Felsic Suite Research Articles

Re-melting of the Neoproterozoic juvenile mafic lower crust: The origin of adakitic and non-adakitic felsic suites in the southwest margin of Yangtze Craton

Eocene-Oligocene adakitic and non-adakitic shoshonitic felsic intrusions from the southwest margin of the Yangtze Craton provide a crucial geological record for studying crust-mantle interactions at the craton boundary. Despite their significance, the petrogenesis and source properties of these intrusions remain debated. This study suggests that the adakitic and non-adakitic features of these intrusions result from the re-melting of Neoproterozoic juvenile lower crust, as evidenced by zircon U-Pb dating, geochemical analyses, and petrogenetic investigations. The Ninglang-Yongsheng shoshonitic felsic intrusions, characterized by high Mg#, MgO, Ni, and Cr contents, exhibit isotopic characteristics akin to mantle-derived magma and juvenile mafic lower crust. Their U-Pb ages span 32–36 Ma, and they show similar patterns in major and trace elements, rare earth elements (REEs), and isotopes, with enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs) like Nb, Ta, and Ti. These intrusions also demonstrate significant fractionation between light and heavy REEs, with a modest negative Eu anomaly. Their (87Sr/86Sr)i and εNd(t) values range from 0.7060 to 0.7069 and − 4.4 to −1.5, respectively, while Pb isotopic compositions vary slightly.This paper, along with prior research, has developed a genetic model for the Ninglang-Yongsheng shoshonitic felsic intrusions. The adakitic quartz monzonite porphyries likely originated from high-degree partial melting of the juvenile lower crust, whereas the non-adakitic shoshonitic syenite porphyries resulted from lower-degree melting. The composition of the juvenile lower crust may be garnet amphibolite-pyroxenite. This suggests distinct compositional variations within the Ninglang-Yongsheng juvenile lower crust. Moreover, the region's shoshonitic felsic intrusions have assimilated mantle-derived magma. In conclusion, the juvenile lower crust on the Yangtze Craton's southwest margin displays zonal characteristics, with the Liuhe-Beiya area's lower crust showing higher potassium content and more enriched isotopic signatures compared to the Ninglang-Yongsheng region.

Formation of Paleo- to Meso-Archean continental crust in the western Dharwar Craton, India: Constraints from U[sbnd]Pb zircon ages and Hf-Pb-Sr isotopes of granitoids and sedimentary rocks

The combination of UPb zircon ages with Hf-Sr-Pb isotopes of different intrusive and extrusive felsic and sedimentary rocks provides constraints on the petrogenetic evolution of the continental crust in the western Dharwar Craton, India. The oldest detrital zircon preserved formed at ∼3.6 Ga and represents a relic of the oldest felsic crustal material in the region. The dominant granitoid units of the western Dharwar Craton contain zircon grains with magmatic ages between 3.4 Ga and 3.0 Ga that indicate the formation of major felsic continental crust during this interval.Trace element abundances of the granitoids indicate that the oldest members of the intermediate to felsic suite derived by partial melting of mafic material at ∼3.6–3.4 Ga. The initial bulk rock Hf isotope compositions of these granitoids are consistent with their formation by melting of even older mafic material that was slightly enriched relative to the depleted mantle composition. This mafic and slightly enriched material formed by mantle melting at ∼ < 3.8 Ga. The Hf isotope compositions of individual zircon grains, obtained by two different analytical techniques (in-situ and complete dissolution followed by chromatographic separation) give evidence for the presence of such older mafic material (<3.8 Ga) that formed the immediate precursor of their granitoid host rocks. Such a mafic source for the granitoids is consistent with PbSr isotope systematics of these that shows no indication of Eoarchean enriched/evolved material in the western Dharwar Craton. The mafic source material of the granitoids thus represents an intermediate stage of crust formation that started after 3.8 Ga with the formation of mafic crust by mantle melting. The combined geochronological and isotopic constraints suggest that the Mesoarchean felsic crust of the Dharwar Craton formed by differentiation of melts derived from an amphibolite/eclogite source rock and included increasing contributions of reprocessed crustal material with time from ∼3.6 to 3.0 Ga. The major interval of growth of felsic continental crust was from 3.4 to 3.0 Ga. The younger generation of granitoids formed mostly by reworking of older intermediate to felsic crust. These different felsic magmatic bodies with distinct petrogeneses and sources, that include the depleted mantle, older mafic crust and the evolved continental crust, became essential elements of the stable continental crust of the western Dharwar Craton, the majority of which was generated from 3.4 to 3.0 Ga.

Evolved magmatic arcs of South Borneo: Insights into Cretaceous slab subduction

Combined U-Pb zircon data of granitic, dioritic, and gabbroic suites reveal development of arc-related magmatism in South Borneo throughout the Late Mesozoic but dominantly in the Cretaceous (144–72 Ma). Cretaceous igneous suites constitute the Schwaner-Singkawang, Kuching-LongPahangai, and Meratus-West Java magmatic arc zones. Arc-related felsic suites are low-K tholeiitic, and calc-alkaline to high-K calc-alkaline. Most of them are metaluminous (A/CNK 0.7–1.1), I-type, and have high large-ion lithophile element (LILE) contents, low high-field strength element (HFSE) contents, and low crystallization temperatures (688–769 °C). A few rocks have adakite characteristics, such as high Sr/Y and low Y. Late Cretaceous granitic suites from South Schwaner, which evolved at elevated temperature (761–852 °C), have diverse compositions that range from I-type transitional to A-type. The highly fractionated chemistry of these samples is attributed to an intraplate stretching tectonic setting. This regime is consistent with tectonic controls of the synchronous tholeiitic suites within North Schwaner. Two subduction systems beneath Sundaland converged and interacted during the Cretaceous. The Paleo-Pacific subduction in the north built arc-trench architectures, as shown by the Early Cretaceous Lupar Line mélange, the Schwaner to Singkawang magmatic arc, and the Late Cretaceous Lupar Line mélange and Kuching-LongPahangai arc. The Meso-Tethys subduction in the southeast produced the Cretaceous Schwaner-Singkawang magmatic arc, Meratus to Luk-Ulo mélanges, and Meratus-West Java magmatic arc. When entered into Late Cretaceous, slab rollback led to crustal stretching in the Schwaner to Singkawang regions, with the production of A-type transitional and tholeiitic suites. Though debatable, we infer the SW Borneo block to be a Sundaland continuation; Semitau shows an affinity for South Cathaysia. Both of the terranes would accrete onto Sundaland during the Indosinian orogeny.

Petrographic evidences of open-system magmatic processes in the felsic rocks of the northern part of the Ditrău Alkaline Massif (Eastern Carpathians, Romania)

Abstract Over the almost 190 years-long research of the Ditrău Alkaline Massif (Eastern Carpathians, Romania), felsic rocks have been regarded as homogeneous, uniform units of the igneous complex. Nevertheless, our detailed textural study revealed that the felsic suite (diorite–alkaline feldspar syenite and nepheline-bearing syenite–granite series) cropping out north of the Jolotca Creek valley is more heterogenous at micro-scale than previously thought. This heterogeneity partly derives from abundant mafic mineral-rich clusters; nevertheless, felsic minerals also exhibit various, remarkable textural features. Outcrop to micro-scale traits of felsic crystal settling, mafic mineral aggregates and flow fabrics along with metamorphic country rock xenoliths suggest that the studied rocks crystallized under dynamic magmatic conditions. Cumulate formation, shear flow, convection currents as well as various open-system magmatic processes (e.g., magma recharge, magma mixing and mingling, crystal or mush transfer and recycling, country rock assimilation) played a significant role in the petrogenesis of the examined felsic suite. Based on field observations as well as on the microtextural relationship of the minerals, two major groups of felsic rocks were distinguished: (1) felsic rocks (lacking or containing sparse mafic minerals) spatially associated with mafic rocks and (2) felsic rocks (with mafic minerals and clots) spatially unassociated with mafic rocks. Rocks of the former group are dominated by plagioclase, accompanied by minor alkaline feldspar, biotite and accessory titanite. Distinct structural and textural features suggest the physical accumulation of the rock-forming phases. Such textural properties can also be observed in some rocks of the second group. Isolated mafic minerals are rather scarce in the latter; nevertheless, different types of aggregates made up of either identical or various mafic phases are more common. Clustered minerals are either intact or show different stages of alteration. A detailed petrographic study of the above-mentioned peculiarities has been implemented in order to define their potential origin(s) and petrogenetic significance.

Magmatism in the Siang window of the Eastern Himalayan Syntaxis, NE India: a vestige of Kerguelen mantle plume activity

Abstract We report new U–Pb zircon ages for mafic plutonic (gabbro) and volcanic (andesite) rocks, along with the whole-rock chemistry of a mafic–felsic suite of volcanic rocks from the Siang window of the Eastern Himalayan Syntaxis, NE India. Field relationships, and mineralogical and geochemical characteristics, of the studied mafic–intermediate–felsic rocks suggest their co-magmatic linkage that was generated in an extensional tectonic environment. Incompatible trace elements and low concentrations of large ion lithophile elements (LILEs) and REE behaviour reflect both the enriched nature of the mafic rocks and the limited influence of crustal contamination in their genesis. Partial melting and fractional crystallization processes have played a major role during the genesis of these felsic volcanics from the parental mafic magma. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages suggest that the mafic plutonic rock was emplaced at c. 121.18 ± 1 Ma and intermediate volcanic rock was emplaced at c. 135.48 ± 0.50 Ma during the Early Cretaceous period. The new ages are consistent with earlier reported zircon U–Pb ages (133.0 ± 1.9–130.7 ± 1.8 Ma) of felsic volcanic rocks from the present study area. Our new field observations, and mineralogical and geochemical characteristics, in conjunction with the U–Pb isotopic database suggest that the major magmatic event in the core of the Siang window of the Eastern Himalaya is coeval with the Rajmahal–Sylhet–Mikir–Shillong flood basalts of eastern and northeastern India, and the Comei–Bunbury Large Igneous Province of southeastern Tibet and SW Australia. These events are related to the break-up of eastern Gondwana and outbreak of the Kerguelen plume.

Petrogenesis and tectonic implications of the Eocene-Oligocene potassic felsic suites in western Yunnan, eastern Tibetan Plateau: Evidence from petrology, zircon chronology, elemental and Sr-Nd-Pb-Hf isotopic geochemistry

In western Yunnan, potassic felsic suites with important Cu mineralization have been interpreted as being genetically associated with the Ailao Shan-Red River (ASRR) shear zone. In this paper, we show that the magmatism is unrelated to the shear zone, but shares many characteristics in common with potassic suites along the Jinsha suture to the north and northwest on the northern Tibetan Plateau, representing a magmatic response to the continued India-Asia convergence since the collision in the early Eocene. These felsic suites include syenite, monzogranite porphyries and granite porphyries. We dated 15 representative samples using zircon U-Pb method that gives a tight emplacement age of ~37–35 Ma. On 45 representative samples, we have done bulk-rock major element, trace element and Sr-Nd-Pb isotope analysis. We use these data, together with zircon Hf isotope data and the literature data on potassic mafic suites in the region to discuss the petrogenesis of these rocks. We conclude that the petrogenesis of the felsic potassic suites is most consistent with a scenario of deep (≥ 40 km) melting of thickened crust. The crustal melting is caused by intrusion and underplating of potassic mafic melts derived from melting of metasomatized mantle lithosphere. The mafic potassic suites in the region result from such mantle melts. Mixing of the mafic melts with the felsic melts explains the syenite and monzogranite porphyries and their extended fractional crystallization gives rise to the granite porphyries. The potassic suites along the Jinsha suture, including those from northern Tibet, western Yunnan and Vietnam, share the same tectonic setting during the Eocene-Oligocene. In response to the continued India-Asia convergence, removal of the lower continental lithospheric mantle triggered by Rayleigh-Taylor convective instability induces asthenosphere upwelling, partial melting of the metasomatized mantle lithosphere. We hypothesize that the eastward migration of the potassic magmatism along the Jinsha suture in the period of Eocene-Oligocene results from eastward thickening and removing of the continental lithospheric mantle along the suture, which, in time and space, correlates with the dischronal subduction of the torn Indian continental slab at ca. 52 Ma along the India-Asia collision zone.

Early Cretaceous gabbro–granite complex from central Inner Mongolia: Insights into initial rifting and crust–mantle interaction in the northern China–Mongolia basin–range tract

Coeval high-K calc-alkaline to shoshonitic bimodal igneous suites hold a key to monitoring important geodynamic processes and crust–mantle interactions within post-collisional to within-plate extensional settings. Here we report a unique early Cretaceous gabbro–granite complex from the poorly exposed Erenhot basin region of central Inner Mongolia, eastern Central Asian Orogenic Belt (CAOB). The mafic suite includes small syenogabbro to monzodiorite stocks, while the felsic suite mainly comprises syenogranites with minor mafic to intermediate microgranular enclaves. The former exhibits a transitional character from alkali-calcic to alkalic, notable enrichment in large ion lithophile elements (LILEs) and weak to moderate depletion in high field strength elements (HFSEs), and possesses low initial 87Sr/86Sr ratios (0.70486–0.70537), positive εNd(t) (+2.1 − +2.6) and zircon εHf(t) (+2.3 − +9.0) values. These features suggest its derivation from low-degree melting of amphibole-bearing lherzolites in the lithospheric mantle that had been metasomatized by subducting slab-derived fluids. The host granites are magnesian, alkali-calcic and peraluminous. They show typical trace elemental features of arc-related felsic melts, with initial 87Sr/86Sr ratios of 0.70499 to 0.70593, εNd(t) of −0.1 to +1.4, zircon εHf(t) of +4.8 to +8.3 and δ18O values from +4.53 to +6.60‰. These elemental and isotopic traits are consistent with partial melts of a mixed protolith composed of newly-underplated basaltic materials and extant older lower crust. The monzonitic enclaves are alkalic, highly enriched in LILEs and depleted in HFSEs, with variable whole-rock SrNd (ISr(t) = 0.70216–0.704678, εNd(t) = +0.2 − +1.8) and zircon Hf (εHf(t) = +0.9 − +7.1) isotopic values. These transitional compositions are compatible with hybridization processes between mantle-derived mafic magma and crust-derived felsic magma. Given its temporal coincidence with initial rifting in the regional basin formation and the closure of Mongol–Okhotsk ocean, Early Cretaceous Erenhot gabbro–granite complex could not only serve as spatial markers for relating magmatic underplating with basin formation, but also catch a unique snapshot into crust–mantle interactions within a post-orogenic extensional regime probably underpinned by lithospheric dripping and gravitational collapse after the Mongol–Okhotsk suturing.

The Pan-African post-collision Hosséré Mana plutonic complex and associated Gapi Stock (Western Cameroon Domain, Cameroon): Petrology, mineralogy and geochemistry

In the Cameroon Pan-African orogenic belt, the West Cameroon Domain (WCD) is characterized by the apparent lack of Archean-Paleoproterozoic relicts, the development of Neoproterozoic basins and abundant plutonic massifs. The Hosséré Mana plutonic complex (HMPC) and the Gapi gabbroic stock are located in the vicinity of the Cretaceous Ngaou Boh granite complex and the Neogene Tchabal Gangbada basanite-trachyte-rhyolite volcanic plateau. They comprise clinopyroxene + amphibole ± olivine leucogabbro, clinopyroxene + amphibole monzogabbro, biotite + amphibole ± clinopyroxene monzodiorite, porphyritic and foliated biotite ± muscovite monzogranite. Two parallel trends of Mg#, displayed by leucogabbro (0.66–0.61) - monzogabbro (0.57–0.37), and by monzodiorite (0.66–0.55) -monzogranite (0.47–0.33), evidence two contrasting igneous suites, magnesian tholeiitic and magnesian high-K alkali-calcic to calc-alkaline (I-type). Primitive mantle-normalized patterns of the tholeiitic suite are fractionated, with no significant HFSE anomalies, suggesting a garnet-bearing enriched peridotite source. Primitive mantle-normalized patterns of the high-K alkali-calcic to calc-alkaline suite include more fractionated felsic types and display NbTa negative anomalies, suggesting another source with amphibole - garnet in the residue and possible origin of felsic rocks by partial melting of mafic lower crust with garnet and titanite-rutile in the residue. In leucogabbro, fairly low Sri (0.70538) and little negative εNd(750 Ma) (+0.2), with a TDM single-stage Nd model age of 1.42 Ga, are consistent with magmas extracted near the lithosphere – asthenosphere boundary from an enriched mantle source close to BSE (Bulk Silicate Earth). In monzodiorite, little higher Sri (0.70628) and strongly negative εNd(560 Ma) (−8.8), with a TDM single-stage Nd model age of 1.64 Ga, point to a supplementary (lithospheric?) mantle source metasomatized by subduction products. HMPC and GS massifs, emplaced in a syn-to post-kinematic environment after the major collision stage, illustrate coeval magnesian tholeiitic and magnesian high-K alkali-calcic calc-alkaline (I-type) igneous suites, in which primary magmas were tapped from different mantle sources, followed by younger felsic suite issued from garnet-bearing lower crustal source.

Geology, alteration, and lithogeochemistry of the Hood volcanogenic massive sulfide (VMS) deposits, Nunavut, Canada

The Hood volcanogenic massive sulfide (VMS) deposits are hosted by the ~2.68 Ga Amooga Booga volcanic belt (ABVB) in the northwestern Archaen Slave Craton and consist of three deposits (Hood 10, 41, and 41A) and three occurrences (46, 461, and 462). The mineralized zones consist of massive to semi-massive pyrrhotite, pyrite, chalcopyrite, sphalerite, and galena hosted predominantly by felsic volcanic flows within the predominantly mafic ABVB. The mineralized lenses occur at different stratigraphic levels and have textural, alteration, and stratigraphic features consistent with formation via subseafloor replacement. The felsic volcanic rocks in the Hood deposits can be subdivided into groups based on immobile trace element geochemistry. The main felsic types (A and B) are petrographically indistinguishable. Type A has higher high field strength element (HSFE) and rare earth element (REE) contents than type B, suggesting a higher temperature of formation. Type A rocks also have higher Nb/Ta values indicative of a greater mantle input in their genesis compared to type B rocks. Mineralization is more closely associated with type A than type B rocks. The two mafic volcanic rock types previously identified in the ABVB, type I and type II, both occur within the Hood deposits. The type II mafic group is interpreted to be the result of variable crustal contamination of type I magma. The volcanic rocks of the ABVB are interpreted to have formed in a continental margin arc/back-arc setting. The genesis of these magmatic suites involved magmatic underplating and emplacement through pre-existing sialic basement that resulted in crustal melting, mantle-crust mixing, and contamination leading to the aforementioned geochemical features in both mafic and felsic suites. This type of extensional tectonic environment was likely associated with high heat flow and is similar to global VMS environments proximal to extending continental margins (e.g., Sturgeon Lake, Bathurst, and Finlayson Lake). The similarities of the ABVB to other Slave Craton greenstone belts further highlights the overall potential for greenstone-hosted VMS mineralization in the Slave Craton. Chlorite-sericite (+/− quartz) is the dominant hydrothermal alteration assemblage in the Hood deposits and is typical of VMS-style mineralization. Mass change calculations illustrate that elemental changes are typical of VMS environments with gains in Fe2O3, MgO, and base metals associated with chlorite alteration near mineralized zones; K2O gains associated with sericite alteration; and losses of Na2O in both alteration types.

Early to late Neoproterozoic magmatism and magma mixing–mingling in Sri Lanka: Implications for convergent margin processes during Gondwana assembly

The Sri Lankan fragment of Gondwana preserves the records of Neoproterozoic tectonothermal events associated with the final assembly of the supercontinent. Here we investigate a suite of magmatic rocks from the Wanni, Kadugannawa and Highland Complexes through geological, petrological, geochemical and zircon U–Pb and Lu–Hf isotopic techniques. The hornblende biotite gneiss, charnockites, metagabbro and metadiorites investigated in this study show geochemical features consistent with calc-alkaline affinity and subduction-related signature including LILE enrichment relative to HFSE coupled with distinct Nb–Ta depletion and weak negative Zr–Hf anomalies. The felsic suite falls in the volcanic arc granites (VAGs) field and the mafic suite shows island arc basalt affinity in tectonic discrimination plots, suggesting that the protoliths of the rocks were derived from arc-related magmas in a convergent margin setting. LA-ICPMS zircon U–Pb analyses show crystallization of charnockite and dioritic mafic magmatic enclave from the Highland Complex during ca. 565 and 576Ma corresponding to bimodal magmatism. The diorite also contains metamorphic zircons of ca. 525Ma. Hornblende–biotite gneiss from the Kadugannawa Complex shows protolith emplacement age at 973–980Ma, followed by new zircon growth during repeated thermal events through late Neoproterozoic. The dioritic enclaves in these rocks are much younger, and form part of a deformed and metamorphosed dyke suite with emplacement ages of 559Ma, broadly coeval with the bimodal magmatism in the Highland Complex at that time. The youngest group of zircons in this rock shows ages of 508Ma, corresponding to the latest thermal event. A charnockite from this locality shows oldest group of zircons at 962Ma, corresponding to emplacement age similar to that of the magmatic protolith of the hornblende biotite gneiss. This rock also shows zircon growth during repeated thermal events at 832Ma, 780Ma, 721Ma and 661–605Ma. The lower intercept age of 543Ma marks the timing of collisional metamorphism. Charnockite from the Wanni Complex shows emplacement age at 1000Ma, followed by thermal event at 570Ma, the latter correlating with the bimodal magmatic event in the Highland Complex. The dioritic enclave within this charnockite shows an age of ca. 980Ma, suggesting intrusion of mafic magma into the felsic magma chamber. Zircons in the diorite also record multiple zircon events during 950 to 750Ma. Zircons in the Highland Complex charnockite possess negative εHf(t) values in the range −6.7 to −12.6 with TDMC of 2039–2306Ma suggesting magma derivation through melting of Paleoproterozoic source. In contrast, the εHf(t) range of −11.1 to 1.6 suggests a mixed source of both of older crustal and juvenile material. The εHf(t) values of −4.5 to 4.5 and TDMC of 1546–1962Ma for the hornblende biotite gneiss also shows magma derivation from mixed sources that included Paleoproterozoic components. The younger dioritic intrusive, however, has a more juvenile magma source as indicated by the mean εHf(t) value of 1.3. The associated charnockite shows a tight positive cluster of εHf(t) from 0.6 to 5.1, suggesting juvenile input. Charnockite from the Wanni Complex shows clearly positive εHf(t) values of up to 13.1, and TDMC in the range 937–1458Ma suggesting much younger and depleted mantle source. The diorite enclave also has positive εHf(t) values with an average value of 8.5 and TDMC in the range of 709–1443Ma clearly suggesting younger juvenile sources. The early and late Neoproterozoic bimodal suites are correlated to convergent margin magmatism associated with the assembly of Sri Lanka within the Gondwana supercontinent.

Geology and U–Pb geochronology of the Espedalen Complex, southern Norway, and its position in the Caledonian nappe systems

Abstract The Espedalen Nappe is located in the east of the Jotun–Valdres Nappe Complex in southern Norway. Its main component, the Espedalen Complex, consists of an early suite of jotunite, charnockite and augengneiss, a main anorthosite–ultramafite–norite suite, and a late gabbronorite suite. Mafic dykes and lamprophyres cut the complex, some being feeders to the Stylskampen mafic to felsic suite at the western margin of the complex. The complex hosts Ni–Cu mineralizations which were mined in past centuries. Two crystalline outliers occur at Ormtjernkampen and Røssjøkollen further south. Caledonian convergence thrust the nappe on to Early Palaeozoic sedimentary rocks. Rocks of the Espedalen Complex are covered and/or tectonically interleaved with Late Precambrian conglomerates and arkoses of the Valdres Group. Dating of zircon and titanite by ID-TIMS U–Pb indicates formation of the complex at 1520–1510 Ma, followed by deformation and metamorphism between 1050 and 950 Ma during the Sveconorwegian orogeny, before the eventual translation during the Caledonian orogeny. These data indicate an affinity of the nappe with Telemarkian crust in the autochthonous Baltic basement, in contrast to the rocks of the overlying Jotun Complex which correlate instead to Gothian crust formed between 1600 and 1700 Ma.

Volcanic Stratigraphy, Geochronology, and Gold Deposits of the Archean Hope Bay Greenstone Belt, Nunavut, Canada

The Archean Hope Bay greenstone belt is located in the Bathurst Block of the northeastern part of the Slave Structural Province, a predominantly Archean, granite-greenstone-metasedimentary terrane. The greenstone belt is dominated by mafic volcanic rocks with less common felsic volcanic and volcaniclastic products and subordinate ultramafic bodies and metasedimentary rocks. Three main Au deposits have been defined in the greenstone belt, with a cumulative resource of over 10 million ounces (Moz) of Au, as of April 2007. The Hope Bay greenstone belt can be divided into a series of coherent panels of strata that tend to be fault bounded and collectively are used to construct a composite stratigraphic column of the belt constrained by U-Pb geochronology on felsic suites. The oldest felsic suite, the Flake Lake suite (ca. 2700 Ma), is a succession of felsic volcanic rocks that has a tholeiitic geochemical affinity, interpreted as a product of rift-related volcanism. Overlying this suite is a series of well-constrained calc-alkaline, mainly felsic volcanic rocks, which have ages of ca. 2690 Ma (Square Lake suite), ca. 2686 Ma (Windy felsic suite), ca. 2677 Ma (Koignuk suite), and ca. 2662 Ma (Clover Lake suite), all of which are interpreted as products of arc volcanism. The transition from rift- to arc-related volcanism divides the greenstone belt into upper and lower volcanic cycles with the transition occurring at about ca. 2690 Ma. The older cycle of volcanic rocks has a distinctive group of mafic rocks consisting of mafic pillowed flows which have an Fe-rich tholeiitic geochemical affinity which is not recognized in the younger volcanic cycle. These Fe-rich tholeiitic mafic volcanic rocks commonly show a spatial relationship to Au mineralization acting as host strata to the main Au deposits in the greenstone belt. The younger cycle (<ca. 2690 Ma) of mafic volcanic rocks tends to have a tholeiitic affinity with some minor andesitic volcanic rocks near the top of the composite stratigraphic column. Detrital zircon work was carried out for all the main sedimentary successions recognized in the Hope Bay greenstone belt. The detrital zircon results show that in each case the sediments were derived mainly from local sources, as very few anomalously old zircons were recognized. A conglomerate outcropping in the north part of the belt is dominated by locally derived material, with detrital zircon work supporting this interpretation. The three main Au deposits at Hope Bay are all associated with the older cycle of volcanic rocks, interpreted as products of rift-related volcanism. The structural setting of each deposit is different but each is related to D2 strain in structural and/or stratigraphic settings which provided dilational environments to localize auriferous vein-related mineralization or localized alteration and mineralization of favorable host lithologies. The older cycle of tholeiitic felsic and Fe-rich mafic volcanic rocks are rift-related volcanic products, which are time-equivalent to the Kam Group, in the Yellowknife Supergroup. Overlying these rocks are calc-alkaline felsic volcanic rocks and tholeiitic mafic volcanic rocks, which represent the transition to calc-alkaline arc-related volcanism, which correlates to the Banting Group of the Yellowknife Supergroup. Overlying the younger (ca. 2677 Ma) felsic volcanic rocks are a succession of conglomerates consisting mainly of locally derived clasts, representing collapse of the arc into a fluvial-dominated environment. This sedimentary succession is possibly equivalent to the regionally extensive Burwash Basin of the Yellowknife Supergroup.

The Late Miocene Guacimal Pluton in the Cordillera de Tilarán, Costa Rica: its nature, age and petrogenesis

*Corresponding author The Guacimal Pluton is situated in the Cordillera de Tilaran in the northwestern Costa Rica. It forms an oval-shaped body strongly elongated in the NW–SE direction. Its dimensions are ~15 × 4–6 km with an exposed surface of 60–70 km 2 . The pluton intruded basic volcanic rocks of the Aguacate Group (Miocene–Pliocene) and is surrounded by a wide thermal aureole of calc-silicate metasomatic rocks. The pluton is mainly formed of monzogranites to granodiorites, which strongly prevail over more basic types occuring scarce and relatively thin dykes and enclaves. The dominant magmatic minerals of this felsic suite are quartz, plagioclase, and K-feldspar with subordinate Mg-rich biotite, amphibole I, and magnetite. Orthopyroxene, Mn-rich ilmenite, Al-poor titanite, rutile, apatite, zircon, thorite, and chalcopyrite are accessories. Secondary minerals, which occur as fillings of miarolitic cavities and interstices, are quartz II, K-feldspar II, epidote, chlorite, actinolite, ilmenite II and Al-rich titanite II. The much less frequent mafic suite (mainly quartz diorite to quartz monzodiorite/monzogabbro) is composed of plagioclase, pargasite, actinolite, K-feldspar, quartz and magnetite, with accessory amounts of opaque minerals, epidote, chlorite, and titanite. The pluton was emplaced at a depth of c. 3 km, crystallized at temperature of c. 760–800 °C under a relatively high oxygen fugacity (1.6–2.1 log units above the NNO buffer). Increased activities of volatiles (H 2 O, F) upon cooling are indicated by the presence of highly aluminous, F-rich titanite and other hydrous silicates in miarolitic cavities. The prevailing, felsic rocks of the Guacimal Pluton are high-K calc-alkaline, whereas the mafic suite is nearly exclusively medi um-K calc-alkaline in nature. Laser ablation ICP-MS dating of zircons from two granite samples yielded statistically identical U–Pb ages of 6.3 ± 0.5 and 6.0 ± 0.4 Ma, respectively. The Sr–Nd isotopic compositions are rather primitive ( 87 Sr/ 86 Sr 6 = 0.70380–0.70413, e 6 Nd ~ +7.3 to +7.9). Narrow range of these values rules out open-system processes such as magma mixing or assimilation of isotopically contrasting upper continental crust. Instead, the felsic suite is interpreted as either having crystallized from a highly fractionated melt extracted from a plagioclase–amphibole-dominated crystal mush in a putative deep crustal reservoir or a product of partial melting of older arc-related rocks, such as intermediate lavas or volcaniclastics or immature psammitic sediments rich in volcanic material. The observed variation in the felsic suite was most likely produced by low degrees of closed-system fractional crystallization of an assemblage dominated by feldspars. At least some of the rocks of the volumetrically subordinate mafic suite may represent litholo gies rich in the complementary cumulates.

Rift and intra-oceanic subduction signatures in the Western Tethys during the Triassic: The case of ultramafic lavas as part of an unusual ultramafic–mafic–felsic suite in Othris, Greece

The Triassic igneous rocks of Othris, Greece, exhibit unusual and extreme compositional variations. Abundant E-MORB and rare OIB alkaline basalts appear to be linked to NW Gondwana (Apulia) margin rifting, Pelagonian microcontinent detachment and opening of the Pindos oceanic strand at Western Tethys. They are spatially associated with wehrlites, ultramafic lavas, picrites, transitional boninites, tholeiitic basaltic andesites and calc-alkaline andesitic to rhyodacitic rocks, whose new geochemical and petrological data suggest that they were formed in a short-lived subduction system, developed shortly after rifting/opening within the Pindos ocean.OIB alkaline basalts seem to have been formed from an enriched, possibly garnet bearing mantle source, while formation of E-MORB basalts may represent partial melts (~20%) of an enriched mantle source. The liquidus temperature for the primary rift-related magma is estimated at ~1330°C, while mantle potential temperature reached ~1435°C, with ~14wt.% MgO. Subduction-related rocks were produced after differentiation of primary picritic magma, generated after ~32% partial melting of a fertile mantle source within the newly formed mantle wedge. Temperature conditions were similar to those calculated for the rift-related primary magma, but with higher MgO contents (~16wt.%). Olivine and clinopyroxene accumulation of a batch of the subduction-related primary magma led to the formation of ultramafic magma under hydrous conditions. Wehrlites represent ultramafic magma stacked at the bottom of a magma chamber. Ultramafic lavas were formed when ultramafic magma was brought to the surface, most likely with the aid of an upwelling asthenospheric E-MORB flow passing through a slab break-off or by the bottom edge of the infant slab. The remainder of the primary picritic magma underwent variable degrees of fractional crystallization forming transitional boninites and tholeiitic basaltic andesites in a front-arc setting and calc-alkaline intermediate and felsic rocks in places closer to the passive margin of the Pelagonian microcontinent.

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.

Tectono-stratigraphic setting of the Moreton’s Harbour Group and its implications for the evolution of the Laurentian margin: Notre Dame Bay, Newfoundland1This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The Moreton’s Harbour Group lies along the Red Indian Line, the fundamental Iapetus suture that separates rocks of peri-Laurentian affinity with rocks of peri-Gondwanan affinity in the Newfoundland Appalachians. Characterization of age and environment of formation of the Moreton’s Harbour Group is an important constraint on evolution of the Laurentian margin during Ordovician closure of Iapetus Ocean and associated marginal basins. The Moreton’s Harbour Group comprises a fault-bounded ophiolitic sequence of layered gabbro, sheeted diabase, pillow basalt, and felsic intrusive rocks. It is offset by high-angle shear zones that were contemporaneous with a 477.4 ± 0.4 Ma syn-tectonic and syn-magmatic suite of trondhjemite and tonalite. Trace element data from the felsic suite indicate formation in a supra-subduction zone setting, although isotopic data from the felsic intrusive rocks (εNd (–5.02) to (–10.53), Tdm 1200–1800 Ma) indicate a significant amount of contamination from Mesoproterozoic or older continental crust. The age and tectonic setting of the Moreton’s Harbour Group suggest that it is the northernmost extent of the ca. 480 Ma Annieopsquotch Ophiolite Belt. We present a model in which the Moreton’s Harbour Group formed in response to propagation of the Annieopsquotch Ophiolite Belt spreading centre into the Dashwoods microcontinent. This ridge propagation model supports the formation of the Annieopsquotch Ophiolite Belt immediately outboard of Dashwoods and explains its rapid accretion to the composite Laurentian margin.

Igneous Geochemistry of Mineralized Rocks of the Baguio District, Philippines: Implications for Tectonic Evolution and the Genesis of Porphyry-Style Mineralization

The Baguio district of the Philippines is one of the world’s premier mineral provinces, containing >35 million ounces (Moz) of gold and 2.7 million metric tons (Mt) of copper in epithermal, porphyry, and skarn deposits that formed in the last 3.5 m.y. Pliocene and Pleistocene magmatic rocks of the Baguio district that are spatially and temporally associated with mineralization can be broadly subdivided into an intermediate to felsic suite of mineralized low to medium K intrusions, some of which have adakitic affinities and a suite of mafic to intermediate, medium K to shoshonitic hornblende-phyric dikes. The geochemical and isotopic characteristics of the dikes are consistent with primitive mantle-derived melts that underwent minimal crustal contamination as they ascended through the arc crust. In contrast, the intermediate to felsic suite has been contaminated by young arc crust, suggesting ponding and fractionation within shallow-crustal magma chambers. The Philippine arc has formed in a complex tectonic environment and is currently sandwiched between two active subduction zones. Eastward-directed subduction of the Scarborough Ridge along the Manila trench is currently associated with flattening of the downgoing slab. The formation of the Mafic dike complex is broadly coeval with the onset of subduction of the Scarborough Ridge and slab flattening. The extinct Scarborough Ridge would have been younger than the downgoing plate and consequently more susceptible to melting. These melts can account for the isotopic recharge of the Pliocene subarc mantle as well as the generation of the primitive melts and adakitic rocks found within the Baguio district. The interaction between primitive mafic melts and the more felsic calc-alkaline rocks has

Evidence for an enriched asthenospheric source for coronitic metagabbros in the Adirondack Highlands

Coronitic metagabbros (CMGs) in the Adirondack Highlands display chemical and isotopic features consistent with derivation from an enriched asthenospheric mantle source and are samples of the parental magma of Adirondack anorthosite. Primary ophitic textures in the CMGs are overprinted by mineral coronas developed during granulite-facies metamorphism of anhydrous olivine gabbronorites during the Ottawan orogeny. Restricted in silica content (45–48 wt%) and olivine normative, the CMGs are predominantly tholeiitic in composition, although a minority display some calc-alkaline features. Unlike older Adirondack mafic and felsic suites, the CMG rocks lack or have greatly reduced, incompatible element patterns (N PM ) generally associated with subduction processes. Rare-earth elements (N CH ) have minor light rare-earth element (LREE) enrichment with La/Sm values from 1.42 to 1.98, compatible with a transitional to enriched mantle source. When CMGs are plotted on various major (TiO 2 versus P 2 O 5 ) and trace-element (Sm versus Cr) diagrams, the CMGs form a continuous field between that of oxide- and apatite-rich gabbros (OGNs and OAGNs) and anorthosites and leucogabbros within the Adirondacks. Initial epsilon Nd (e Nd ) values are +3.13 to +3.69, generally higher than Adirondack anorthosite values, but significantly less than contemporaneous depleted mantle. Neodymium T DM model ages that are ∼400 million years older than their crystallization age and enriched compositional trends indicate that they were not derived from depleted mantle. These data indicate Adirondack CMGs were derived from a previously untapped and enriched asthenospheric source. Asthenospheric upwelling was triggered by lithospheric delamination following Shawinigan orogenesis at ca. 1160 Ma and provides a link between tectonism, mantle geodynamics, and massif anorthosite petrogenesis in the Grenville allochthonous monocyclic belt.

An overview on geochemistry of Proterozoic massif-type anorthosites and associated rocks

A critical study of 311 published WR chemical analyses, isotopic and mineral chemistry of anorthosites and associated rocks from eight Proterozoic massif anorthosite complexes of India, North America and Norway indicates marked similarities in mineralogy and chemistry among similar rock types. The anorthosite and mafic-leucomafic rocks (e.g., leuconorite, leucogabbro, leucotroctolite, anorthositic gabbro, gabbroic anorthosite, etc.) constituting the major part of the massifs are characterized by higher Na2O + K2O, Al2O3, SiO2, Mg# and Sr contents, low in plagioclase incompatible elements and REE with positive Eu anomalies. Their δ 18O‰ (5.7–7.5), initial 87Sr/86Sr (0.7034–0.7066) and ɛ Nd values (+1.14 to +5.5) suggest a depleted mantle origin. The Fe-rich dioritic rocks occurring at the margin of massifs have isotopic, chemical and mineral composition more close to anorthosite-mafic-leucomafic rocks. However, there is a gradual decrease in plagioclase content, An content of plagioclase and XMg of orthopyroxene, and an increase in mafic silicates, oxide minerals content, plagioclase incompatible elements and REE from anorthosite-mafic-leucomafic rocks to Fe-rich dioritic rocks. The Fe-rich dioritic rocks are interpreted as residual melt from mantle derived high-Al gabbro melt, which produced the anorthosite and mafic-leucomafic rocks. Mineralogically and chemically, the K-rich felsic rocks are distinct from anorthosite-mafic-leucomafic-Fe-rich dioritic suite. They have higher δ 18O values (6.8–10.8‰) and initial 87Sr/86Sr (0.7067–0.7104). By contrast, the K-rich felsic suites are products of melting of crustal precursors.

Mesozoic collision-related magmatism and crust–mantle interaction along the Anhui segment of the Yangtze River Valley, east-central China

The middle segment of the Yangtze River Deep Fault Belt, located in the foreland of the Dabie orogen, contains widely exposed volcanic–intrusive complexes that formed during two episodes of magmatism (post-collisional and post-orogenic), reflecting crust–mantle interactions during the Late Jurassic (J3) to Early Cretaceous (K1). This article summarizes research on the Mesozoic igneous suites and xenolith suites in the area along the Yangtze River. ‘Post-collisional magmatism’ occurred during lithospheric extension at ∼145–130 Ma. Its beginning and end are marked by gabbroic xenoliths and pyroxene cumulates within intrusions at Tongling, and by alkali-rich magmatic rocks. The association includes peraluminous silicic rocks and metaluminous mafic–felsic igneous suites, ranging from medium-K to high-K calc-alkaline to shoshonitic compositions. Taking the Tongling region as an example, quartz monzodiorite yields a sensitive high resolution ion microprobe (SHRIMP) zircon U–Pb age of 139.5 ± 2.9 Ma, and granodiorite yields an age of 135.5 ± 4.4 Ma. These intrusive rocks contain 52.79–66.46 wt.% SiO2, 13.12–17.73 wt.% Al2O3, 1.37–4.62 wt.% MgO, 3.86–6.84 wt.% FeOT, and 4.71–7.87 wt.% total alkalis (Na2O + K2O). ACNK values range from 0.62 to 1.20, and ANK values from 1.45 to 3.48. ‘Post-orogenic magmatism’ occurred during lithospheric delamination at ∼130–120 Ma. The start of magmatism was marked by the formation of gabbro containing spinel lherzolite xenoliths in the Nanjing–Wuhu Basin (NWB), and its end was marked by the generation of feldspathoid phenocryst-bearing phonolite in the NWB and the Lujiang–Zongyang Basin (LZB), respectively. The association that formed during this episode ranges from alkaline to peralkaline. Taking the Niangniangshan Formation in the NWB as an example, the Nosite phonolite yields a whole-rock monomineral Rb–Sr isochron age of 120 ± 9 Ma, and contains 49.92–60.09 wt.% SiO2, 17.67–20.65 wt.% Al2O3, 0.08–2.45 wt.% MgO, 1.32–6.62 wt.% FeOT, and 9.24–13.92 wt.% total alkalis (Na2O + K2O). ACNK values range from 0.72 to 1.24, and ANK values from 1.03 to 1.35. The two magmatisms correspond to two episodes of crust–mantle interaction. The first involved intensive interaction between middle–lower crust and underplated basaltic magma derived from the upper mantle lithosphere, whereas the second involved minor interaction between the middle–lower crust and basaltic magma derived from the lower lithospheric mantle.