using eclogitic and UHP-crustal garnet as an indicator mineral.

Magmatism in the Iberian Pyrite Belt: petrological constraints on a metallogenic model

Igneous formations associated with massive sulphide deposits in the Iberian Pyrite Belt (IPB) are essentially composed of basic lavas and dolerites, and dacitic to rhyolitic volcanites; intermediate lavas are subordinate. The basic rocks show variable geochemical characteristics: lavas and dolerites comparable to recent within-plate alkaline basalts seem restricted to the western and southern parts of the IPB, whereas basic rocks comparable to continental tholeiites or arc-related basalts occur across the whole belt. The felsic rocks are classified as calc-alkaline and belong to the “low-Al2O3 and high-Yb type”. At given SiO2, Al2O3 and TiO2 contents, they show variable Zr, Nb, and HREE contents. Heavy-rare-earth element fractionation decreases from the dacites to the rhyolites ([Gd/Yb]N ∼ 1), whereas the negative Eu-anomaly becomes more pronounced. The characteristics of the rhyolites are typical of sulphide-fertile volcanic packages. Trace-element modelling suggests that the felsic rocks evolved from a dacitic parent magma through fractional crystallization of hornblende and plagioclase. Partial melting of an amphibolite protolith, which appears as the most probable model for the origin of this dacitic magma, requires a high T/P gradient in the crust. The occurrence of alkaline basalts and continental tholeiites is consistent with formation of the IPB in a tensional tectonic setting. However, the associated island-arc tholeiites suggest a location in a domain of plate convergence. Emplacement in a fore-arc basin over a recently accreted crustal segment is envisaged as a possible hypothesis to account for the geological and petrological constraints. A high geothermal gradient and eruption in a submarine tensional basin could have been two key ingredients for the development of massive sulphide deposits within the IPB.

Arc-rift transition volcanism in the Puertecitos Volcanic Province, northeastern Baja California, Mexico

The Neogene Puertecitos Volcanic Province of northeastern Baja California records a transition from arc-related volcanic activity to rift volcanism associated with opening of the Gulf of California. The eastern Puertecitos Volcanic Province is divided into three volcanic sequences based on mapping, petrology, and ^(40)Ar/^(39)Ar geochronology. The lowest sequence comprises early to middle Miocene (20–16 Ma) arc-related andesitic lava flows, volcanic necks, and proximal pyroclastic and epiclastic deposits up to 400 m in thickness, with minor basaltic lava flows. Following the initiation of crustal extension in the region (11–6 Ma), synrift volcanism produced two rhyolitic sequences that discordantly overlie the arc-related rocks. The older synrift sequence (6.4–5.8 Ma) is composed of rhyolite domes and a series of pyroclastic flows up to 300 m thick. The upper sequence (3.2–2.7 Ma) consists of ash-flow tuffs and pumice-lapilli pyroclastic flows, collectively up to 200 m thick. Minor andesite eruptions followed each episode of silicic synrift volcanism. Synvolcanic faults produced topographic relief that controlled deposition of the pyroclastic flows and caused gentler dips upsection. Rhyolite domes are aligned parallel to the predominant north-northwest to north-northeast fault pattern. All three volcanic sequences are calc-alkaline. However, the synrift andesite is characterized by lower K_(2)O, lower incompatible element concentrations, and less fractionation of light rare earth elements than the arc-related basalt and andesite. This suggests that the primary melts were more primitive for synrift andesite than for the arc-related rocks.