Phase equilibrium constraints on a deep crustal metamorphic field gradient: metapelitic rocks from the Ivrea Zone (NW Italy)

The metamorphic rocks of the Ivrea Zone in NW Italy preserve a deep crustal metamorphic field gradient. Application of quantitative phase equilibria methods to metapelitic rocks provides new constraints on the P–T conditions recorded in Val Strona di Omegna, Val Sesia and Val Strona di Postua. In Val Strona di Omegna, the metapelitic rocks show a structural and mineralogical change from mica‐schists with the common assemblage bi–mu–sill–pl–q–ilm ± liq at the lowest grades, through metatexitic migmatites (g–sill–bi–ksp–pl–q–ilm–liq) at intermediate grades, to complex diatexitic migmatites (g–sill–ru–bi–ksp–pl–q–ilm–liq) at the highest grades. Partial melting in the metapelitic rocks is consistent with melting via the breakdown of first muscovite then biotite. The metamorphic field gradient in Val Strona di Omegna is constrained to range from conditions of ～3.5–6.5 kbar at ≈650 °C to ～10–12 kbar at >900 °C. The peak P–T estimates, particularly for granulite facies conditions, are significantly higher than those of most earlier studies. In Val Sesia and Val Strona di Postua, cordierite‐bearing rocks record the effects of contact metamorphism associated with the intrusion of a large mafic body (the Mafic Complex). The contact metamorphism occurred at lower pressures than the regional metamorphic peak and overprints the regional metamorphic assemblages. These relationships are consistent with the intrusion of the Mafic Complex having post dated the regional metamorphism and are inconsistent with a model of magmatic underplating as the cause of granulite facies metamorphism in the region.     

青藏高原东南缘哀牢山构造带泥质高压麻粒岩的发现及其构造意义

哀牢山构造带泥质高压麻粒岩主要由石榴石、夕线石、钾长石和斜长石变斑晶及尖晶石、铁假蓝宝石、蓝晶石、石英、金红石和钛铁矿包裹体组成,为确定印支地块和华南地块的边界提供了关键性标志。石榴石-黑云母-斜长石-石英地质温压计(GBPQ)计算结果及标志性高温矿物组合(Spl+Qz)表明泥质高压麻粒岩的形成和演化经历了高压/高温进变质到中温/低压退变质的顺时针P-T演化过程。其中:1)高压/高温进变质阶段的矿物组合为Ky+Sil+Grt1+Kf1+Pl1+Spr+Ter(Kf+Pl)+Bt1+Spl+Qtz+Ilm1+Rut1,形成于850～919℃,≥10.4kbar;2)中温/低压退变质阶段的矿物组合为Grt2+Bt2+Pl2+Ms+Qtz+Ilm2+Rut2,早期和晚期的温压条件分别为664～754℃,4.9～6.5kbar和572～576℃,3.5～3.9kbar。反映陆壳物质在碰撞过程中俯冲到地下深处(≥30km)经高压高温变质后快速折返到中上地壳的动力学演变轨迹。     

Complex geometry of the Cenozoic magma plumbing system in the central Sahara,NW Africa

Topographic uplifts in the central Sahara occur in the Hoggar-Aïr and Tibesti-Gharyan swells that consist of Precambrian rocks overlain by Cenozoic volcanic rocks. The swells and associated Cenozoic volcanism have been related either to mantle plumes or to asthenospheric upwelling and to partial melting due to rift-related delamination along pre-existing Pan-African mega-shears during the collision between Africa and Europe. The Cenozoic volcanic rocks in the Hoggar generally range from Oligocene tholeiitic/transitional plateau basalts, which occur in the centre of the dome, to Neogene alkali basalts characterized by a decrease in their degree of silica undersaturation and an increase in their La/Yb ratios. The alkali basaltic rocks occur mainly along the margins of the dome and typically have less radiogenic Nd and Sr isotopic ratios than the tholeiitic/transitional basalts. The geochemistry of the most primitive basaltic rocks resembles oceanic island basalt (OIB) tholeiitic – in particular high-U/Pb mantle (HIMU)-type – and is also similar to those of the Circum-Mediterranean Anorogenic Cenozoic Igneous (CiMACI) province. These characteristics are consistent with, but do not require, a mantle plume origin. Geophysical data suggest a combination of the two mechanisms resulting in a complex plumbing system consisting of (a) at depths of 250–200 km, an upper mantle plume (presently under the Aïr massif); (b) between 200 and 150 km, approximately 700 km northeastward deflection of plume-derived magma by drag at the base of the African Plate and by mantle convection; (c) at approximately 150 km, the magma continues upwards to the surface in the Tibesti swell; (d) at approximately 100 km depth, part of the magma is diverted into a low S-wave velocity corridor under the Sahara Basin; and (e) at approximately 80 km depth, the corridor is tapped by Cenozoic volcanism in the Hoggar and Aïr massifs that flowed southwards along reactivated Precambrian faults.     

Geochronologic-petrochemical studies of the Hongshishan mafic–ultramafic intrusion,Beishan area,Xinjiang (NW China): petrogenesis and tectonic implications

The Hongshishan mafic–ultramafic intrusion (SIMS zircon U–Pb age 286.4 ± 2.8 Ma) consists of dunite, clinopyroxene peridotite, troctolite, and gabbro. Major elements display systematic correlations. Trace elements have identical distribution patterns, including flat rare-earth element (REE) patterns with positive Eu anomalies and enrichments in large ion lithophile elements (LILE) but depletions in Nb and Ta, indicating fractional crystallization as a key factor in magmatic evolution. Petrologic and geochemical variations in drill core samples demonstrate that minor assimilation and progressive magma injections were closely associated with Ni–Cu mineralization. Mass balance estimates and Sr–Nd isotopes reveal that the Hongshishan parental magmas were high-Mg and low-Ti tholeiitic basalts and were derived from a lithospheric mantle source that had been modified by subducted slab metasomatism before partial melting.

Southward subduction of the Palaeo-Tianshan–Junggar Ocean is further constrained by a compilation of inferred, subduction-induced modifications of mantle sources in mafic–ultramafic intrusions distributed in the eastern Tianshan–Beishan area. Integrating the regional positive ?_Nd(t) granites, high-Mg and low-Ti basaltic magmas (mafic–ultramafic intrusions), and slightly later high-Ti basalts in NW China suggests that their petrogenesis could be attributed to Permian mantle plume activities.     

Timing of granitic magma mixing in the southeastern Gyeongsang Basin,Korea: SHRIMP-RG zircon data

Late Cretaceous calc-alkaline granites in the Gyeongsang Basin evolved through the mixing of mafic and felsic magmas. The host granites contain numerous mafic magmatic/microgranular enclaves of various shapes and sizes. New SHRIMP-RG zircon U–Pb ages of both granite and mafic magmatic/microgranular enclaves are 75.0?±?0.5 Ma and 74.9?±?0.6 Ma, respectively, suggesting that they crystallized contemporaneously after magma mixing. The time of injection of mafic melt into the felsic magma chamber can be recognized as approximately 75 Ma by field relations, petrographic features, geochemical evolution, and SHRIMP-RG zircon dating. This Late Cretaceous magma mixing event in the Korean Peninsula was probably related to the onset of subduction of the Izanagi (Kula)–Pacific ridge.     

Geochemical and isotopic constraints on the age and origin of mafic dikes from eastern Shandong Province,eastern North China Craton

Post-orogenic mafic dikes are widespread across eastern Shandong Province, North China Craton, eastern China. We here report new U–Pb zircon ages and bulk-rock geochemical and Sr–Nd–Pb isotopic data for representative samples of these rocks. LA-ICP-MS U–Pb zircon analysis of two mafic dike samples yields consistent ages of 118.7 ± 0.25 million years and 122.4 ± 0.21 million years. These Mesozoic mafic dikes are characterized by high (⁸⁷Sr/⁸⁶Sr) _i ranging from 0.7082 to 0.7087, low ?_Nd(t) values from??17.0 to??17.5, ²⁰⁶Pb/²⁰⁴Pb from 17.14 to 17.18, ²⁰⁷Pb/²⁰⁴Pb from 15.44 to 15.55, and ²⁰⁸Pb/²⁰⁴Pb from 37.47 to 38.20. Our results suggest that the parental magmas of these dikes were derived from an ancient, enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials prior to magma generation. The mafic dikes underwent minor fractionation during ascent and negligible crustal contamination. Combined with previous studies, these findings provide additional evidence that intense lithospheric thinning beneath eastern Shandong occurred at ～120 Ma, and that this condition was caused by the removal/foundering of the lithospheric mantle and lower crust.     

Petrochemistry of Mesozoic mafic intrusions related to the Paraná Magmatic Province,Uruguay

The Cuaró Formation is part of the sequence of Mesozoic mafic intrusions related to the Early Cretaceous break-up of Gondwana and represents the southernmost occurrences within the Paraná Magmatic Province in Uruguay. We present field data, petrography and lithogeochemical results regarding these dike swarms and sills that crop out in the southern extreme of the Paraná Basin. Dolerites and sills mainly exhibit glomeroporphyritic textures; the phenocrysts consist of plagioclase, clinopyroxene, relicts of olivine and titaniferous magnetite. Bulk-rock geochemical analyses allowed their classification as low-Ti subalkaline tholeiitic basalts and andesitic basalts. Trace element data indicated that the protoliths of these intrusions include the subcontinental lithospheric mantle, as is generally recognized for other Gondwana-related continental flood basalt provinces.     

Early subduction–exhumation and late channel flow of the Greater Himalayan Sequence: implications from the Yadong section in the eastern Himalaya

Based on metamorphic studies of the Yadong high-pressure (HP) granulite and multiple thermochronological investigations of granitoids from both upper and lower parts, the Yadong section in the eastern Himalaya constrains the Cenozoic tectonic evolution of the Greater Himalayan Sequence (GHS). The Yadong HP granulite, located at the top of the GHS, underwent a peak-stage HP granulite facies metamorphism and two stages of retrograde metamorphism. Granulite and hornblende facies retrograde metamorphism took place at 48.5 and 31.8 Ma, respectively, marking the time of exhumation of the subducted Indian slab to lower and middle crustal levels. Subsequently, an average young zircon U–Pb age obtained from the Yadong HP granulite indicated that this unit was captured by its surroundings in a partially molten condition at 16.9 Ma. In addition, three granitoids from both the lower and the upper parts of the GHS yielded biotite ⁴⁰Ar/³⁹Ar ages of 11.0, 11.3, and 11.5 million years. These consistent ages suggest that the GHS along the Yadong section was laterally extruded and synchronously cooled to ～300°C at ～11.3 Ma. Furthermore, the granitic gneisses yield apatite fission track ages of ～7 million years, documenting the cooling of the GHS to ～110°C. A two-stage model describes the Cenozoic tectonic evolution of the GHS: (1) the Indian slab had subducted under Tibet before ～55 Ma, and was exhumed to the lower crust (50-40 km) at 48.5 Ma, and to the middle crust (22-15 km) at 31.8 Ma; and (2) the partial melting occurred at middle crustal levels during the period 31.8 to 16.9 Ma, causing channel flow. In the late stage, the GHS was laterally extruded by ductile mid-crustal flow during the period 16.9 to ～7 Ma, characterized by a fast cooling rate of ～2 mm per year.     

Geochronology,geochemistry, Hf isotopic compositions and formation mechanism of radial mafic dikes in northern Tibet

A large mafic dike swarm is radially distributed in southern Qiangtang. Three typical samples were selected for geochronology, geochemistry, and Hf isotopic analysis. Zircon U–Pb dating indicates that the three dikes formed at 291 ± 2, 292 ± 3, and 300 ± 2 Ma. Whole-rock compositions show that the southern Qiangtang mafic dikes are alkaline, Fe + Ti rich, and exhibit relative enrichment in light rare-earth elements. The ratios of incompatible elements are similar to those of oceanic island and Emeishan basalts. Geochemical diagrams show that the dikes erupted in an intraplate environment. Zircon Hf isotopic data suggest that magma that produced the mafic dikes was derived from a depleted mantle source. The geochemical characteristics of the dikes approximate that of eruption products of a brief period of mantle plume activity (300–280 Ma). According to eight geologic maps of Qiangtang, the mafic dikes crop out over an area of 150 km from north to south and 500 km from east to west, radiating outward from Mayigangri. We conclude that the mafic dikes in southern Qiangtang are related to the combined effect of Permian plate motions and mantle plume activity, and the Mayigangri area overlies the hot spot. Furthermore, the mantle plume in southern Qiangtang may have propelled the closing of the Palaeo-Tethys Ocean.     

Geochemistry of Permian Mafic Igneous Rocks from the Napo‐Qinzhou Tectonic Belt in Southwest Guangxi,Southwest China: Implications for Arc‐Back Arc Basin Magmatic Evolution

The Napo-Qinzhou Tectonic Belt (NQTB) lies at the junction of the Yangtze, Cathaysia and Indochina (North Vietnam) Blocks, which is composed of five major lithotectonic subunits: the Qinzhou-Fangcheng Suture Zone (QFSZ), the Shiwandashan Basin (SB), the Pingxiang-Nanning Suture Zone (PNSZ), the Damingshan Block (DB) and the Babu-Lingma Suture Zone (BLSZ). On the basis of geochemical compositions, the Permian mafic igneous rocks can be divided into three distinct groups: (1) mafic igneous rocks (Group 1) from the Longjing region in the PNSZ and Hurun region in the BLSZ, which are characterized by intermediate Ti, P and Zr with low Ni and Cr contents; (2) mafic igneous rocks (Group 2) from the Naxiao and Chongzuo region in the DB, characterized by low-intermediate Ti, P and Zr with high Ni and Cr concentrations; and (3) mafic igneous rocks (Group 3) from the Siming region in the Jingxi carbonate platform of the northwestern margin of the NQTB, with intermediate-high Ti, P and Zr and low Ni and Cr contents. The Group 1 rocks yield a weighted mean 206Pb/238U age of 250.5±2.8 Ma and are geochemically similar to basalts occurring in back-arc basin settings. The Group 2 rocks exhibit geochemical features to those basalts in island arcs, whereas the Group 3 rocks show geochemical similarity to that of ocean island basalts. All three groups are characterized by relatively low εNd(t) values (–2.61 to +1.10) and high initial 87Sr/86Sr isotopic ratios (0.705309–0.707434), indicating that they were derived from a subduction-modified lithospheric mantle and experienced assimilation, fractional crystallization, and crustal contamination or mixing during magmatic evolution. Accordingly, we propose the existence of an arc-back arc basin system that developed along the NQTB at the border of SW Guangxi Province (SW China) and northern Vietnam, and it was formed by continued northwestward subduction of the Cathaysian (or Yunkai) Block under the Yangtze Block, and northeastward subduction of the Indochina Block beneath the Yangtze Block during Permian time.