Melting Of Subcontinental Lithospheric Mantle Research Articles

Constraints on the source of Siberian Trap magmas from Mo isotope evidence

The relative contributions of asthenospheric mantle, lithospheric mantle, and continental crust in the genesis of the Siberian Traps Large Igneous Province (ST-LIP) remain poorly constrained. Most models invoke partial melting of asthenospheric mantle within a mantle plume with an inventory of recycled crustal material, with or without melting of subcontinental lithospheric mantle, and crustal contamination during ascend through the continental lithosphere. A greater understanding of this topic is of fundamental importance because the ST-LIP basalts are associated with the large Permian-Triassic extinction and the world’s largest magmatic Ni-Cu-platinum group element sulfide resources (Ni-Cu-PGE), the Norilsk-Talnakh mining camp. The ∼250 ± 2 Ma Siberian Traps at Norilsk contain a classic sequence of basaltic rocks that provide a spatial and stratigraphic context for changing chemistry of the eruptive products. We present a detailed geochemical and Sr-Nd-Hf-Mo isotopic investigation of ST-LIP volcanic rocks and associated sedimentary rocks, including coal and anhydrite, from the Norilsk area. The 98Mo/95Mo isotope ratios (reported as δ98Mo ratio relative to NIST SRM 3134) vary significantly from −0.62 to 0.07 ‰ for the older basalt formations and −0.41 to 0.03 ‰ for the younger basalt formations. The range of δ98Mo and its correlation with the other geochemical tracers cannot be explained by post-magmatic alteration, magmatic differentiation, or sulfide fractionation as Mo behaves as a lithophile element under these magmatic conditions. We suggest that the δ98Mo range can be explained by interaction of the plume with subcontinental lithospheric mantle modified by subduction processes. This is particularly prominent in the earlier Ivakinsky to Gudchikhinsky formations where light δ98Mo values coupled with low Mo/Ce can be explained by contributions from a dehydrated eclogitic component, whereas more rare heavy δ98Mo values and high Mo/Ce likely require contributions from a fluid metasomatized mantle source. Later magmas of the Nadezhdinsky formation show clear evidence of crustal contamination in their combined Mo-Sr-Nd-Hf isotope and trace element systematics, while the later more voluminous Morongovsky type magmas are shallower, large degree melts with limited crustal interaction. Our data shows the usefulness of Mo isotopes in deciphering magma sources of large igneous provinces eruptions.

Cu-Au-Platinum Group Element Mineralization in the Mbesa Prospect, Southern Tanzania: Unconventional Magmatic Sulfides

Abstract High-grade Cu mineralization was recently discovered in the Mbesa prospect in southern Tanzania, but its origin is poorly constrained. Herein, we present new major element, trace element, rare earth element (REE), and platinum group element (PGE) geochemistry and petrographic data to constrain the origin of the mineralization. The sulfide mineralization is dominated by chalcopyrite and bornite and characterized by high Cu/Ni over 100 in the massive and disseminated ores. The mineralization lacks pyrrhotite and pentlandite, which are found commonly in magmatic Ni-Cu-PGE sulfide deposits. Platinum group minerals present are michenerite (PdBiTe) and sudburyite (PdSb). PGE concentrations expressed as Pd + Pt + Au are up to 1.31 and 1.04 ppm in the massive and disseminated ores, respectively. The δ34S of chalcopyrite separates range from –3.9 to –0.6‰, bornite separates vary from –1.8 to 1.1‰, and bornite-chalcopyrite mixtures yield –1.3 and 0.6‰, mostly consistent with a magmatic sulfur origin. The sulfides are hosted by coarse-grained plagioclase amphibolite and hornblende gneiss with recrystallization textures suggestive of metamorphism. Both rocks are characterized by enrichment in large ion lithophile elements (LILEs), with negative anomalies of Zr and Nb, consistent with crustal contamination of magmas derived from partial melting of subcontinental lithospheric mantle (SCLM). An SCLM origin is further suggested by the close association of Cu sulfides with carbonates and apatite. It is suggested that the sulfides crystallized from the fractionated Cu-rich sulfide melt enriched in intermediate solid solution (iss) at shallow crustal levels. Sulfide were likely mobilized as droplets attached to CO2 vapor phase as expressed by close association between carbonates and apatite with the Cu-rich sulfides. Nickel-rich sulfides may remain undiscovered at deeper portions around the Mbesa prospect.

Origin of Silurian granite in the Baiganhu field, Eastern Kulun Terrane, NW China: Implications for the tectonic setting of Sn-W mineralization

The Baiganhu area in the Eastern Kunlun Terrane on the northern margin of the Tibetan Plateau contains world-class Sn-W deposits in close association with monzogranite. However, the origin of granite and the tectonic setting for the Sn-W mineralization have not been well constrained. Here we present the petrological and geochemical data of the granite (431 Ma) associated with the Sn-W mineralization to evaluate the geological-tectonic setting for the mineralization. The monzogranite has high SiO2 contents, ranging from 65.8 to 73.5 wt%, and shows a geochemical affinity with A2-type granites. Relatively high zircon saturation temperatures, ranging between 750 °C and 921 °C, suggest that the parental magmas were hot. The whole-rock εNd (t = 431 Ma) values range from –2.7 to –3.1 with TDM ages between 1810 and 1040 Ma, and zircon εHf (t = 431 Ma) values range from + 0.7 to + 4.1 with TDM ages between 1040 and 900 Ma. The data suggest that the magmas had significant contributions of old rocks. The granitic rocks intruded into quartz-muscovite schist of the Proterozoic Xiaomiao Complex, which underlies the vast area. Zircon grains in the schist are magmatic in origin based on sharp oscillatory zoning and high Th/U ratios (>0.2). The 207Pb-206Pb zircon ages (n = 25) of the schist range from 1898 ± 2 to 1700 ± 6 Ma, suggesting that the zircon grains originate from a Paleoproterozoic magmatic belt. Such ancient rocks, either as lower crustal component or subcontinental lithospheric mantle, contributed ∼ 30 % to the parental magmas. We conclude that post-collisional extension within the Eastern Kunlun Terrane was accompanied by upwelling and adiabatic melting of the asthenospheric mantle which induced partial melting of subcontinental lithospheric mantle and lower crustal rocks and formed granitic magmas. Aqueous fluids separated from the granitic magma which preferentially incorporate W and Sn contributed to the mineralization in and around the granites.

Geochronology, petrogenesis, and tectonic setting of amphibolitic rocks from the Tutak metamorphic Complex, Sanandaj-Sirjan Zone, Iran

Amphibolitic rocks of the Tutak Metamorphic Complex (TMC), in the southern part of the Sanandaj Sirjan Zone (SaSZ), are associated with meta-dolerites, felsic intrusions, mica-schists, and marbles as bodies and interlayers. Rutile grains from an amphibolite (ES-18-1) yield a U-Pb age of 56.08 ± 1.13 Ma (Late Paleocene-Early Eocene) while Zr-in-rutile thermometry indicates peak temperatures ∼ 616 to 674 °C. These data are inferred to reflect the timing of medium-pressure/medium-temperature (MP/MT) amphibolite facies metamorphism in Late Paleocene-Early Eocene rather than upper Cretaceous time in the TMC complex. Geochemically, the amphibolites are from an Nb-enriched basalt-andesite to basalt protolith with sub-alkaline to alkaline affinity. The amphibolites have moderate 87Sr/86Sr(t) ratios (0.702451 to 0.712720), 143Nd/144Nd(t) ratios (0.512122 to 0.512209), positive ƐNd(535 Ma) (3.38 to 5.08) values and two-stage Nd model ages (TDM) between 0.84 and 1.3 Ga. The geochemical trends and (La/Yb)cn> 3, Nb/La> 0.6, Sm/Nd< 0.3, Zr/Y (4.42 to 7.47, avg. 5.72) ratios with moderate 87Sr/86Sr(535) ratios and positive ƐNd(535 Ma) values indicate that the mafic protoliths of the amphibolites are probably derived from the low partial melting of subcontinental lithospheric mantle in a spinel lherzolite field, with enriched mantle (E-MORB/OIB-like) nature. Subsequently, they experienced minor crustal contamination in the early stages of the development of intra-continental-type back-arc basin (BABB). Lack of subduction signals in the amphibolites with E-MORB/OIB-like features may result from the roll-back of subducting Proto-Tethys oceanic slab during the Cambrian similar to other peri-Gondwana terranes.

Early Ordovician seamounts preserved in the Canadian Cordillera: Implications for the rift history of western Laurentia

Abstract The breakup of the supercontinent Rodinia and development of the western Laurentian rifted margin are in part recorded by Neoproterozoic to mid-Paleozoic igneous and sedimentary rock successions in the Canadian Cordillera. New bedrock mapping and volcanic facies analysis of Early Ordovician mafic rocks assigned to the Menzie Creek Formation in central Yukon allow reconstruction of the depositional environment during the volcanic eruptions, whole-rock geochemical data constrain the melting depth and crust-mantle source regions of the igneous rocks within the study area, and zircon U-Pb age studies provide determination of the precise timing of submarine eruptions. Menzie Creek Formation volcanic rocks are interlayered with continental slope strata and show lithofacies consistent with those of modern seamount systems. Representative seamount facies contain several kilometers of hyaloclastite breccia and pillow basalt with rare sedimentary rocks. Menzie Creek Formation seamounts form a linear array parallel to the Twopete fault, an ancient extensional or strike-slip fault that localized magmatism along the nascent western Laurentian margin. Zircon grains from two volcanic successions yielded high-precision chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) dates of ca. 484 Ma (Tremadocian), which are interpreted as the age of eruption. Menzie Creek Formation rocks are alkali basalt and have oceanic-island basalt–like geochemical compositions. The whole-rock trace element and Nd-Hf isotope compositions are consistent with the partial melting of subcontinental lithospheric mantle at ~75–100 km depth. Post-rift, Early Ordovician seamounts in central Yukon record punctuated eruptive activity along a rift-related fault, the separation of a continental fragment from western Laurentia, or the oblique post-breakup kinematics from the counterclockwise rotation of Laurentia that facilitated local extension in the passive margin.

Geochemical Constraints on the Origin of Primitive Potassic Lavas in the Eastern Virunga Volcanic Province

AbstractYoung mafic lavas from the East African Western Rift record melting of subcontinental lithospheric mantle that was metasomatically modified by multiple tectonic events. We report new isotope data from monogenetic cinder cones near Bufumbira, Uganda, in the Virunga Volcanic Field: 87Sr/86Sr = 0.7059–0.7079, εNd = −6.5 to −1.3, εHf = −6.3 to +0.9, 208Pb/204Pb = 40.1–40.7, 207Pb/204Pb = 15.68–15.75, and 206Pb/204Pb = 19.27–19.45. Olivine phenocrysts from the Bufumbira lavas have 3He/4He = 6.0–7.4 RA. The isotopic data, in conjunction with major and trace element systematics, indicate that primitive Bufumbira magmas are derived from two different metasomatized lithospheric source domains. Melts generated by lower degrees of melting record greater contributions from ∼1 to 2 Ga isotopically enriched garnet‐amphibole‐phlogopite pyroxenite veins within the lithosphere. As melting progresses, these vein melts become increasingly diluted by melts that originate near the lithosphere/asthenosphere boundary, shifting the isotopic compositions toward the common lithospheric mantle (CLM) proposed by Furman and Graham (1999, https://doi.org/10.1016/s0024-4937(99)00031-6). This ∼450–500 Ma source domain appears to underlie all Western Rift volcanic provinces and is characterized by 87Sr/86Sr ∼ 0.705, εNd ∼ 0, εHf ∼ +1 to +3, 206Pb/204Pb ∼ 19.0–19.2, 208Pb/204Pb ∼ 39.7, and 3He/4He ∼ 7 RA. Basal portions of the dense subcontinental lithospheric mantle may become gravitationally unstable and founder into underlying warmer asthenosphere, exposing surfaces where melting of locally heterogeneous veins produces small‐volume, alkaline mafic melts. Mafic lavas from all Western Rift volcanic provinces record mixing between the CLM and locally variable metasomatized source domains, suggesting this style of melt generation is fundamental to the development of magma‐poor rifts.

AN UNUSUAL EARLY EOCENE, SYNCOLLISIONAL CARBONATITE COMPLEX AND RELATED RARE EARTH ELEMENT DEPOSIT IN THE INDIA-ASIA COLLISION ZONE, NORTHWESTERN VIETNAM

Abstract Carbonatite-related rare earth element (REE) deposits, the most significant source of REEs globally, are normally generated in extensional settings, such as intracontinental rifts, mantle plume-related environments, or postcollisional orogens. Syncollisional orogens represent overall compressional regimes, so carbonatites and related REE deposits are rarely identified in such a setting. However, this study reports an anomalous syncollisional carbonatite-related REE deposit, Dong Pao, in the India-Asia collision zone in northwestern Vietnam. The Dong Pao deposit is dated at ca. 52 to 51 Ma through zircon and bastnäsite Th-U-Pb chronometers. The ore-hosting carbonatites were emplaced as stocks with associated syenite. The carbonatite-syenite complex is significantly enriched in light REEs, Ba, and Sr and depleted in high-field strength elements, and has high (87Sr/86Sr)i ratios (&amp;gt;0.707) and low εNd(t) values (–6.5 to –5.6). These geochemical signatures imply that the carbonatite-syenite complex was derived from partial melting of subcontinental lithospheric mantle previously metasomatized and fertilized by REE- and CO2-bearing fluids. Timing of the REE-rich carbonatite-syenite complex indicates that it was related to a far-field stress within the early Eocene main-collision stage at 52 to 51 Ma rather than the late-collision stage at 42 to 35 Ma as previously thought. Collisional tectonism involving block rotation and fault activation are interpreted to have induced disturbance of the lithosphere mantle and created localized, transtensional/extensional environments oblique to the trend of the orogen that facilitated emplacement of the REE-rich carbonatitic magmas. Dong Pao appears to be the first identified, high-tonnage REE deposit that formed in the syncollisional geodynamic setting. Such a finding highlights that tectonic disturbance of an REE-rich lithosphere mantle distal to collision sutures has the potential to generate REE deposits, even during prominent convergence and collision of continents. As such, it defines additional search spaces for exploration of other REE orebodies of this style in complex collisional orogens.

Modification of the lithospheric mantle induced by recycled crustal components: Insights from Early Cretaceous appinites from the Liaodong Peninsula, NE China

AbstractThe term “appinites” refers to amphibolerich mafic rocks, as one of the most typical products of partial melting of a metasomatic lithospheric mantle. This paper presents results of integrated geochemical and isotopic analyses for six Early Cretaceous (125–119 Ma) appinitic plutons from the Liaodong Peninsula, northeastern North China Craton (NCC), to not only investigate the properties of the metasomatic lithospheric mantle induced by variable recycled crustal components but also to discuss a hydrous mantle source. The appinites originated from partial melting of sub-continental lithospheric mantle (SCLM), and can be geochemically categorized into three groups distinguished by the input of different recycled crustal components (e.g., terrigenous sediments and delaminated lower continental crust) on the basis of two distinct Hf-O isotopic arrays. Based on the occurrence of voluminous amphiboles (50–60 vol%) and minor clinopyroxene remnants, a petrogenetic model was proposed to interpret the formation of appinites. Asthenospheric upwelling caused by the subduction and roll-back of the Paleo-Pacific plate firstly resulted in partial melting of the SCLM to yield basaltic magmas. Subsequently, numerous hydration reactions between early anhydrous minerals (e.g., olivine and pyroxene) of basaltic magmas and hydrous melts derived from terrigenous sediments and delaminated lower continental crust occurred to produce the studied appinites. Ultimately, the recycled crustal materials can serve as the metasomatic agent not only to transfer the enriched isotopic signatures into the studied appinites but also to represent hydrous components to play the part of hydrous reactions. The high abundance of amphibole in the studied appinites is indicative of a hydrous lithospheric mantle beneath the eastern NCC.

SHRIMP U–Pb ages, mineralogy, and geochemistry of carbonatite–alkaline complexes of the Sillai Patti and Koga areas, NW Pakistan: Implications for petrogenesis and REE mineralization

Carbonatites and related alkaline igneous rocks are important hosts of rare earth elements (REEs). Carbonatite–alkaline complexes in NW Pakistan represent rare examples of carbonatite emplaced in a collisional orogenic belt. However, the precise age, magmatic source, and petrogenesis of carbonatites emplaced in collisional settings and associated REE mineralization remain uncertain. The present study focuses on the origin and REE mineralization potential of alkaline–carbonatite complexes exposed in the Koga and Sillai Patti (SP) areas, NW Pakistan.The Koga carbonatites occur as minor plug-like intrusions in syenites. SHRIMP U–Pb dating of zircons from the Koga syenites, which are spatially associated with carbonatites, yield weighted mean ages of 283.6 ± 1.7(1σ), 281.9 ± 1.6(1σ), 282.6 ± 1.6(1σ), and 306.5 ± 3.7 Ma (1σ). The Koga syenites are characterized by high SiO2 (62.79–65.58 wt%) and Al2O3 (16.92–18.55 wt%) and low TiO2 (0.10–0.59 wt%) contents and are enriched in alkalis (Na2O > K2O). Sr and Nd isotopes from the Koga carbonatite complex are characterized by positive εNd(t) values (+3.10 to + 3.47) and low (87Sr/86Sr)i values (0.70233–0.70382), which are consistent with rift-related carbonatites.The SP carbonatites yield high total REE contents (1768–1920 ppm) and are enriched in light REEs (LREEs) relative to heavy REEs (HREEs; LREE/HREE = 20.0–20.7). The results of Sr–Nd isotope analyses show that the SP carbonatite has high Sr and low Nd isotopic ratios with negative εNd(t) values ranging from − 1.71 to − 2.21, variable 207Pb/206Pb ratios, and anomalous Pb–Sr and Pb–Nd patterns, suggesting Pb contamination during ascent of the primary carbonatite melts. Values of δ18OSMOW and δ13CPDB (‰) for the SP carbonates vary from + 14.45‰ to + 18.12‰ and from − 6.57‰ to − 9.70‰, respectively, which can be attributed to low-temperature alteration.On the basis of data for bulk-rock trace elements and Sr–Nd–Pb–C–O isotopes, it is concluded that the Koga carbonatite–alkaline complex was formed during the rifting and break-up of the northern margin of Gondwanaland. In contrast, carbonatite of the SP carbonatite–alkaline complex was probably generated by partial melting of sub-continental lithospheric mantle in a post-collisional tectonic setting. Although the SP carbonatite contains abundant REEs, it does not host significant REE mineralization owing to the lack of a mature sequence of evolutionary phases and to possible phosphate mineral saturation in the primary magmas, which caused REE depletion in the later carbonatitic magmas through fractional crystallization. Therefore, we conclude that carbonatites that are relatively rich in apatite but poor in fluorite and barite may not be prospective for REE mineralization.

Inhomogeneous crust-mantle interaction and Triassic tectonic escape of a Proterozoic microplate: A tale of the Bikou Terrane

Compressional stress in collisional orogenic belts open a free boundary for the lateral extrusion of rigid blocks. Here we trace the history of tectonic escape of a Proterozoic microplate and the associated inhomogeneous crust-mantle processes from magmatic records in the Bikou Terrane, which is the largest microblock located among the North China Block, South China Block and Songpan-Ganzi Orogenic Belt. Zircon U-Pb geochronology indicates that the Triassic granitoid intrusions in the Bikou Terrane emplaced at 220–210 Ma. Petrographical, whole-rock geochemical and isotopical features suggest that the granitoids derived from the partial melting of lower crust, with inhomogeneous contribution of mafic magma derived from partial melting of subcontinental lithospheric mantle. Compared with the Yangba pluton in northeastern Bikou Terrane, the five intrusions in the southwestern Bikou Terrane display relatively lower REE concentrations, more negative Eu anomalies (δEu = 0.24–0.93), and lower Nb/Ta and Zr/Hf ratios with the values of 14–20 and 37–45. Furthermore, they display higher (87Sr/86Sr) i from 0.7050 to 0.7075 and lower εNd(t) from −9.4 to −4.7. These features imply that the granitic intrusions in southwestern Bikou Terrane were primary derived from the partial melting of the lower crust with lesser input of mantle materials than the Yangba pluton, indicating inhomogeneous crust-mantle interaction in the Bikou Terrane. We calculated the Moho depth of northeast and southwest Bikou Terrane as ~60 km and ~ 40–50 km, respectively. We propose that the Bikou Terrane escaped southwestward in Late Triassic following a transition in the stress regime of compression from NE-SW to NW-SE, in addition to the slab break-off from east to west. The lateral extrusion resulted in the inhomogenous crust-mantle interaction.

Zircon U-Pb ages, geochemistry, and Sr-Nd isotope ratios for early cretaceous magmatic rocks, southern Saqqez, northwestern Iran

Sanandaj-Sirjan Zone (SaSZ) is one of the most dynamic structural zones of Iran, which is divided into three main parts: Northern, Central and Southern. The northern SaSZ has been affected by deformation due to fault activities near the Zagros suture zone, and mylonitic structures have overprinted these rocks and was affected by three episodes of magma injection during the Permian-Carboniferous, Early Cretaceous and Cenozoic. In this study, the rock units investigated that have been considered Precambrian-Paleozoic basement on geological maps. This paper considers zircon U-Pb dating, whole-rock chemistry and Sr-Nd isotope ratios of Cretaceous magmatic rocks in the N-SaSZ to develop a new geodynamic model for the evolution of these magmatic rocks. The new zircon U-Pb ages obtained in this study show that the magmatic rocks crystallized at 115–107 Ma in the Early Cretaceous (Aptian-Albian) and are much younger than the supposed ages presented on geological maps. This complex classified into two main groups of basic-intermediate and acidic rocks based on SiO2 contents. The whole-rock chemistry of the basaltic and andesitic rocks, which are interbedded with marine shallow-water sedimentary deposits, shows their typical calc-alkaline affinity and subordinate tholeiitic series on an active margin. The positive εNd(t) of approximately +4 for some undifferentiated basalts with negative Ti and Nb anomalies shows the relation of these rocks to calc-alkaline magmatism and was generated by the partial melting of subcontinental lithospheric mantle (SCLM). Granitoid rocks with some affinity to the peraluminous group with a negative εNd(t) value (-3.2) mainly and negative Ti and Nb anomalies plot in an active margin tectonic setting. Simultaneous mafic calc-alkaline volcanism and the generation of granitic intrusions in the Early Cretaceous could have occurred on an active margin. Due to the absence of Jurassic arc related magmatic rocks in northern SaSZ and presence of Cretaceous calc alkaline magmatic activity, which are not observed in the central SaSZ, support the idea that the subduction of the Neotethys beneath the northern SaSZ started in the Early Cretaceous.

Zircon U-Pb, geochemical and isotopic constraints on the age and origin of A- and I-type granites and gabbro-diorites from NW Iran

The continental crust of NW Iran is intruded by Late Cretaceous I-type granites and gabbro-diorites as well as Paleocene A-type granites. SIMS and LA-ICPMS U-Pb analyses of zircons yield ages of 100–92 Ma (Late Cretaceous) for I-type granites and gabbro-diorites and 61–63 Ma (Paleocene) for A-type granites. Late Cretaceous gabbro-diorites (including mafic microgranular enclaves; MMEs) from NW Iran show variably evolved signatures. They show depletion in Nb and Ta on N-MORB-normalized trace-element spider-diagrams and have high Th/Yb ratios, suggesting their precursor magmas were generated in a subduction-related environment. Gabbro-diorites have variable zircon εHf(t) values of +1.2 to +8, δ18O of 6.4 to 7.4‰ and bulk rock εNd(t) of −1.4 to ~ +4.9. The geochemical and isotopic data attest to melting of subcontinental lithospheric mantle (SCLM) to generate near-primitive gabbros with radiogenic Nd isotopes (εNd(t) = ~ +4.9) and high Nb/Ta and Zr/Hf ratios, similar to mantle melts (Nb/Ta ~ 17 and Zr/Hf ~ 38). These mafic melts underwent further fractionation and mixing with crustal melts to generate Late Cretaceous evolved gabbro-diorites.Geochemical data for I-type granites indicate both Nb-Ta negative and positive anomalies along with enrichment in light REEs. These rocks are peraluminous and have variable bulk-rock εNd(t) (−1.4 to +1.3), zircon εHf(t) (+2.8 to +10.4) and δ18O (4.7–7.3‰) values, but radiogenic bulk rock Pb isotopes. The geochemical and isotopic signatures of these granites suggest interaction of mantle-derived mafic magmas (similar to near-primitive Oshnavieh gabbros) with middle-upper crust through assimilation-fractional crystallization (AFC) to produce Late Cretaceous I-type granites.Paleocene A-type granites have distinctive geochemical features compared to I-type granitoids, including enrichment in Nb-Ta, high bulk rock εNd(t) (+3.3 to +3.9) and zircon εHf(t) (+5.1 − +9.9) values. Alkaline granites are ferroan; they have low MgO, CaO, Sr, Ba and Eu concentrations and high total Fe2O3, K2O, Na2O, Al2O3, Ga, Zr, Nb-Ta, Th and rare earth element (REE) abundances and Ga/Al ratios. These rocks might be related to fractionation of a melt derived from a sub-continental lithospheric mantle, but which interacted with asthenosphere-derived melts.We suggest that subduction initiation and the resultant slab roll-back caused extreme extension in the overlying Iranian plate, induced convection in the mantle wedge and led to the decompression melting of SCLM. Rising mantle-derived magmas assimilated middle-upper crustal rocks. Fractionating mantle-derived magmas and contamination with crustal components produced evolved gabbro-diorites and I-type granites. In contrast, asthenospheric upwelling during the Paleocene provided heat for melting and interaction with SCLM to generate the precursor melts to the A-type granites.

Petrological and geochemical study of the Sylhet trap basalts, Shillong plateau, N.E. India: Implications for petrogenesis

Sylhet Traps exposed along the southern margin of Shillong plateau, Northeast India are subalkaline tholeiitic basalts. The basalts are generally massive but occasionally contain large amygdules of zeolites and chalcedony. Microscopically, some basalts show porphyritic texture with olivine phenocrysts. Phenocryst assemblage of plagioclase ± clinopyroxene ± olivine implies crystallization at shallow level. SEM-EDX analysis shows occurrences of spinel with Ni and Cr within the basalts therefore indicating partial melting of the subcontinental lithospheric mantle as the possible source materials for the basalts. The multi-element plot for the basalts shows two distinct trends: one with significant enrichment of LILE and depletion of HFSE and plot similar to OIB (Type 1) while the other trends are chara cterized by slight enrichment of LILE and negative anomalies at Nb, P and Ti (Type 2). Chondrite-normalized REE patterns for Type 1 basalt shows very high enrichment of LREE and a strong right dip HREE pattern and also plots similar to typical OIB while Type 2 show a slight enrichment of LREE over HREE with small Eu anomaly. The geochemical signatures suggest crustal contamination by plume-derived magma produced by low degree of partial melting for Type 1 basalt. Type 2 basalt was produced by partial melting of subcontinental lithospheric mantle, which may be triggered by plume upwelling.

Geochronological and geochemical studies of adakites from Tethyan Belt, Western Pakistan: A clue to geodynamics and Cu-Au mineralization

ABSTRACT Large porphyry Cu-Au deposits are associated with Early Miocene intrusive rocks in Tethyan belt, discovered along Chagai magmatic arc in Western Pakistan, adjacent to Southeast Iran. Two types of rocks were discriminated as granodiorite and monzodiorite from Saindak area. The granodiorites are associated with regional large Cu-Au mineralization, while the monzodiorites are mostly ore-barren. LA-ICP-MS U-Pb dating yielded weighted mean 206Pb/238U ages of 22.16–24.2 Ma for granodiorites and ca. 22.6 Ma for a monzodiorite. These intrusive rocks in the Saindak area are all calc-alkaline rocks, with enriched LILEs and depleted HFSEs, and without Eu negative anomalies. The felsic granodiorites are characterized by high Sr contents and Sr/Y ratios, with intermediate to high (La/Yb)N ratios that identify them as typical high-silica adakites. In contrast, the basaltic-andesitic monzodiorites are just normal arc-related rocks, showing less fractionated REE patterns than the granodiorites, with systematically lower LREE and higher MREE and HREE. Low K2O/Na2O ratios and decoupled Sr/Y-(La/Yb)N ratios indicate the characteristics of slab-derived adakites for the granodiorites, most likely originated through partial melting of the subducted Neotethys oceanic crust beneath Eurasian continent followed by subsequent mantle interaction. The almost simultaneously melting of subcontinental lithospheric mantle induced by dehydration of Neotethys plate gave rise to the formation of the barren monzodiorites. These intrusive rocks in the Saindak area are characterized by similar Sr-Nd-Pb isotopes with high 87Sr/86Sri ratios, slightly negative εNd(t) values and radiogenic Pb isotopes, plotting in the field between the MORB and EM-II mantle endmembers or the Average Cadomian Lower Crust, suggesting subducting sediments or old continental crustal materials have contributed into the compositions of these rocks by source enrichment or crustal contamination. The slab-melting derived adakite is favourable for regional massive Cu-Au mineralization in the Saindak area.

Timing and origin of silicic volcanism in northwestern Mexico: Insights from zircon U-Pb geochronology, Hf isotopes and geochemistry of rhyolite ignimbrites from Palmarejo and Guazapares in Southwest Chihuahua

Precise timing and constraints for magma source(s) are critical to understand the post-subduction tectonomagmatic evolution of the world's largest Cenozoic Silicic Large Igneous Province in western Mexico. We provide in-situ zircon U–Pb ages and Hf isotope compositions of rhyolitic ignimbrites exposed in the Palmarejo and Guazapares at the northwestern extremity of Sierra Madre Occidental. Four rhyolite ignimbrite samples from Palmarejo yielded zircon weighted mean U–Pb ages between 23 and 24 Ma. However, five individual analyses from one sample yielded the oldest age of 25.09 ± 0.41 Ma (MSWD = 1.7, n = 5). A rhyolitic tuff from the Guazapares district yielded older individual dates varying from 25.7 to 28.6 Ma with a main population yielding a weighted mean age of 27.31 ± 0.36 Ma. Zircon U–Pb ages suggest that the ignimbrite flare-up in southwestern Chihuahua remained restricted to the Early Middle Oligocene to Early Miocene. We suggest that the zircon U–Pb dates >24 Ma (particularly from Guazapares) are the antecrystic zircons included from the underlying older ignimbrites. The zircon U–Pb dates of 25–28 Ma are related to the culmination of syn-extensional first ignimbrite flare-up and bimodal volcanism started as early as 35 Ma. The first ignimbrite flare-up is possibly resulted from the detachment of the leading edge of the Farallon slab around 35–28 Ma. The younger age of 24–23 Ma from Palmarejo can be correlated to the second episode of ignimbrite flare-up erupted during the pervasive regional extension, normal faulting and onset of formation of metamorphic core complexes. This second flare-up is resulted by the subsequent events of break off/rollback of the Farallon plate. The positive Hf isotope composition (εHf (t) = +1.0 to +5.8) and concomitant mafic volcanism suggest partial melting of subcontinental lithospheric mantle and the overlying Laramide crust by upwelling of decompressing asthenosphere during post-Laramide regional extension. This interpretation is consistent with the successive detachments, fragmental removal and/or rollback of the leading edges of the Farallon slab beneath the western continental margin of North America. We did not find any xenocrystic zircon inheritance neither from the Laramide plutonic basement/roots of the Jurassic Guerrero terrane nor from the Proterozoic North American crust. However, zircon inheritance of Laramide age has been reported from the Guazapares district. Partial melting of Laramide basement was possibly facilitated by the heat advection and ponding of subcontinental lithospheric mantle-related magmas in the crust during the Cenozoic regional extension.

The nature and evolution of mantle upwelling at Ross Island, Antarctica, with implications for the source of HIMU lavas

The multiple, proximal, young and/or active volcanic centers of Ross Island, Antarctica, provide a unique opportunity to investigate both deep and shallow processes of alkaline magma genesis and the length scales of mantle heterogeneity. Ross Island, Antarctica is an assembly of four silica-undersaturated alkaline volcanic centers, including the active phonolitic Erebus volcano (1.2 to 0 Ma). Mt. Terror, Mt. Bird, and Hut Point Peninsula surround Erebus on the periphery of Ross Island and are mostly older (∼0.3 to 4 Ma) and mainly basanitic in composition. While the geochemical compositions and HIMU isotopic signature of Erebus lavas are well characterized, the geochemistry of the peripheral volcanic centers was, until this study, poorly known. To further investigate the nature of mantle sources and magmatic processes beneath Ross Island, we therefore measured major and trace element and Sr, Nd, Hf, and Pb isotope compositions on fifty-seven samples from the three volcanoes peripheral to Erebus volcano and compared the results to previous geochemical and isotopic studies of the latter. Mt. Terror, Mt. Bird, and Hut Point Peninsula have 87Sr/86Sr ranging from 0.702907 to 0.703147, εNd ranging from +4.3 to +6.3, εHf ranging from +5.6 to +8.6, and 206Pb/204Pb ranging from 19.282 to 20.241. The Sr, Nd, Hf, and Pb isotope compositions of all four volcanoes (Erebus, Terror, Bird, and Hut Point Peninsula) fall on a mixing line between the HIMU and DMM mantle end-members, with a minor contribution from an EM component. Small, but distinct differences in the isotopic compositions of the four volcanoes, most notably Mt. Bird, imply mantle source heterogeneity beneath Ross Island on a length scale comparable to or smaller than inter-volcano distances (i.e., less than 50 km). The major and trace element systematics and isotopic signatures of the Ross Island volcanic samples are distinct from those for samples from Zealandia and older Antarctic samples that typify magmas from the Cenozoic alkaline magmatic province. The chemical and isotopic compositions of the Ross Island lavas and tephras are best explained by small-degree melting of an ancient asthenospheric source containing residual garnet. Additionally, recent global tomographic modeling indicates low shear wave velocity anomalies with depths down to ∼1200 km beneath Ross Island. This suggests a deep-seated upwelling plume impinging on shallower metasomatized mantle which has been gradually eroded away and is no longer a potential source of Ross Island magmatism. Rather, the source of Ross Island lavas is old (Archean to early Proterozoic) material upwelling from the deep mantle as opposed to melting of subcontinental lithospheric mantle metasomatized by Paleozoic subduction. While our findings have local significance in terms of understanding Ross Island volcanism and providing a new perspective on the evolution of volcanism in the surrounding region during the Phanerozoic, they also have important global implications for gauging the nature, age, and evolution of the HIMU mantle source.

Zircon U-Pb dating, geochemistry and evolution of the Late Eocene Saveh magmatic complex, central Iran: Partial melts of sub-continental lithospheric mantle and magmatic differentiation

The Saveh magmatic complex is a composite intrusion of diorite to tonalite. It is part of the Urumieh-Dokhtar magmatic arc in central Iran. Zircon UPb dating indicates that emplacement and cooling occurred 37–40 Ma ago in late Eocene (Bartonian) time. Saveh igneous rocks define a continuous mafic-felsic continuum in terms of SiO2 (55.4–70.8 wt%), Al2O3 (14.2–19.8 wt%) and CaO (6.4–17.1 wt%) contents. The initial values of 87Sr/86Sr and ɛNd(t) are 0.7048 to 0.7062 and + 2.6 to +3.4 (Mean = +2.9). Nd model ages (TDM = 555 to 616 Ma) are consistent with derivation from Iranian continental crust and subcontinental lithospheric mantle of Cadomian age (500–600 Ma). Saveh igneous rocks are enriched in K, Cs and Pb and depleted in P, Nb and Ta, indicating that the magma source was moderately affected by slab-derived fluids, as expected for the Eocene tectonic environment. The continuum of rock types from diorite to graphic eutectic tonalite indicates that magmatic evolution was controlled by the F (FeO + MgO + MnO)-Anorthite (An)-Orthoclase (Or) cotectic with minor contamination by supra-crustal components. Similar initial ratios of 87Sr/86Sr and 143Nd/144Nd for all of the rocks and the slightly younger age of tonalitic rocks (2–3 Ma younger) further indicate that the crystallization of quartz and plagioclase with graphic texture occurred in the final stage of magmatic differentiation. Saveh magmas reflected partial melting of Iranian subcontinental lithospheric mantle due to extension above the Neo-Tethys subduction zone.

Nature and origin of Mesozoic granitoids and associated mineralization in the Sanjiang Tethys Orogeny, SW China: the Xiuwacu complex example

ABSTRACTThe magmatic generation for the Late Triassic–Early Jurassic (~215–200 Ma) and Early Cretaceous–Late Cretaceous (~108–79 Ma) post-collisional granites in the Sanjiang Tethys orogeny remain enigmatic. The Xiuwacu complex, located in the southern Yidun Terrane, consists of biotite granite with a weight mean 206Pb/238U age of 199.8 ± 2.5 Ma, aplite granite of 108.2 ± 2.3 Ma, monzogranite porphyry of 80.8 ± 1.0 Ma, and diorite enclaves of 79.2 ± 0.9 Ma and 77.9 ± 0.8 Ma. The Late Triassic biotite granites show I-type granite affinities, with high SiO2 contents, high Mg# values, high zircon δ18O values, and negative whole-rock ƐNd(t) values, indicating a predominant ancient crustal source with the input of juvenile materials. Their fractionated REE patterns and concave-upward middle-to-heavy REE patterns require garnet-bearing amphibolite as the melt source. The Cretaceous highly fractionated aplite granites and monzogranite porphyries have relatively high SiO2 contents, high (Na2O + K2O)/CaO ratios, high zircon δ18O values, and enriched whole-rock Sr–Nd isotopic signatures, suggesting that their parent magmas were likely originated from the ancient middle- to lower crust. Their significant negative Eu anomalies and obvious depletions in Nb, Sr, and Ti demonstrate that the Cretaceous granitic magmas had experienced more fractionation than the Late Triassic felsic magmas. The Late Cretaceous diorite enclaves show low SiO2 contents, high Mg# values, and high zircon δ18O values, suggesting that they were probably derived from the partial melting of subcontinental lithospheric mantle enriched by the Late Triassic subduction. The Late Triassic–Early Jurassic and Early Cretaceous–Late Cretaceous magmatism witnessed the post-collisional setting and intraplate extensional setting in response to the slab break-off and lithospheric-scale transtensional faulting, respectively. The partial melting of subduction-modified lithospheric mantle or/and residual sulphide cumulates within the lower crust during the origination of Late Cretaceous magmas could have provided metals for the formation of Xiuwacu deposit.

Lithium content and isotopic composition of the juvenile lower crust in southern Tibet

The concentrations and isotopic geochemistry of Li are potentially useful geochemical tracers of geological processes. To fully utilize Li isotopes as geochemical tracers, it is necessary to characterize the Li isotopic compositions of the various geological reservoirs. However, the Li isotopic composition of the juvenile lower crust is currently poorly constrained. Given that lithospheric architecture of the Tibetan Plateau includes Indian upper/lower crust, Tibetan upper/lower crust and juvenile lower crust, it is necessary to determine the Li isotopic composition for each geological endmember underneath southern Tibet. Among them, the juvenile lower crust was formed directly by underplating of mantle-derived basaltic magma, which is likely to be the critical factor to control the Cu-Au mineralization in southern Tibet and is responsible for crustal thickening beneath southern Tibet. Here, we report the Li concentration and isotopic composition of the juvenile lower crust in southern Tibet. Based on whole-rock major element, trace element, and Sr–Nd–Pb isotopic data, we infer that the Yeba basalts and Gangdese gabbros were derived from partial melting of metasomatized lithospheric mantle, and have compositions similar to the juvenile lower crust. In contrast, the Dianzhong andesites and Gangdese diorites originated from partial melting of the juvenile lower crust. Therefore, these units may be considered representative of the juvenile lower crust. The juvenile lower crust has Li concentrations of 7.1–37.2ppm (mean=15.4ppm), consistent with the Li concentration for the lower crust (13ppm). Li isotopic compositions (δ7Li) vary from +0.8‰ to +6.6‰ (mean=3.0‰), similar to values for the EMI/EMII mantle. The Li isotopic compositions of the analyzed samples were not significantly affected by alteration, metamorphism, crustal assimilation, or magmatic differentiation, and therefore represent the isotopic compositions of the juvenile lower crust. The Li systematics of the juvenile lower crust may be attributed to partial melting of subcontinental lithospheric mantle that has undergone metasomatism by Li-rich fluids derived from subducted oceanic crust and marine sediments. Our study also demonstrates near-identical Li isotopic compositions for juvenile lower crust and metasomatized lithospheric mantle, resulting from the lack of Li isotope fractionation during basalt generation and differentiation.

Devonian alkaline magmatic belt along the northern margin of the North China Block: Petrogenesis and tectonic implications

Some Devonian magmatic rocks have been identified from the northern margin of the North China Block (NCB) in recent years. However, their petrogenesis and tectonic setting are still highly controversial. Here we present new geochronological, Sr-Nd-Hf isotopic and whole-rock chemical data on several newly identified and previously reported Devonian alkaline complexes, including mafic-ultramafic rocks (pyroxenites and gabbros), alkaline rocks (syenites, monzonites) and alkaline granites in the northern NCB. We firstly identified some mafic-ultramafic rocks coeval with monzonite and quartz monzonite in the Sandaogou and Wulanhada alkaline intrusions. New zircon U-Pb dating of 16 samples from the Baicaigou, Gaojiacun, Sandaogou, Wulanhada and Chifeng alkaline intrusions combined with previous geochronological results indicate that the Devonian alkaline rocks emplaced during the early-middle Devonian at around 400–380 Ma and constitute an E–W-trending alkaline magmatic belt that extend ca. 900 km long along the northern margin of the NCB. Whole-rock geochemical and Sr-Nd-Hf isotopic data reveal that the Devonian alkaline rocks were mainly originated from partial melting of a variably enriched lithospheric mantle with different involvement of ancient lower crustal component and fractional crystallization. The Devonian alkaline magmatic belt rocks in the northern NCB are characterized by very weak or no deformations and were most likely related to post-collision extension after arc-continent collision between the Bainaimiao island arc and the northern margin of North China Craton during the latest Silurian. Partial melting of subcontinental lithospheric mantle to produce the Devonian alkaline magmatic rocks suggests that the northern North China Craton has an inhomogeneous, variably enriched subcontinental lithospheric mantle and was characterized by significant vertical crustal growth during the Devonian period.