Medium-grade Metamorphic Rocks Research Articles

Microfabric, 3D-strain geometry and kinematic vorticity analysis of Au-bearing shear zone, Central Domain of the Borborema Province, NE Brazil

In the Central Domain of the Borborema Province, mineralized quartz veins type lode-gold deposits (orogenic gold deposits) occur hosted in shear zones. The gold mineralizations are associated with medium-grade metamorphic country rocks, mainly gneisses, and mica schists, consisting of several occurrences and small deposits closely related to the deformational fabric. The Itapetim Shear Zone (ISZ) is one of these shear zones hosting gold mineralizations, which is characterized by heterogeneous and asymmetric deformation fabric. Gold mineralizations occur in laminated quartz veins (shear veins), generally restricted to the innermost portion of the shear zone, extending along the strike for a few kilometers. The microstructures in mineralized quartz veins and mylonitic rocks show evidence of crystal plastic deformation and annealing, indicating that static recrystallization took place after deformation ceased. The strain magnitude, strain symmetry, and kinematic vorticity data show that the shear zone has experienced a deformation history of flattening to plane strain with contributions of both simple (Wm ≥ 0.9) and pure shear (Wm ≤ 0.6) deformation components. It is concluded that the partitioning of kinematic flow contributed to the localized occurrence of mineralized veins preferentially along a segment of the shear zone marked by a dominant simple shear deformation regime.

Coupled U–Pb and Lu–Hf zircon data of Late Devonian magmatism from Bajo de La Leona (Deseado Massif, Argentina): Tracking the southernmost exposure of the Devonian magmatic arc of Patagonia

In Bajo de La Leona, located in the eastern Deseado Massif, pre-Permian basement rocks of the Río Deseado Complex crop out. They consist of high- to medium-grade metamorphic rocks intruded by two Paleozoic granitoids, namely the Mina de La Leona Granite and the Laguna Larga Granite. The latter is the main goal of this work. It is a calc-alkaline to high-K calc-alkaline, ferroan, calcic to alkali-calcic, and mainly peraluminous monzoleucogranite which plots within the I&S-type and volcanic-arc fields of the tectonic discrimination diagrams. Its trace elements composition (e.g., Sr/Y, LaN/YbN, Sm/Yb, Eu/Eu*, Dy/Dy*, Dy/Yb) suggests plagioclase ± clinopyroxene fractionation in a shallow source at low pressures. Assimilation could have played a role in magma differentiation, thus the magmatic development of the Laguna Larga Granite is likely a consequence of assimilation-fractional crystallization (AFC) processes. Coupled LA-ICP-MS U–Pb and Lu–Hf zircon data for the studied granite, as well as Sm–Nd whole-rock data, are here presented for the first time. A Famennian U–Pb zircon crystallization age of 361.9 ± 1.7 Ma was obtained, together with εHf(t) zircon values from -15 to +6 with TDM-Hf model ages between 2.0 and 0.9 Ga. A thin crust, as suggested by low values of thickness proxies such as Sr/Y (1.06–1.83) and LaN/YbN (3.97–4.96), would allow emplacement of juvenile mantle-derived material, giving rise to the suprachondritic εHf(t) values. A mantle-derived source is also suggested by a suprachondritic εNd(t) whole-rock value of +2.34 (TDM-Nd = 0.93 Ga). On the other hand, the subchondritic εHf(t) values could be related to wall rock assimilation. In a regional context, the obtained results for the Laguna Larga Granite allow to correlate it with coeval rocks from northern Patagonia (e.g., San Martín de los Andes batholith, Paleozoic Central Patagonian igneous-metamorphic belt). Therefore, the Devonian magmatic arc might have extended from the North Patagonian Andes and the western North Patagonian Massif to the eastern Deseado Massif along a NNW-SSE-striking belt, confirming previous proposals.

Fluid Evolution and Ore Genesis of the Songjianghe Au Deposit in Eastern Jilin Province, NE China: Constraints from Fluid Inclusions and H-O-S-Pb Isotope Systematics

The medium-sized Songjianghe Au deposit is located in the southeastern part of the Jiapigou-Haigou gold belt (JHGB) in central eastern Jilin Province, NE China. The gold mineralization is primarily characterized by disseminated-style ores and hosted in the low-/medium-grade metamorphic rocks of the Seluohe Group. The ore bodies are governed by NNW-striking brittle-ductile structures and spatially correlated with silicic and sericitic alterations. Four alteration/mineralization stages have been distinguished: (I) Quartz-pyrrhotite-pyrite, (II) quartz-polymetallic sulfides, (III) quartz-pyrite, and (IV) quartz-calcite. The fluid inclusion (FI) assemblage in quartz from Stage I comprises C1-type, C2-type, C3-type, and VL-type FIs, with total homogenization temperatures (Th-total) of 292.8 to 405.6 °C and salinities of 2.8 to 9.3 wt% NaCl eqv. Quartz from Stage II (main ore stage) developed C2-, C3-, and VL-type FIs, with a Th-total of 278.5 to 338.9 °C and salinities of 2.8 to 8.1 wt% NaCl eqv. Stage III is characterized by coexisting C3- and VL-type FIs in quartz, with a Th-total of 215.9 to 307.3 °C and salinities of 2.4 to 7.2 wt% NaCl eqv. Only VL-type FIs are observed in Stage IV, with a Th-total of 189.5 to 240.4 °C and salinities of 3.7 to 5.7 wt% NaCl eqv. The Laser Raman spectroscopic results demonstrated minor CH4 in the C-type FIs from Stages I and II. The results suggest that ore fluids may have evolved from a medium-high temperature, low-salinity immiscible CO2-NaCl-H2O ± CH4 system to a low temperature, low-salinity homogeneous NaCl-H2O system. Fluid immiscibility caused by the rapid drop in pressure may have been the main trigger for gold-polymetallic sulfide precipitation. The Songjianghe Au deposit may have been formed under 352–448 °C and 850–1380 bar pressure, based on the isochore intersection for Stage II fluid inclusions. The H-O isotopic compositions (Stage I: δ18Ofluid = 5.6 to 5.8‰, δD = −96.2 to −95.7‰; Stage II: δ18Ofluid = 3.7 to 4.2‰, δD = −98.7 to −89.8‰; Stage III: δ18Ofluid = 1.2 to 1.4‰, δD = −103.5 to −101.2‰) indicate that the hydrothermal fluids are dominated by magmatic water in the early stages (Stages I and II) and mixed with meteoric water since Stage III. The pyrite S-Pb isotope data (δ34S: −2.91 to 3.40‰; 206Pb/204Pb: 16.3270 to 16.4874; 207Pb/204Pb: 15.2258 to 15.3489; 208Pb/204Pb: 36.6088 to 36.7174), combined with Pb isotopic compositions of the intrusive rocks and wall rocks (the Seluohe Group) in the ore district, indicate that the ore-forming materials at Songjianghe are predominantly from a magmatic source and may have been affected by the contamination of the Seluohe Group. In accordance with the features of ore geology, ore-forming fluids and metals, and geodynamic setting, the Songjianghe Au deposit belongs to a mesothermal magmatic hydrothermal vein gold deposit, which formed in the intermittent stage of Paleo-Pacific plate subduction during the Late Jurassic.

Multivariate statistical analysis of trace elements in apatite: Discrimination of apatite with different origins

Partial least squares-discriminant analysis (PLS-DA) was used to construct the link between trace element contents of magmatic or hydrothermal apatite and deposit and rock types. Apatite with different origins is discriminated by characteristic chemical composition. In average, ore magmatic apatite has higher light REE and Th, and lower Sr than barren rock apatite, independent of deposit types. Hydrothermal apatite has lower La, Ce, Pr, and Nd contents than magmatic apatite, probably because fluids prefer migrating light rare earth element out of apatite. Hydrothermal apatite from iron oxide copper gold (IOCG) and iron oxide apatite (IOA) deposits is discriminated by high Mn and Sr relative to magmatic apatite, probably because these elements are fluid mobile. Barren magmatic apatite from granitoid-related deposits is well separated from ore magmatic apatite by higher Sr, Eu, Gd, Tb, Dy, and lower Mn contents, which are likely related to the relatively low degree of differentiation and high aluminum saturation index of parental magma.Magmatic apatite from IOA deposits is discriminated by relatively high Nd, Sm, Gd, Tb, Dy, and low Mn and U contents, which are possibly related to the crystallization of Mn-compatible magnetite microlites and peraluminous magmas. Magmatic apatite from granitoid-related W deposits is relatively rich in Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The relative enrichment of Sr in magmatic apatite from granitoid-related Cu-Pb-Zn and Pb-Zn deposits is likely due to less fractionation of Sr-compatible plagioclase. Melt of sediment component accounts for high Mn and Th in magmatic apatite from granitoid-related Mo deposits. Magmatic apatite from porphyry deposits is discriminated by relatively high V, Sr, and Eu contents. Magmatic apatite from porphyry Mo deposits has relatively high Pr, Nd, and Sm and low Sr contents compared to other types of porphyry deposits due to the more evolved magma. Hydrothermal apatite from IOA deposits is discriminated by high V and Sr, whereas those from IOCG deposits have high Sm, Eu, Gd, Tb, and U contents, because the lower temperature of IOCG cause precipitation of REE in Cl-rich fluid. Relatively low Gd and Tb in hydrothermal apatite from skarn deposits are possibly due to the crystallization of amphibole.Apatite in carbonatite has high contents of trace elements possibly because high contents of Ca and P in melts cause high apatite-melt partition coefficients of trace elements. Apatite from sedimentary and metamorphic rocks is discriminated by relative enrichment of Sr and Eu. Compared to igneous rocks, apatite from sedimentary rocks has low REE contents, reflecting low REE budget in surface waters. Breakdown of some minerals to release and remobilize REE and U during metamorphism can interpret high heavy REE and U contents in apatite from high-grade metamorphic rocks relative to those from low- and medium-grade metamorphic rocks. The ability to discriminate apatite from different types of deposits and rocks indicates the application potential of apatite chemistry in mineral exploration. A flowchart was proposed to identify apatite with unknown origins.

The Li-Bearing Pegmatites from the Pampean Pegmatite Province, Argentina: Metallogenesis and Resources

The Li-bearing pegmatites of the Pampean Pegmatite Province (PPP) occur in a rare-element pegmatite belt developed mainly in the Lower Paleozoic age on the southwestern margin of Gondwana. The pegmatites show Li, Rb, Nb ≤ Ta, Be, P, B, Bi enrichment, and belong to the Li-Cs-Ta (LCT) petrogenetic family, Rare-Element-Li (REL-Li) subclass; most of them are of complex type and spodumene subtype, some are of albite-spodumene type, and a few of petalite subtype. The origin of the pegmatites is attributed predominantly to fractionation of fertile S-type granitic melts produced by either fluid-absent or fluid-assisted anatexis of a thick pile of Gondwana-derived turbiditic sediments. Most of the pegmatites are orogenic (530–440 Ma) and developed during two overlapped collisional orogenies (Pampean and Famatinian); a few are postorogenic (~370 Ma), related to crustal contaminated A-type granites. The pegmatites were likely intruded in the hinterland, preferably in medium-grade metamorphic rocks with PT conditions ~200–500 MPa and 400–650 °C, where they are concentrated in districts and groups. Known combined resources add up 200,000 t of spodumene, with variable grades between 5 and 8 wt.% Li2O. The potential for future findings and enlargement of the resources is high, since no systematic exploration program has yet been developed.

The importance of parameter selection in studies of detrital zircon provenance: An example from Mesozoic deposits of the Bohemian Massif foreland (Poland)

Detrital zircons are commonly used to identify sediment provenance, but usually only their ages are employed for interpretation. We here test the combination of four data types: crystallization ages, Th/U values, cathodoluminescence-induced internal textures, and grain shapes. Six samples of zircons from Triassic and Cretaceous deposits from the northeastern foreland of the Bohemian Massif (Poland) were used to identify as accurately as possible source rocks and their paleolocations, changes in erosional levels of source areas, and transport directions. The samples contain ten age populations, among which Carboniferous and Neoproterozoic zircons dominate, but the use of four parameters allowed subtle differences between the samples to be recognized; this makes interpretation more accurate and more reliable. A comparative analysis based on over 20,000 data points collected from ~200 contributions allowed us to identify the equivalents of crystalline source rocks, which were high-grade and medium-grade metamorphic rocks of the Bohemian Massif (mostly granulites) in the case of complex-textured zircons, and Bohemian magmatic rocks along with pyroclastics in the case of oscillatory-zoned grains. The source rocks were located in the northeastern and eastern parts of the massif, where the proportions of the specific rocks exposed to erosion have changed significantly from the Mesozoic to the present. We found distinct changes in the erosional levels of source areas caused by removal of sedimentary cover and the erosion of crystalline rocks, combined with tectonic movements, particularly in the Olenekian and Coniacian. The transport of debris from the southwest and west during the Induan–Anisian, as well as from the south and north during the Norian, were largely controlled by paleogeography. The Cretaceous transgression resulted in the supply of grains mainly from the nearest units during the Cenomanian and Coniacian. We strongly recommend using a combination of at least these four parameters in provenance studies.

Magnetotelluric imaging of the shallow-seated magma reservoir beneath the Karadağ stratovolcano, Central Anatolia, Turkey

The central Anatolian Volcanic Province encompasses many volcanoes elongated in a northeast-southwest direction. Karadağ is one of the dormant Quaternary stratovolcanoes of the region at the southern end of the province. We acquired broadband magnetotelluric (MT) data in the period range 0.0001–251 s at 124 locations to obtain a three-dimensional electrical resistivity model to identify the associated magmatic-hydrothermal system beneath the Karadağ stratovolcano. The resistivity model provides evidence for low resistivity anomalies that include (1) a near-surface localised low resistivity (~ 20 Ωm) zone situated directly beneath the crater centre that is interpreted to be lacustrine sediments of an extinct crater lake, (2) a shallow low resistivity (~ 10 Ωm) zone of probable hydrothermal alteration in the eastern part of the stratovolcano and (3) a deep and large zone of low resistivity (<10 Ωm) attributed to a shallow-seated magma reservoir at a depth of 3 to 7 km beneath the volcanic edifice. A few hundred meters thick resistive surface layer corresponds to the Karadağ adakitic volcanics (i.e., andesites and dacites), and a resistive basement representing Precambrian to Mesozoic high- and medium-grade metamorphic rocks on the flanks is observed in the resistivity model.

Spiral-shaped fabrics in metamorphic rocks: A new example of rotation during progressive deformation

This paper deals with a new example of spiral geometry in medium-grade metamorphic rocks. In this example, multi-microlayers are wrapped into millimeter-to-centimeter scale spirals and form pod-shaped structures 3 cm to >10 cm long and 1.5–2 cm across which are outlined by layers up to 3 mm thick and are named spiral pods. Their long axes are oriented at angles of 40–90° to mineral lineations, and are sub-parallel to the average orientation of curvilinear hinge lines of shear-related folds. The wrapping sense coincides with the sense determined by shear sense indicators and shear-related fold vergence. We conclude that the spiral pods developed during a single progressive non-coaxial deformation. Their development began with the formation of buckle microfolds and small shear-related tight to isoclinal folds with curvilinear hinge lines. Shear-related folds experienced refolding and subsequent disarticulating of their refolded hinges. The sense of rotation of hinge lines and rolling of hinges is consistent with the shear sense and these processes led to the formation of spiral pods at all stages of progressive non-coaxial deformation. Thus the spiral pods evolved from refolded folds with curvilinear hinge lines whereas spiral slump folds with a similar structure from cylindrical refolded folds.

Apatite Eu/Y-Ce discrimination diagram: A big data based approach for provenance classification

磷灰石广泛分布于火成岩、沉积岩和变质岩中，是一种常见的、包含丰富微量元素的副矿物。磷灰石晶格可容纳丰富的微量元素，且因其形成的物理化学条件不同会表现出差异明显的微量元素特征。利用磷灰石微量元素特征可以追踪物质来源和演化。现在常用的方法是利用磷灰石的微量元素绘制二元判别图解，经典判别图解包括Sr-Y、Sr-Mn、Y-（Eu/Eu^*）和（Ce/Yb）_N-REE图解。随着微区测试技术发展，磷灰石微量元素数据日渐丰富，同时由于磷灰石化学成分的复杂性，传统图解已逐渐无法有效利用这些数据所携带的信息，进而无法准确判别其生成环境。建立能准确判别磷灰石物源的新型判别图解故而迫切。近年来，磷灰石微量元素数据的大量积累，为运用以大数据为依托，准确判别磷灰石物源奠定了数据基础。本研究将大数据技术与地球化学数据相结合，共收集整理了1925个代表性磷灰石测试点的微量元素数据，对富碱性火成岩、超镁铁质岩石、镁铁质火成岩、长英质花岗岩、中-低级变质岩、高级变质岩六种类型中磷灰石微量元素数据进行穷举端元处理，共获得7140个磷灰石物源判别图解端元组合，在轮廓系数限定下，进一步有效筛选并提取出能判别磷灰石物源类型的最优图解端元。本文构建了Eu/Y-Ce磷灰石判别新图解，相较于之前的磷灰石判别图解，其涵盖了更全面的物源类型，可以更准确地判别源区类型。;Apatite is a common and important accessory mineral in rocks. Apatite comprises rich various trace elements, which are formed in different apatite-generation environments. Constructing apatite-associated trace element discrimination diagrams is a classic and efficient technique for the investigation of apatite provenance. The typical examples include the diagrams of Sr-Y, Sr-Mn, Y-(Eu/Eu^*) and (Ce/Yb)_N-REE. However, the current discrimination diagrams cannot precisely distinguish apatite types due to the massive increase of apatite trace elements that are detected by the developing microscale analyses, which therefore cannot precisely predict the apatite-formation environment. Fortunately, the significant increase of apatite trace element data advances our understanding in apatite provenance by using the big data analysis technique. In this study, we applied the big data study for the collected 1925 published apatite trace element data, analyzing six apatite-associated rock types, which are alkali-rich igneous rocks, ultramafic rocks, mafic igneous rocks, felsic granitoids, low- and medium-grade metamorphic rocks and high-grade metamorphic rocks. The yield 7140 apatite-associated discrimination diagrams were further detailed evaluated by introducing the Silhouette coefficient. By the process, we proposed an Eu/Y-Ce discrimination diagram to distinguish apatite.

Application of IOR techniques in fractured basement reservoirs – A case study

As the production of oil and gas from conventional reservoirs is declining steadily, the interest in development of naturally fractured basement reservoirs has been on the rise in the past few decades. The naturally fractured reservoirs category comprises basement reservoirs which in turn includecrystalline formations ranging from intrusive and extrusive magmatic bodies (particularly granites) to the low to medium-grade metamorphic rocks. Basement refers to a set of igneous (intrusive or extrusive) and metamorphic rocks that sit under an unconformity at the bottom of a sedimentary sequence. In crystalline fractured and weathered crystalline basement reservoirs, natural fractures play an important role in hydrocarbon migration and storage. Naturally fractured reservoirs account for a huge share of the world's remaining hydrocarbon reserves. The hydrocarbon industry has known for a long time that naturally fractured basement reserves exist. They were once thought to be non-productive and were frequently labelled as having “no economic potential.” They may, however, be found in a wide range of petroliferous basins all over the world. Many IOR techniques are used and studied in this technical paper for developing a favourable model for improved hydrocarbon recovery from naturally fractured reservoirs, including hydraulic proppant fracturing, advanced emulsion acidizing stimulation, directional drilling application, gel treatment, and other favourable IOR techniques.

Petrogenesis and tectonic significance of Paleoproterozoic metavolcanic rocks in the Khondalite Belt, North China Craton

As a typical continent–continent collision belt, the Khondalite Belt (KB) of the North China Craton (NCC) represents a Paleoproterozoic orogenic belt that underwent subduction and collision. Although many high-grade metamorphic rocks have been well studied, the low to medium grade metamorphic rocks well preserved in KB are less studied, which hinders a better understanding of the initiation of Paleoproterozoic subduction and collision processes. The Erdaowa Group is a representative of such a metamorphic rock series. Zircon U-Pb dating of the metavolcanic rocks in the Erdaowa Group yields the eruption ages of 2465 ± 24 Ma and 2350 ± 34 Ma, and the metamorphic ages of 1933 ± 32 Ma and 1844 ± 30 Ma. The geochemical results show that the metavolcanic rocks of the Erdaowa Group belong to tholeiitic basalts series. The enrichment of large ion lithophile elements (LILEs) such as Rb and Ba, and the depletion of high field strength elements (HFSEs) such as Ta, Nb and Ti indicate that the metavolcanic rocks have arc geochemical affinities and may have been formed at an active continental margin associated with subduction. Zircon Lu-Hf isotopic analysis shows that εHf(t) values are −0.15 – + 8.72, and the single-stage depleted mantle model age is 2448 – 2707 Ma, indicating that the primary magma originated from partial melting of the depleted lithospheric mantle and experienced slight crustal contamination. The results further prove that the low and medium grade rocks were directly involved in the subduction and collision between the Yinshan Block and the Ordos Block.

Crystallization Conditions and Petrogenetic Characterization of Metaluminous to Peraluminous Calc-Alkaline Orogenic Granitoids from Mineralogical Systematics: The Case of the Cambrian Magmatism from the Sierra de Guasayán (Argentina)

The Sierra de Guasayán (Eastern Sierras Pampeanas, Argentina) is formed by low to medium grade metamorphic rocks intruded by Cambrian metaluminous (La Soledad quartz-diorite), slightly peraluminous (Guasayán, El Escondido and El Martirizado granodiorite plutons), and strongly peraluminous (Alto Bello granodiorite) granitoids of the Pampean magmatic arc. Chemical compositions of amphibole, plagioclase, biotite, and titanite indicate that these granitoids were emplaced at low pressure (mostly &lt;3 kbar) and temperature (&lt;770 °C) under oxidizing conditions (QFM + 1 and QFM + 2), which are similar to the emplacement conditions reported for other granites of the Pampean magmatic arc. Mineral assemblages and whole-rock and mineral chemistry of the granitoids from the Sierra de Guasayán indicate an I-type affinity for the La Soledad quartz-diorite (amphibole, biotite, and titanite), S-type affinity for the Alto Bello granodiorite (biotite, muscovite, cordierite, and sillimanite), and a hybrid nature for the main Guasayán and El Escondido plutons (biotite, monazite, and magnetite). This hybrid nature is supported by the presence of abundant mafic microgranular enclaves and rapakivi texture and by published zircon Hf-isotope data (εHfi ranging from −4.76 to −0.12). This suggests, in turn, the involvement of hybridization in the genesis of these granitoids, which seems to be a common mechanism operating in the Pampean magmatic arc.

Těžké minerály rozsypových ložisek zlata vázaných na khantaishirský ofiolitový komplex poblíž měst Altaj a Khaliun (jihozápadní Mongolsko)

Detailed morphological and chemical studies of heavy minerals from two localities fluvial sediments in the area of the khantaishir ophiolitic complex near the towns Altai and Khaliun (Southwestern Mongolia) allowed the interpretation possible source region for the gold. The heavy mineral spectrum from the sediments near the Altai town is dominated by magnetite (32 %), chromite (27 %), epidote (11 %), apatite (6 %), and clinopyroxene (5 %). We assume that these minerals come from the ultrabasic and basic igneous rocks in the Neoproterozoic khantaishir ophiolitic complex. The relatively undeformed and three-dimensional shape of gold particles indicating short distance their transport. Rare is native gold enclosed in dolomite or quartz, which indicates that potential gold sources are listvenite. The heavy mineral spectrum from the fluvial sediments in the small creek near the Khaliun town is different. The studied sample includes magnetite (31 %), amphibole (19 %), zircon (18 %), pyrite (13 %), apatite (5 %), epidote (4 %), titanite (4 %), clinopyroxene (2 %), monazite (1 %), ilmenite (1 %), garnet (1 %), and barite (0.1 %). Large variations in the mineral composition heavy mineral spectrum indicate a wide source area which includes basic to intermediate igneous rocks Cambrian-Ordovician Ikh-Mongol Arc System and medium-grade metamorphic rocks (metapelite). The subspherical rounded shape of the gold particles indicates fluvial transport. In the case of small and geologically simple drainage area as creek near the Altai town represents heavy minerals a good tool for determination of the origin of placer gold. There is a contrast between the heavy mineral spectrum from the localities near the Altai and Khaliun towns. The shape of gold particles as well as a simple heavy mineral spectrum from sediments near the Altai indicates short transport from the limited draining area (approximately 6 km2). Gold probably originating from the ultramafic rocks (listvenite), according to associated dolomite and simple spectrum of heavy minerals. Whereas the origin of gold from the placer deposits near Khalinun remains unclear and most probably could originate from the hydrothermal veins in intermediate or basic igneous rocks (presence of barite associated with abundant pyrite).

Diachronous onset and polyphase cooling of the Taili-Yiwulüshan metamorphic core complex corridor, NE China, and its relationships to the formation of adjacent extensional basins

The aim of the study to investigate the cooling and exhumation history of deep crustal rocks along the Taili-Yiwulüshan metamorphic core complex (MCC) corridor and correlate these with volcanic-clastic deposition in the Late Jurassic to Early Cretaceous supra-detachment Fuxin-Yixian basin in the west and the mostly Eocene Liaohe basin in the east. This corridor exposes low- to medium-grade metamorphic Neoarchean rocks and Late Triassic, Late Jurassic and Early Cretaceous granitic rocks, which exhumed from the middle to shallow crustal levels. New 40Ar/39Ar ages of muscovite, biotite, K-feldspar and apatite (U-Th-Sm)/He ages in combination with compiled published geochronological data from the granitic rocks constrain three major stages of cooling exhumation and deformation from Late Jurassic through Early Cretaceous to Eocene interrupted by Late Cretaceous tectonic quiescence. The onset of activity and exhumation and cooling is diachronous along the MCC corridor. At Taili in the south, the main activity of the sinistral shear zone is constrained between 160 and 140 Ma with major cooling from crystallization temperatures of granitoids from 700–750 °C to 160 °C (first stage). Along the Waziyu ductile detachment at the western border of the Yiwulüshan MCC major cooling and exhumation is bracketed between 130 Ma and 100 Ma (second stage) and thus indicates a northward progradation of deformation and exhumation activity. However, cooling ages as old as 159 Ma along the eastern margin of the Yiwulüshan MCC indicate an earlier initiation of cooling and westward rolling-hinge type propagation (first stage). Late Cretaceous and Paleogene times are characterized by a tectonically quiet period with only minor cooling and exhumation (stage three). The fourth and final stage of exhumation and cooling to very low temperatures occurred during Late Paleocene to earliest Eocene times and is associated with the transtensional or extensional opening of the Liaohe basin.

U–Pb detrital zircon ages from Gorgoglione Flysch sandstones in Southern Apennines (Italy) as provenance indicators

AbstractTwenty-one sandstone samples belonging to the intermediate part of Gorgoglione Flysch (GF) dated Middle-Miocene, cropping out in the Southern Apennines (Italy), have been studied to highlight the detritus provenance. Petrographic and chemical composition indicates that the successions consist of feldspatho-quartzose and litho-feldspatho-quartzose arenites interbedded with pelitic and calciclastic layers and reveals a provenance from a basement formed by low- to medium-grade metamorphic rocks with abundant granitoids covered by sedimentary rocks in which a volcanic component was also present. In the Mediterranean area, basements with these characteristics are widespread both in western and southwestern domains. The supply provenance of Gorgoglione Flysch has been better detailed utilizing U–Pb detrital zircon ages recording the geological history of the source rocks. Fifty-eight crystals from six samples of coarse- and fine-grained sandstones have been analysed using the U–Pb isotopic systematic (LA-ICP-MS). They produce 70 concordant zircon ages forming three defined clusters at 672 ± 28 Ma, 458 ± 9 Ma and 297 ± 8 Ma, and four zircon ages corresponding to 24 ± 1 Ma. An evaluation of the entire collected data suggests that the provenance area is better identified in northwestern sectors of the Mediterranean area in which the Sardinia–Corsica (pro-part) block plays a fundamental role.

Actual performance analysis of a double shield TBM through sedimentary and low to medium grade metamorphic rocks of Ghomrood water conveyance tunnel project (lots 3 and 4)

In this paper, after a brief description of engineering geological properties of Ghomrood water conveyance tunnel project (lots 3 and 4 with a total length of 18 km), the actual performance of the machine during the excavation of tunnel through sedimentary and low to medium grade metamorphic rocks has been analyzed. Subsequently, using data collected during pre-construction and construction phases of the project, the relationships between rock mass properties and measured machine performance parameters have been investigated and proposed as a prediction model which can be used in future projects in which similar geological conditions are encountered. Results of analyses indicated that there are strong relationships between TBM performance and engineering geological conditions of the tunnel alignment.

Kinematics and Timing Constraints in a Transpressive Tectonic Regime: The Example of the Posada-Asinara Shear Zone (NE Sardinia, Italy)

Detailed geological field mapping, integrated with meso- and microstructural investigations, kinematic of the flow and finite strain analyses, combined with geochronology, are fundamental tools to obtain information on the temperature–deformation–timing path of crystalline rocks and shear zone. The Posada-Asinara shear zone (PASZ) in northern Sardinia (Italy) is a steeply dipping km-thick transpressive shear zone. In the study area, located in the Baronie region (NE Sardinia), the presence of mylonites within the PASZ, affecting high- and medium-grade metamorphic rocks, provides an opportunity to quantify finite strain and kinematic vorticity. The main structures of the study area are controlled by a D2 deformation phase, linked to the PASZ activity, in which the strain is partitioned into folds and shear zone domains. Applying two independent vorticity methods, we detected an important variation in the percentage of pure shear and simple shear along the deformation gradient, that increases from south to north. We constrained, for the first time in this sector, the timing of the transpressive deformation by U–(Th)–Pb analysis on monazite. Results indicate that the shear zone has been active at ~325–300 Ma in a transpressive setting, in agreement with the ages of the other dextral transpressive shear zones in the southern Variscan belt.

Heavy Minerals as Indicators of the Source and Stratigraphic Position of the Loess-Like Deposits in the Orava Basin (Polish Western Carpathians)

This study is focused on the loess-like deposits accumulated on glaciofluvial fans of the Czarny Dunajec River in the Orava Basin (Southern Poland). The deposition of these sediments took place during three cold intervals of the Pleistocene: Würm, Riss, and Günz/Mindel. So far, the provenance and age of the deposits has not been precisely defined, even though the development of each fan is believed to be related to the successive glacial periods in the Tatra Mountains. Heavy minerals were studied to determine the source of the deposits. Heavy mineral analyses revealed that zircon, tourmaline, rutile, garnet, amphibole, epidote, and apatite are the typical constituents of the heavy mineral fraction. Abundances of heavy minerals differ in each of the Pleistocene fans of the Czarny Dunajec River, especially the amphibole content. However, the chemical composition of garnet, amphibole, and tourmaline is rather uniform. This research showed that mainly medium-grade metamorphic rocks with a subordinate share of high-grade metamorphics, and granitic rocks are the dominant source rocks of the deposits studied. Such rocks are exposed in the Western Tatra Mountains, which most probably supplied the Orava Basin with clastic material. Change in abundances of heavy minerals in the succession may reflect the progressive erosion of the source area. Grain-size distribution and textural features of the sampled sediments suggest fluvial and aeolian modes of transportation. Additionally, this study indicated that heavy minerals may be used to correlate the loess covers in the Orava Basin.

Fast cooling of normal-fault footwalls: Rapid fault slip or thermal relaxation?

Abstract Rapid rock exhumation in mountain belts is commonly associated with crustal-scale normal faulting during late-orogenic extension. The process of normal faulting advects hot footwall rocks toward Earth’s surface, which shifts isotherms upwards and increases the geothermal gradient. When faulting stops, this process is reversed and isotherms move downwards during thermal relaxation. Owing to these temporal changes of the geothermal gradient, it is not straightforward to derive the history of faulting from mineral cooling ages. Here, we combine thermochronological data with thermokinematic modeling to illustrate the importance of syntectonic heat advection and posttectonic thermal relaxation for a crustal-scale normal fault in the European Alps. The north-south–trending Brenner fault defines the western margin of the Tauern window (Austria) and caused the exhumation of medium-grade metamorphic rocks during Miocene orogen-parallel extension of the Alps. We analyzed samples from a 2-km-thick crustal section, including a 1000-m-long drill core. Zircon and apatite (U-Th)/He ages along this transect increase with elevation from ca. 8 to ca. 10 Ma and from ca. 7 to ca. 9 Ma, respectively, but differ by only ∼1 m.y. in individual samples. Thermokinematic modeling of the ages indicates that the Brenner fault became active at 19 ± 2 Ma and caused 35 ± 10 km of crustal extension, which is consistent with independent geological constraints. The model results further suggest that the fault slipped at a total rate of 4.2 ± 0.9 km/m.y. and became inactive at 8.8 ± 0.4 Ma. Our findings demonstrate that both syntectonic heat advection and posttectonic thermal relaxation are responsible for the cooling pattern observed in the footwall of the Brenner normal fault.

Regional sediment sources versus the Indus River system: The Plio-Pleistocene of the Peshawar Basin (NW-Pakistan)

Sedimentary archives within the Peshawar Basin in northwest Pakistan preserve a record of siliciclastic supply patterns during the Plio-Pleistocene epoch. The basin fill consists of fluvial-floodplain clays overlain by thick lacustrine-glaciofluvial rhythmites with deposits of multiple alluvial fans in the southern parts of the basin and topped by loess deposits in ideal vertical successions. Framework mineralogical composition and bulk sediment geochemical proxies of these sediments indicate detritus derivation from dominantly acidic igneous sources with contribution from medium-grade metamorphic rocks. There appears to be no significant contribution from ultramafic-mafic or very high grade metamorphic sources. In addition, sediments from the southern part of the basin support supply from low-grade metamorphic rocks and carbonate sediments. Overall sedimentation occurred in an active continental margin setting under conditions of insignificant chemical weathering. The uplift of the Attock–Cherat Range during the late Pliocene (2.8 Ma) restricted the Indus River from shedding its detritus into the basin. Therefore, the sediments of the Peshawar Basin do not reflect signatures of the Indus River siliciclastics and thus lack the basic igneous geochemical signal, which is otherwise typical for the Himalayas hinterland. On the contrary, the geochemical signatures of these sediments favour predominant detritus flux from the acidic rocks of the Peshawar Plain Alkaline Igneous Complex (PPAIC) and associated metasediments. Therefore, the Kabul-Swat River and other local tributaries of this smaller river system that originated within the PPAIC and associated mountains are considered the major transporting agents which contributed siliciclastics into the Peshawar Basin during the Plio-Pleistocene. Additionally, in the southern part of the basin the uplifting Attock–Cherat Range appeared to have contributed detritus in the form of alluvial-fluvial fan deposits.