GENERALIZED FLOW LAWS OF POLYPHASE ROCKS: AN OVERVIEW

Rheological properties of polyphase rocks play an important role in the dynamics of the lithosphere and asthenosphere. However, such fundamental issues have not been well resolved. A theoretical analysis has been made to develop expressions for the flow laws of polyphase rocks in terms of the volume fractions and flow laws of their constituent phases. The flow strengths predicted by the proposed model for common crustal and upper mantle rocks such as granite, diorite, diabase and lherzolite, and for synthetic two-phase composites such as forsterite-enstatite and water ice-ammonia dehydrate aggregates are in good agreement with previously determined experimental values. The proposed theoretical model allows one to calculate, to the first approximation, the flow laws of a large number of polyphase rocks at geologic conditions based on the experimentally determined flow laws of a relatively small number of monomineralic aggregates.     

GENERALIZED FLOW LAWS OF POLYPHASE ROCKS: AN OVERVIEW

Rheological properties of polyphase rocks play an important role in the dynamics of the lithosphere and asthenosphere. However, such fundamental issues have not been well resolved. A theoretical analysis has been made to develop expressions for the flow laws of polyphase rocks in terms of the volume fractions and flow laws of their constituent phases. The flow strengths predicted by the proposed model for common crustal and upper mantle rocks such as granite, diorite, diabase and lherzolite, and for synthetic two-phase composites such as forsterite-enstatite and water ice-ammonia dehydrate aggregates are in good agreement with previously determined experimental values. The proposed theoretical model allows one to calculate, to the first approximation, the flow laws of a large number of polyphase rocks at geologic conditions based on the experimentally determined flow laws of a relatively small number of monomineralic aggregates.     

Rheological and kinematical responses to flow of two-phase rocks

Numerical simulations have been performed to investigate the strain-dependent behaviour of rheological and kinematical responses to flow of two-phase rocks using the commercial finite-difference program FLAC2D. It was assumed that the two phases have Maxwell rheology. Plane strain and velocity boundary condition, which produces a simple shear deformation, were also assumed. Two types of geometries were considered: strong phase supported (SPS) and weak phase supported (WPS). We calculated strain-dependent variations of effective viscosity and partitioning of strain rate, vorticity and kinematic vorticity number during deformation in both SPS and WPS structure models.The results show that the strain-dependent behaviour is largely influenced by the geometry of the composite. SPS models show both strain hardening and strain softening during the simulations, with strain hardening preceding strain softening. A critical shear strain is necessary to begin the strain softening behaviour. Strain hardening and strain softening are accompanied by a reduction and an increase of the partition of strain rate into the weak phase, respectively. On the other hand, WPS models show only weak strain hardening and strain softening, being the strain-dependent behaviour close to a steady state flow. In addition, the following results are obtained on vorticity and kinematic vorticity number; (1) in both SPS and WPS models the partition of vorticity into weak phase increases with progressive shear strain, i.e. the strong phase becomes less rotational, (2) in SPS models weak inclusions changes from sub-simple shear to super-simple shear with progressive strain, whereas the strong matrix changes from super-simple shear to sub-simple shear, (3) in WPS models the strong inclusions with high viscosity contrasts are less rotational but can be in super-simple shear condition to high strains.The observed strain-dependent behaviours have been compared with previous proposed analytical models. The degree of agreement is variable. Balshin and Ryshkewitch–Duckworth models are only applicable to SPS models. Ji-generalized mixture rule model is applicable to both models.The results suggest that polyphase rocks with SPS structure during ductile shear deformation respond as strain softening materials, after an initial strain hardening stage that may drive to the strain localization into the material.     

Reconstruction of deformation and flow parameters from deformed vein sets

In any homogeneously deformed material, sets of material lines with a similar stretch history occupy sectors with a scale and orientation that depend on finite strain, area change rate and vorticity of the parent flow. If the size and shape of these “material line sectors” are known, they can be used to determine the sense of shear and to calculate deformation and flow parameters, even if actual stretches along material lines are unknown. In rocks which have undergone near-homogeneous deformation on a large scale, the orientation of sets of boudinaged folded and folded-boudinaged or boudinaged-folded veins can be used for such calculations. A general theoretical background to the problem is presented and a Mohr circle construction to carry out the deformation and flow analysis is introduced. Among other statements, the theory predicts that shortened or folded boudins must develop in non-coaxial monophase deformation, with considerable area change in the plane of observation, or in polyphase deformation.     

Alpine polyphase tectono-metamorphic evolution of the South Carpathians: A new overview

The main terrains involved in the Cretaceous–Tertiary tectonism in the South Carpathians segment of the European Alpine orogen are the Getic–Supragetic and Danubian continental crust fragments separated by the Severin oceanic crust-floored basin. During the Early–Middle Cretaceous times the Danubian microplate acted initially as a foreland unit strongly involved in the South Carpathians nappe stacking. Multistage folding/thrusting events, uplift/erosion and extensional stages and the development of associated sedimentary basins characterize the South Carpathians during Cretaceous to Tertiary convergence and collision events. The main Cretaceous tectogenetic events responsible for contraction and crustal thickening processes in the South Carpathians are Mid-Cretaceous (“Austrian phase”) and Latest Cretaceous (“Laramide” or “Getic phase”) in age. The architecture of the South Carpathians suggests polyphase tectonic evolution and mountain building and includes from top to bottom: the Getic–Supragetic basement/cover nappes, the Severin and Arjana cover nappes, and Danubian basement/cover nappes, all tectonically overriding the Moesian Platform. The Severin nappe complex (including Obarsia and Severin nappes) with Late Jurassic–Early Cretaceous ophiolites and turbidites is squeezed between the Danubian and Getic–Supragetic basement nappes as a result of successive thrusting of dismembered units during the inferred Mid- to Late Cretaceous subduction/collision followed by tectonic inversion processes.

Early Cretaceous thick-skinned tectonics was replaced by thin-skinned tectonics in Late Cretaceous. Thus, the former Middle Cretaceous “Austrian” nappe stack and its Albian–Lower Senonian cover got incorporated in the intra-Senonian “Laramide/Getic” stacking of the Getic–Supragetic/Severin/Arjana nappes onto the Danubian nappe duplex. The two contraction events are separated by an extensional tectonic phase in the upper plate recorded by the intrusion of the “Banatitic” magmas (84–73 Ma). The overthrusting of the entire South Carpathian Cretaceous nappe stack onto the fold/thrust foredeep units and to the Moesian Platform took place in the Late Miocene (intra-Sarmatian) times and was followed by extensional events and sedimentary basin formation.     

Carbon–oxygen isotope and trace element constraints on how fluids percolate faulted limestones from the San Andreas Fault system: partitioning of fluid sources and pathways

Understanding the way fluids flow in fault zones is of prime importance to develop correct models of earthquake mechanics, especially in the case of the abnormally weak San Andreas Fault (SAF) system. Because fluid flow can leave detectable signatures in rocks, geochemistry is essential to bring light on this topic. The present detailed study combines, for the first time, C–O isotope analyses with a comprehensive trace element data set to examine the geometry of fluid flow within a significant fault system hosted by a carbonate sequence, from a single locality across the Little Pine Fault–SAF system. Such a fault zone contains veins, deformation zones, and their host rocks. Stable isotope geochemistry is used to establish a relative scale of integrated fluid–rock ratios. Carbonate δ¹⁸O varies between 28‰ and 15‰ and δ¹³C between 5‰ and −7‰. From highest to lowest delta values, thus from least to most infiltrated, are the host rocks, the vein fillings, and the deformation zone fillings, respectively. Infiltration increases toward fault core. The fluids are H₂O–CO₂ mixtures. Two fluid sources, one internal and the other external, are found. The external fluid is inferred to come essentially from metamorphism of the Franciscan formation underneath. The internal (local) fluid is provided by a 30% volume reduction of the host limestones resulting from pressure solution and pore size reduction. Most trace elements, including the lanthanides, show enrichment at the 100-m scale in host carbonate rocks as fluid–rock ratios increase toward the fault core. In contrast, the same trace element concentrations are low, relative to host rocks, in veins and deformation zone carbonate fillings, and this difference in concentration increases as fluid–rock ratio increases toward the fault core. We suggest that the fluid trace elements are scavenged by complexation with organic matter in the host rocks. Elemental complexation is especially illustrated by large fractionation of Y–Ho and Nb–Ta geochemical pairs. Complexation associated with external fluid flow has a significant effect on trace element enrichment (up to 700% relative enrichment) while concentration by pressure solution associated with volume decrease of host rocks has a more limited effect (up to 40% relative enrichment). Our observations from the millimeter to the kilometer scale call for the partitioning of fluid sources and pathways, and for a mixed focused–pervasive fluid flow mechanism. The fluid mainly flows within veins and deformation zones and, simultaneously, within at least 10 cm from these channels, part of the fluid flows pervasively in the host rock, which controls the fluid composition. Scavenging of the fluid rare earth elements (REE) by host rocks is responsible for the formation of REE-depleted vein and deformation zone carbonate fillings. Fluid flow is not only restricted to veins or deformation zones as commonly believed. An important part of fluid flow takes place in host rocks near fault zones. Hence, the nature of the lithologies hosting fault zones must be considered in order to take into account the role of fluids in the seismic cycle.     

Early Cretaceous gabbroic rocks from the Taihang Mountains: Implications for a paleosubduction-related lithospheric mantle beneath the central North China Craton

SHRIMP zircon U–Pb ages and geochemical and Sr–Nd–Pb isotopic data are presented for the gabbroic intrusive from the southern Taihang Mountains to characterize the nature of the Mesozoic lithospheric mantle beneath the central North China Craton (NCC). The gabbroic rocks emplaced at 125 Ma and are composed of plagioclase (40–50%), amphibole (20–30%), clinopyroxene (10–15%), olivine (5–10%) and biotite (5–7%). Olivines have high MgO (Fo = 78–85) and NiO content. Clinopyroxenes are high in MgO and CaO with the dominant ones having the formula of En_42–46Wo_41–50Fs_8–13. Plagioclases are dominantly andesine–labradorite (An = 46–78%) and have normal zonation from bytownite in the core to andesine in the rim. Amphiboles are mainly magnesio and actinolitic hornblende, distinct from those in the Precambrian high-pressure granulites of the NCC. These gabbroic rocks are characterized by high MgO (9.0–11.04%) and SiO₂ (52.66–55.52%), and low Al₂O₃, FeOt and TiO₂, and could be classified as high-mg basaltic andesites. They are enriched in LILEs and LREEs, depleted in HFSEs and HREEs, and exhibit (⁸⁷Sr/⁸⁶Sr)_i = 0.70492–0.70539, ε_Nd(t) = − 12.47–15.07, (²⁰⁶Pb/²⁰⁴Pb)_i = 16.63–17.10, Δ8/4 = 70.1–107.2 and Δ7/4 = − 2.1 to − 9.4, i.e., an EMI-like isotopic signatures. Such geochemical features indicate that these early Cretaceous gabbroic rocks were originated from a refractory pyroxenitic veined-plus-peridotite source previously modified by an SiO₂-rich melt that may have been derived from Paleoproterozoic subducted crustal materials. Late Mesozoic lithospheric extension might have induced the melting of the metasomatised lithospheric mantle in response to the upwelling of the asthenosphere to generate these gabbroic rocks in the southern Taihang Mountains.     

Composite flow laws derived from high temperature experimental data on limestone and marble

An interesting feature of recently published experimental data on high temperature deformation of Solnhofen limestone and Carrara marble is that it is not possible, for either rock, to fit isothermal points on a log strain-rate vs. log stress plot to a single straight line as required for a flow law of the familiar form e = Aexp(- H/RT)σⁿ. Instead for Solnhofen limestone the data can be well fitted to two straight line segments suggesting a change from power law with high stress exponent (at high stress) to power law with low stress exponent at low stress. However, the constant strain-rate data are even better fitted by a single composite flow law formed by addition of the two power laws; a single flow law operates throughout but the strain-rate contributions of the two components change in response to changing stress. Published microstructural evidence supports this composite flow law approach.For Carrara marble constant data provides much poorer control and it is possible to propose several composite flow laws (formed by addition of two or three separate power-law components) all of which provide reasonable correspondence with the data. Stress relaxation data is then used both to test these flow models and to suggest others. Flow models that are broadly compatible with constant and stress relaxation data can then be tested against microstructural measurements.It is suggested that, by treating a set of composite flow laws as alternative hypotheses to be tested against all available data, a more realistic Theological model will result. Composite flow laws have the major advantage of being able to represent a smooth transition from one dominant deformation mechanism to another irrespective of how wide the transition zone may be.     

Weakening and strain localization produced by syn-deformational reaction of plagioclase

There are many observations in naturally deformed rocks on the effects of mineral reactions on deformation, but few experimental data. In order to study the effects of chemical disequilibrium on deformation we have investigated the hydration reaction plagioclase + H₂OM more albitic plagioclase + zoisite + kyanite + quartz. We utilized fine-grained (2-6 µm) plagioclase aggregates of two compositions (An54 and An60), both dried and with 0.1-0.4 wt% H₂O present, in shear deformation experiments at two sets of conditions: 900 °C, 1.0 GPa (in the plagioclase stability field) and 750 °C, 1.5 GPa (in the zoisite stability field). Dry samples and those deformed in the plagioclase stability field underwent homogeneous shearing by dislocation creep, but samples with 0.1 to 0.4 wt% water deformed in the zoisite stability field showed extreme strain localization into very narrow (~1-3 µm) shear bands after low shear strain. In these samples the microstructures of reaction products in the matrix differ from those in the shear bands. In the matrix, large (up to 400 µm) zoisite crystals grew in the direction of finite extension, and relict plagioclase grains are surrounded by rims of recrystallized grains that are more albitic. In the shear bands, the reaction products albitic plagioclase, zoisite, white mica, and traces of kyanite form polyphase aggregates of very fine-grained (<0.1 µm) dislocation-free grains. Most of the sample strain after % ~2 has occurred within the shear bands, within which the dominant deformation mechanism is inferred to be diffusion-accommodated grain boundary sliding (DAGBS). The switch from dislocation creep in dry samples deformed without reaction to DAGBS in reacted samples is associated with a decrease in flow stress from ~800 to <200 MPa. These experiments demonstrate that heterogeneous nucleation driven in part by chemical disequilibrium can produce an extremely fine-grained polyphase assemblage, leading to a switch in deformation mechanism and significant weakening. Thus, localization of deformation in polyphase rocks may occur on any pressure (P),temperature (T)-path where the equilibrium composition of the constituent minerals changes.     

Magma–host interactions during differentiation and emplacement of a shallow-level, zoned granitic pluton (Tarçouate pluton, Morocco): implications for magma emplacement

The Tarçouate pluton (Anti-Atlas, Morocco) is an inversely zoned laccolith emplaced 583 Ma ago into low-grade metasediments, with the following succession: leucocratic granites, biotite–granodiorites (±monzodiorites), hornblende–granodiorites (±monzodiorites) and monzodiorites syn-plutonic dykes. These rocks form two distinct, chemically coherent, units:

(1) A main unit consists of layered (572<59 wt.%) and homogeneous (632<67%) hornblende–granodiorites, biotite–granodiorites (672<72%) and aplites (702<76%). All these rocks are metaluminous to peraluminous and display fractionated HREE depleted patterns (La/Yb_N=14–61; Yb_N=0.7–6.8). Initial ⁸⁷Sr/⁸⁶Sr ratios (0.7072 to 0.7080) increase, whereas _Nd(t) values (−1.7 to −2.8) decrease from the hornblende– to the biotite–granodiorites. Monzodiorites occur as mafic microgranular enclaves or syn-plutonic dykes.

(2) A subordinate unit consists of leucocratic, distinctly peraluminous, muscovite-bearing granites (722<75%) occurring at the northern edge of the pluton and as dykes in the surrounding schists towards the top of the pluton. These rocks are free of monzodioritic enclaves. They display less fractionated patterns with higher HREE contents (La/Yb_N=2–19; Yb_N=11–18), a distinct _Nd(t) value (−11.8) and a ⁸⁷Sr/⁸⁶Sr initial ratio (0.7480) within those of the surrounding schists (0.7393–0.7819).

Magma–host interactions are closely related to differentiation and occurred at different levels, but mainly before emplacement. Field relationships and petrogenetic modelling show that the bt–granodiorites formed at levels deeper than the level of emplacement, by fractional crystallisation (0.65

These data preclude any significant material transfer process for the emplacement of the Tarçouate pluton, but rather suggest assembly of successive pulses of variably differentiated crystal-poor magmas. These shallow level granitic plutons can be considered as an end-member of magma emplacement with minimum interactions with the country rocks.     

Subsolidus deuteric/hydrothermal alteration of eudialyte in lujavrite from the Pilansberg alkaline complex, South Africa

The most evolved rocks of the Pilansberg alkaline complex are aegirine lujavrites in which three varieties of eudialyte are recognized on the basis of textural relationships and composition. Manganoan eudialyte-I is a relict orthomagmatic phase occurring as poikilitic plates or as relict grains in pseudomorphed euhedral phenocrysts. Late eudialyte-II ranges in composition from manganoan eudialyte through kentbrooksite to taseqite-like varieties and is considered to be formed by cation exchange with eudialyte-I and alkaline fluids. Eudialyte-III is a hydrothermal phase replacing eudialyte-II, and has either taseqite-like (5–7.3 wt.% SrO, < 2.0 wt.% REE₂O₃) or kentbrooksite (< 1.5 wt.% SrO,  8.5 wt.% REE₂O₃) compositions. Three styles of replacement of eudialyte-I and -II are recognizable. Type 1 involves replacement by complex aggregates of zircon, fergusonite-(Ce), allanite-(Ce), britholite-(Ce), titanite, pyrochlore, albite and potassium feldspar, i.e. a “miaskitic” paragenesis. Type 2 alteration consists of complex aggregates dominated by deuteric Na–Zr-silicates (?catapleiite), stronalsite, strontium-apatite and lamprophyllite replacing eudialyte-I and -II and relicts of the “miaskitic paragenesis”, i.e. a highly sodic “agpaitic-to-hyperagpaitic” paragenesis. Type 3 replacement involves mantling of any residual eudialyte-II and zircon, and replacement of deuteric Na–Zr-silicates by eudialyte-III together with barytolamprophyllite as late hydrothermal phases. Further alteration and replacement resulted in the superposition of natrolite, britholite, pyrochlore, allanite and diverse Ba- and Mn-based minerals onto the types 2 and 3 assemblages, and ultimately to the deposition of allanite-(La), La-dominant REE carbonates and rarely a silica phase. All of the alteration styles are considered to have occurred in situ under subsolidus conditions (< 450 °C) by interaction of pre-existing eudialyte and other minerals with deuteric, sodium- and chlorine-bearing aqueous fluids. The evolution of the replacement products is from a miaskitic through an agpaitic to a hyperagpaitic paragenesis and ultimately back to a low agpaitic-to-miaskitic assemblage, reflecting changes in the a(Na⁺)/a(Cl⁻) ratio and alkalinity of the deuteric/hydrothermal fluids.     

Nd isotopic data reveal the material and tectonic nature of the Main Central Thrust zone in Nepal Himalaya

The Main Central Thrust (MCT) is a tectono-metamorphic boundary between the Higher Himalayan crystallines (HHC) and Lesser Himalayan metasediments (LHS), reactivated in the Tertiary, but which had already formed as a collisional boundary in the Early Paleozoic. To investigate the nature of the MCT, we analyzed whole-rock Nd isotopic ratios of rocks from the MCT and surrounding zones in the Taplejung–Ilam area of far-eastern Nepal, Annapurna–Galyang area of central Nepal, and Maikot–Barekot area of western Nepal. We define the MCT zone as a ductile–brittle shear zone between the upper MCT (UMCT) and lower MCT (LMCT). The protoliths of the MCT zone may provide critical constraints on the tectonic evolution of the Himalaya. The LHS is lithostratigraphically divided into the upper and lower units. In the Taplejung–Ilam area, different lithologic units and their ε_Nd (0) values are as follows; HHC (− 10.0 to − 18.1), MCT zone (− 18.5 to − 26.2), upper LHS unit (− 17.2), and lower LHS unit (− 22.0 to − 26.9). There is a distinct gap in the ε_Nd (0) values across the UMCT except for the southern frontal edge of the Ilam nappe. In the Annapurna–Galyang and Maikot–Barekot areas, different lithologic units and their ε_Nd (0) values are as follows; HHC (− 13.9 to − 17.7), MCT zone (− 23.8 to − 26.2 except for an outlier of − 12.4), upper LHS unit (− 15.6 to − 26.8), and lower LHS unit (− 24.9 to − 26.8). These isotopic data clearly distinguish the lower LHS unit from the HHC. Combining these data with the previously published data, the lowest ε_Nd (0) value in the HHC is − 19.9. We regard rocks with ε_Nd (0) values below − 20.0 as the LHS. In contrast, rocks with those above − 19.9 are not always the HHC, and some parts of them may belong to the LHS due to the overlapping Nd isotopic ratio between the HHC and LHS. Most rocks of the MCT zone have Nd isotopic ratios similar to those of the LHS, but very different from those of the HHC. The spatial patterns in the distribution of ε_Nd (0) value around the UMCT suggest no substantial structural mixing of the HHC and LHS during the UMCT activities in the Tertiary. A discontinuity in the spatial distribution of ε_Nd (0) values is laterally continuous along the UMCT throughout the Himalayas. These facts support the theory that the UMCT was originally a material boundary between the HHC and LHS, suggesting the MCT zone was mainly developed with undertaking a role of sliding planes during overthrusting of the HHC in the Tertiary.     

Contribution of syncollisional felsic magmatism to continental crust growth: A case study of the Paleogene Linzizong volcanic Succession in southern Tibet

The Linzizong volcanic succession (~ 65–45 Ma) and the coeval batholiths (~ 60−40 Ma) of andesitic to rhyolitic composition represent a magmatic response to the India–Asia continental collision that began at ~ 70–65 Ma and ended at ~ 45–40 Ma with convergence continuing to present. These syncollisional felsic magmatic rocks are widely distributed along much of the > 1500 km long Gangdese Belt immediately north of the India–Asia suture (Yarlung–Zangbo) in southern Tibet. Our study of the Linzizong volcanic rocks from the Linzhou Basin (near Lhasa) suggests that syncollisional felsic magmatism may in fact account for much of the net contribution to continental crust growth. These volcanic rocks show a first-order temporal change from the andesitic lower Dianzhong Formation (64.4–60.6 Ma), to the dacitic middle Nianbo Formation (~ 54 Ma), and to the rhyolitic upper Pana Formation (48.7–43.9 Ma). The three formations show no systematic but overlapping Nd–Sr isotope variations. The isotopically depleted samples with ε_Nd(t) > 0 indicate that their primary sources are of mantle origin. The best source candidate in the broad context of Tethyan ocean closing and India–Asia collision is the remaining part of the Tethyan ocean crust. This ocean crust melts when reaching its hydrous solidus during and soon after the collision in the amphibolite facies, producing andesitic melts parental to the Linzizong volcanic succession (and the coeval batholiths) with inherited mantle isotopic signatures. Ilmenite as a residual phase (plus the effect of residual amphibole) of amphibolite melting accounts for the depletion of Nb, Ta and Ti in the melt. The effect of ocean crust alteration plus involvement of mature crustal materials (e.g., recycled terrigeneous sediments) enhances the abundances of Ba, Rb, Th, U, K and Pb in the melt, thus giving the rocks an “arc-like” geochemical signature. Residual amphibole that possesses super-chondritic Nb/Ta ratio explains the sub-chondritic Nb/Ta ratio in the melt; residual plagioclase explains the slightly depleted, not enriched, Sr (and Eu) in the melt, typical of continental crust. These observations and reasoning plus the remarkable compositional similarity between the andesitic lower Dianzhong Formation and the model bulk continental crust corroborates our proposal that continental collision zones may be sites of net crustal growth (juvenile crust) through process of syncollisional felsic magmatism. While these interpretations are reasonable in terms of straightforward petrology, geochemistry and tectonics, they require further testing.     

Lunar geochemistry as told by lunar meteorites

About 36 lunar meteorites have been found in cold and hot deserts since the first one was found in 1979 in Antarctica. All are random samples ejected from unknown locations on the Moon by meteoroid impacts. Lithologically and compositionally there are three extreme types: (1) brecciated anorthosites with high Al₂O₃ (26–31%), low FeO (3–6%), and low incompatible elements (e.g., <1 μg/g Th), (2) basalts and brecciated basalts with high FeO (18–22%), moderately low Al₂O₃ (8–10%) and incompatible elements (0.4–2.1 μg/g Th), and (3) an impact-melt breccia of noritic composition (16% Al₂O₃, 11% FeO) with very high concentrations of incompatible elements (33 μg/g Th), a lithology that is identified as KREEP on the basis of its similarity to Apollo samples of that designation. Several meteorites are polymict breccias of intermediate composition because they contain both anorthosite and basalt. Despite the large range in compositions, a variety of compositional parameters together distinguish lunar meteorites from terrestrial materials. Compositional and petrographic data for lunar meteorites, when combined with mineralogical and compositional data obtained from orbiting spacecraft in the 1990s, suggest that Apollo samples identified with the magnesian (Mg-rich) suite of nonmare rocks (norite, troctolite, dunite, alkali anorthosite, and KREEP) are all products of a small, geochemically anomalous (noritic, high Th) region of crust known as the Procellarum KREEP Terrane and are not, as generally assumed, indigenous to the vast expanse of typical feldspathic crust known as the Feldspathic Highlands Terrane. Magnesian-suite rocks such as those of the Apollo collection do not occur as clasts in the feldspathic lunar meteorites. The misconception is a consequence of four historical factors: (1) the Moon has long been viewed as simply bimodal in geology, mare or highlands, (2) one of the last, large basin-forming bolides impacted in the Procellarum KREEP Terrane, dispersing Th-rich material, (3) although it was not known at the time, the Apollo missions all landed in or near the anomalous Procellarum KREEP Terrane and collected many Th-rich samples formed therein, and (4) the Apollo samples were interpreted and models for lunar crust formation developed without recognition of the anomaly because global data provided by orbiting missions and lunar meteorites were obtained only years later.     

Deformation mechanism maps for feldspar rocks

Deformation mechanism maps for feldspar rocks were constructed based on recently published constitutive laws for dislocation and grain boundary diffusion creep of wet and dry plagioclase aggregates. The maps display constant temperature contours in stress-grain size space for strain rates ranging from 10⁻¹⁶ to 10⁻¹² s⁻¹.Two fields of dominance of grain boundary diffusion-controlled creep and dislocation creep are separated by a strongly grain size-sensitive transition zone. For wet rocks, diffusion-controlled creep dominates below a grain size of about 0.1–1 mm, depending on temperature, stress, strain rate and feldspar composition. Plagioclase aggregates containing up to 0.3 wt.% water as often found in natural feldspars are more than 2 orders of magnitude weaker than dry rocks. The strength of water-bearing feldspar rocks is moderately dependent on composition and water fugacity.For a grain size range of about 10–50 μm commonly observed in natural ultramylonites, the deformation maps predict that diffusion-controlled creep is dominant at greenschist to granulite facies conditions. Low viscosity estimates of 10¹⁸–10¹⁹ Pa·s from modeling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures >900 °C or, at lower temperatures, diffusion creep of fine-grained rocks possibly localized in abundant high strain shear zones. For similar thermodynamic conditions and grain size, lower crustal rocks are predicted to be less than order of magnitude weaker than upper mantle rocks.     

Origin and geodynamic significance of Tertiary postcollisional basaltic magmatism in Serbia (central Balkan Peninsula)

Tertiary basaltic magmatism in Serbia occurred through three episodes: (i) Paleocene/Eocene, when mostly east Serbian mafic alkaline rocks (ESPEMAR) formed, (ii) Oligocene/Miocene, dominated by high-K calc–alkaline basalts, shoshonites (HKCA–SHO) and ultrapotassic (UP) rocks, and (iii) Pliocene episode when rocks similar to (ii) originated. In this study, the geodynamics inferred from petrogenesis of the (i) and (ii) episodes are discussed.

The ESPEMAR (62–39 Ma) occur mainly as mantle xenolith-bearing basanites. Their geochemical features, such as the REE patterns, elevated HFSE contents and depleted Sr–Nd isotope signatures, indicate a relatively small degree of melting of an isotopically depleted mantle source. Their mantle-normalized trace element patterns are flat to concave and “bell-shaped”, characteristic of an OIB source free of subduction component. ⁸⁷Sr/⁸⁶Sr_i and ¹⁴³Nd/¹⁴⁴Nd_i isotope ratios (0.7030–0.7047 and 0.5127–0.5129, respectively) indicate a depleted source for the ESPEMAR similar to the European Asthenospheric Reservoir (EAR).

The HKCA–SHO rocks (30–21 Ma) occur as basalts, basaltic andesites and trachyandesites. They show enrichment in LILE and depletion in HFSE with all the distinctive features of calc–alkaline arc-type magmatism. This is coupled with somewhat enriched Sr–Nd isotope signature (⁸⁷Sr/⁸⁶Sr_i=0.7047–0.7064, ¹⁴³Nd/¹⁴⁴Nd_i=0.5124–0.5126). All these features are characteristic of subduction-related metasomatism and fluxing of the HKCA–SHO mantle source with fluids/melts released from subducted sedimentary material.

UP rocks (35–21 Ma) appear as (i) Si-rich lamproites and related rocks and (ii) olivine leucitites and related rocks. UP rocks have high-LILE/HFSE ratios with enrichment for some LILE around 1000× primitive mantle, troughs at Nb and Ti, and peaks of Pb in their mantle-normalized patterns. They also show highly fractionated REE patterns (La/Yb up to 27, La_N up to 400). The isotopic ratios approach crustal values (⁸⁷Sr/⁸⁶Sr_i=0.7059–0.7115 and ¹⁴³Nd/¹⁴⁴Nd_i=0.5122–0.5126), and that signature is typical for ultrapotassic rocks worldwide.

The Paleocene/Eocene episode and formation of the ESPEMAR is referred to as asthenospheric-derived magmatism. This magmatism originated through passive riftlike structures related to possible short relaxational phases during predominantly collisional and compressional conditions. The Oligocene/Miocene episode and formation of HKCA–SHO and UP rocks were dominated by lithospheric-controlled magmatism. Its origin is connected with the activity of a wide dextral wrench corridor generated along the axis of the Dinaride orogen which collapsed in response to thickened crust caused by earlier compressional processes.

To explain conditions of these two magmatic events, a three-stage geodynamic model has been proposed: (1) subduction–termination/collision stage (Paleocene/Eocene), (2) collision stage (Eocene) and (3) postcollision/collapse stage (Oligocene/early Miocene).     

Petrogenesis of Cretaceous adakitic and shoshonitic igneous rocks in the Luzong area, Anhui Province (eastern China): Implications for geodynamics and Cu–Au mineralization

Both adakitic and shoshonitic igneous rocks in the Luzong area, Anhui Province, eastern China are associated with Cretaceous Cu–Au mineralization. The Shaxi quartz diorite porphyrites exhibit adakite-like geochemical features, such as light rare earth element (LREE) enrichment, heavy REE (HREE) depletion, high Al₂O₃, MgO, Sr, Sr / Y and La / Yb values, and low Y and Yb contents. They have low ε_Nd(t) values (− 3.46 to − 6.28) and high (⁸⁷Sr / ⁸⁶Sr)_i ratios (0.7051–0.7057). Sensitive High-Resolution Ion Microprobe (SHRIMP) zircon analyses indicate a crystallization age of 136 ± 3 Ma for the adakitic rocks. Most volcanic rocks and the majority of monzonites and syenites in the Luzong area are K-rich (or shoshonitic) and were also produced during the Cretaceous (140–125 Ma). They are enriched in LREE and large-ion lithophile elements, and depleted in Ti, and Nb and Ba and exhibit relatively lower ε_Nd(t) values ranging from − 4.65 to − 7.03 and relatively higher (⁸⁷Sr / ⁸⁶Sr)_i ratios varying between 0.7057 and 0.7062. The shoshonitic and adakitic rocks in the Luzong area have similar Pb isotopic compositions (²⁰⁶Pb / ²⁰⁴Pb = 17.90–18.83, ²⁰⁷Pb / ²⁰⁴Pb = 15.45–15.62 and ²⁰⁸Pb / ²⁰⁴Pb = 38.07–38.80). Geological data from the Luzong area suggest that the Cretaceous igneous rocks are distributed along NE fault zones (e.g., Tanlu and Yangtze River fault zones) in eastern China and were likely formed in an extensional setting within the Yangtze Block. The Shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of > 50 km (i.e., 1.5 GPa), possibly leaving rutile-bearing eclogitic residue. The shoshonitic magmas, in contrast, originated mainly from an enriched mantle metasomatized by subducted oceanic sediments. They underwent early high-pressure (> 1.5 GPa) fractional crystallization at the boundary between thickened (> 50 km) lower crust and lithospheric mantle and late low-pressure (< 1.5 GPa) fractional crystallization in the shallow (< 50 km) crust. The adakitic and shoshonitic rocks appear to be linked to an intra-continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along NE fault zones (e.g., Tanlu and Yangtze River fault zones) in eastern China. Both the shoshonitic and adakitic magmas were fertile with respect to Cu–Au mineralization.     

The origin of basic rocks of the Korosten AMCG complex, Ukrainian shield: Implication of Nd and Sr isotope data

The Korosten complex is a Paleoproterozoic gabbro–anorthosite–rapakivi granite intrusion which was emplaced over a protracted time interval — 1800–1737 Ma. The complex occupies an area of about 12 000 km² in the north-western region of the Ukrainian shield. About 18% of this area is occupied by various mafic rocks (gabbro, leucogabbro, anorthosite) that comprise five rock suites: early anorthositic A₁ (1800–1780 Ma), main anorthositic A₂ (1760 Ma), early gabbroic G₃ (between 1760 and 1758 Ma), late gabbroic G₄ (1758 Ma), and a suite of dykes D₅ (before 1737 Ma). In order to examine the relationships between the various intrusions and to assess possible magmatic sources, Nd and Sr isotopic composition in mafic whole-rock samples were measured. New Sr and Nd isotope measurements combined with literature data for the mafic rocks of the Korosten complex are consistent and enable construction of Rb–Sr and Sm–Nd isochronous regressions that yield the following ages: 1870 ± 310 Ma (Rb–Sr) and 1721 ± 90 Ma (Sm–Nd). These ages are in agreement with those obtained by the U–Pb method on zircons and indicate that both Rb–Sr and Sm–Nd systems have remained closed since the time of crystallisation. In detail, however, measurable differences in isotopic composition of the Korosten mafic rock depending on their suite affiliation were revealed. The oldest, A₁ rocks have lower Sr (⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70233–0.70288) and higher Nd (εNd₍₁₇₆₀₎ = 1.6–0.9) isotopic composition. The most widespread A₂ anorthosite and leucogabbro display higher Sr and lower Nd isotopic composition: ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70362, εNd₍₁₇₆₀₎ varies from 0.2 to − 0.7. The G₃ gabbro–norite has slightly lower εNd₍₁₇₆₀₎ varying from − 0.7 to − 0.9. Finally, G₄ gabbroic rocks show relatively high initial ⁸⁷Sr/⁸⁶Sr (0.70334–0.70336) and the lowest Nd isotopic composition (εNd₍₁₇₆₀₎ varies from − 0.8 to − 1.4) of any of the mafic rocks of the Korosten complex studied to date. On the basis of Sr and Nd isotopic composition we conclude that Korosten initial melts may have inherited their Nd and Sr isotopic characteristics from the lower crust created during the 2.05–1.95 Ga Osnitsk orogeny and 2.0 Ga continental flood basalt event. Indeed, εNd₍₁₇₆₀₎ values in Osnitsk rocks vary from 0.0 to − 1.9 and from 0.2 to 3.4 in flood basalts. We suggest that these rocks being drawn into the upper mantle might melt and give rise to the Korosten initial melts. ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ values also support this interpretation. We suggest that the Sr and Nd isotopic data currently available on mafic rocks of the Korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by Paleoproterozoic amalgamation of Sarmatia and Fennoscandia.     

Evolution of a post-batholith dike swarm in central coastal Queensland, Australia: arc-front to backarc?

A swarm of felsic and mafic dikes cuts a Late Carboniferous–Permian batholith called the Urannah Suite in central coastal Queensland. Late Permian–Triassic westward thrusting (Hunter–Bowen Orogeny) exposed this mid-crustal Suite and the crosscutting dikes, thus enabling examination of dikes that range in age from syn- to post-batholithic. Although both mafic and felsic dikes have the same dominant northerly strike, field, geochronologic and geochemical examination reveal that the swarm is composite; felsic dikes are older (285 Ma) and geochemically and isotopically distinct from mafic dikes (273–229 Ma). Dike compositions are compared to those of the host plutonic rocks, and to volcanic rocks the same age as the dikes. Whereas the felsic (older) dikes are compositionally similar to their host granites (initial ⁸⁷Sr/⁸⁶Sr>0.7045), the mafic (younger) dikes are isotopically (Sr, Nd, Pb) less radiogenic. Moreover, several different types of mafic dikes are evident based on geochemistry; most of these have mixed characteristics in terms of tectonic classification. All but two dikes of basalt and basaltic andesite composition classify as ‘with-in plate' on Ti–Zr–Y tectonic classification plots. Many of the basalts have high TiO₂ contents (1.5–3.0 wt.%). Most of these have REE and spider diagram patterns like calc-alkaline tholeiites, the exceptions being a few alkali basalts recognized by their alkali content, and high Ti, Ce, Nb and Zr contents. When put into the context of all plutonic rocks in the area (late Paleozoic and Mesozoic), these dikes record a transition at 280 Ma, after which time, all magmatism in the region is less isotopically evolved (initial ⁸⁷Sr/⁸⁶Sr=0.7033–0.7044). A model of slab retreat and hinge movement to the east in the latest Permian explains the change of geochemical signature from arc-front to backarc at about 280 Ma.     

Origin and emplacement of ultramafic–mafic intrusions in the Erro-Tobbio mantle peridotite (Ligurian Alps, Italy)

The Erro-Tobbio peridotites (Voltri Massif, Ligurian Alps) represent subcontinental lithospheric mantle tectonically exhumed during Permo–Mesozoic extension of the Europe–Adria lithosphere. Previous studies have shown that exhumation started during Permian times, and occurred along kilometer-scale lithospheric shear zones which enhanced progressive deformation and recrystallization from spinel- to plagioclase-facies conditions. Ongoing field and petrologic investigations have revealed that the peridotites experienced, during uplift, a composite history of diffuse melt migration and multiple episodes of ultramafic–mafic intrusions. In this paper we present the results of field, structural and petrologic–geochemical investigations into a sector of the Erro-Tobbio peridotite unit that preserves well this multiple intrusion history. Melt impregnation in the peridotites is evidenced by significant plagioclase enrichment and crystallization of unstrained orthopyroxene replacing kinked mantle olivine and clinopyroxene; impregnating melts were thus opx-saturated. Melt–rock interaction caused chemical changes in mantle minerals (e.g. Al decrease and REE increase in cpx; Ti and Cr# enrichment in spinel). Nevertheless, clinopyroxenes still exhibit LREE depletion (Ce_N/Sm_N = 0.006–0.011), indicating a depleted signature for the percolating melts. Melt impregnation was thus related to diffuse porous flow migration of depleted MORB-type melt fractions that modified their compositions towards opx saturation by mantle–melt interaction during ascent. The impregnated peridotites are intruded by a hectometer-scale stratified cumulate body, mostly consisting of troctolites and plagioclase wehrlites, showing gradational, interfingered contacts with the host mantle rocks. Subsequent intrusion events are revealed by the occurrence of olivine gabbros as decameter-wide lenses, variably thick (centimeter- to meter-scale) dykes and thin dykelets, which crosscut both the peridotite foliation and the magmatic layering in the cumulates. Overall, major and trace element compositions of minerals in the intrusives indicate that they represent variably differentiated cumulus products crystallized from rather primitive N-MORB-type aggregated melts. Slightly more evolved compositions are shown by olivine gabbros, relative to the troctolites and plagioclase wehrlites of the cumulate body. Mineral chemistry features (e.g. the Fo–An correlation and high Na, Ti, Mg# in cpx) indicate that the studied intrusive rocks crystallized at moderate pressure conditions (3–5 kbar, i.e. 9–15 km depth). Our study thus points to a progressive transition from porous flow melt migration to emplacement of magmas in fractures, presumably related to progressive change of lithospheric mantle rheology during extension-related uplift and cooling.