Volcanic-plutonic Belt Research Articles

Mineralogy, Geochemistry, Fluid Inclusion and Oxygen Isotope Investigations of Epithermal Cu ± Ag Veins of the Khur Area, Lut Block, Eastern Iran

The Khur metallogenic district is located in a volcanic–plutonic belt in the central Lut Block (central eastern Iran). Mineralization occurs in Middle Eocene andesitic tuff and along four main vein systems trending northwest–southeast (Shurk, Mir‐e‐Khash, Shikasteh Sabz and Ghar‐e‐Kaftar veins). Microscopic studies reveal that the veins contain bornite, chalcocite, pyrite, tennantite together with minor sphalerite and chalcopyrite as hypogene minerals and chalcocite, digenite, covellite, valleriite, malachite, azurite, atacamite, hematite, and goethite as supergene minerals. The ore bodies are accompanied by narrow but intensely developed wall rock alterations of argillization, carbonatization and silicification. Copper content reaches 6.5, 2.4, 4.2 and 5% in Mir‐e‐Khash, Shikasteh Sabz, Ghar‐e‐Kaftar and Shurk, respectively. Microthermometric measurements of quartz‐ and calcite‐hosted fluid inclusions indicate that the mineralization might be derived from a moderately saline hydrothermal fluid at temperatures between 175–316°C. Calculated δ18O values of water in equilibrium with quartz and calcite for Khur veins suggest that the fluid might have had a magmatic source, but the 18O‐depletion was developed through mixing with meteoric water. Copper deposition in Khur veins is believed to have been largely caused by mixing, although wall rock reactions may also have occurred. The Khur veins are classified as volcanic‐subvolcanic hydrothermal‐related vein deposits.

Carboniferous continental arc in the Hegenshan accretionary belt: Constrains from plutonic complex in central Inner Mongolia

The architecture and tectonic evolution of the Hegenshan accretionary belt in the Central Asian Orogenic Belt (CAOB) remains debated. Here we present an integrated study of zircon U–Pb isotopic ages, whole rock major-trace elements, and Sr–Nd–Pb isotopic data from the Hegenshan volcanic-plutonic belt in central Inner Mongolia. Field observations and zircon U–Pb ages allow us to divide the intrusive complex into an early phase at 329–306 Ma and a late phase at 304 to 299 Ma. The intrusive bodies belong to two magma series: calc-alkaline rocks with I-type affinity and A-type granites. The early intrusions are composed of granodiorite, monzogranite and porphyritic granite, and the late calc-alkaline intrusions include gabbro though diorite to granodiorite. The calc-alkaline intrusive rocks exhibit a well-defined compositional trend from gabbro to granite, reflecting continuous fractional crystallization. These rocks show obvious enrichment in LILEs and LREEs and relative depletion of HFSEs, typical of subduction-related magma. They also exhibit isotopic characteristics of mantle-derived magmas such as low initial 87Sr/86Sr (0.7029–0.7053), positive ɛNd(t) values (0.06–4.76) and low radiogenic Pb isotopic compositions ((206Pb/204Pb)I = 17.907–19.198, (207Pb/204Pb)I = 15.474–15.555, (208Pb/204Pb)I = 37.408–38.893). The marked consistency in geochemical and isotopic compositions between the intrusive rocks and the coeval Baoligaomiao volcanic rocks define a Carboniferous continental arc. Together with available regional data, we infer that this east-west trending continental arc was generated by northward subduction of the Hegenshan ocean during Carboniferous. The late alkali-feldspar granites and the high-Si rhyolites of the Baoligaomiao volcanic succession show similar geochemical compositions with high SiO2 and variable total alkali contents, and low TiO2, MgO and CaO. These rocks are characterized by unusually low Sr and Ba, and high abundances of Zr, Th, Nb, HREEs and Y, comparable to the features of typical A2-type granites including their high ratios of FeOT/MgO, Ga/Al and Y/Nb. Our study suggests that the A-type granite was derived from a distinct magma source rather than through fractional crystallization of the coeval calc-alkaline magmas. Their Nd–Pb isotopic compositions are similar to those of calc-alkaline arc rocks and are compatible with partial melting of pre-existing juvenile basaltic crust in the continental arc. Notably, the widespread eruptions of A2-type rhyolitic magmas (305.3 Ma–303.4 Ma) following a short period of magmatic quiescence was temporally and spatially associated with bimodal magmatism with mantle-derived gabbro-diorites and A-type granites (304.3 Ma–299.03 Ma) in the pre-existing arc volcanic-plutonic belt (329 Ma–306 Ma). Such a marked change in the magma affinity likely indicates subducted slab break-off resulting in a change of the regional stress field to an extensional setting within the Carboniferous continental arc that runs E-W for few thousands of kilometers. Thus, the onset of the late magmatism (305–299 Ma) likely represents the maximum age for the cessation of the northward subduction in the Hegenshan ophiolite-arc-accretion belt.

Sources of Matter and Ore-Producing Fluid of the Tamunyer Gold–Sulfide Deposit (Northern Urals): Isotope Results

The Tamunyer deposit is a typical example of gold–sulfide mineralization located in the lower lithologic–stratigraphic unit (S2–D1) of the Auerbach volcanic–plutonic belt. The latter comprises island–arc andesitic volcano–sediments, volcanics, and comagmatic intrusive formations. Carbonates have demonstrated intermediate values of δ13C between marine limestone and mantle. The quartz δ18O is in the range of 15.3–17.2‰. The δ34S of sulfides from the beresitized volcano-sedimentary rocks and ores varies widely from –7.5 to 12‰. The calculated isotope compositions of H2O, CO2, and H2S of the ore-bearing fluid imply two major sources of matter contributing to ore genesis: local rocks and foreign fluid. The ore-bearing fluid was formed by interaction and isotope equilibration between a deep magmatic fluid and marine carbonates (W/R ~ 1), with the contribution of sulfur from the volcano-sedimentary rocks.

Porphyry-Style Petropavlovskoe Gold Deposit, the Polar Urals: Geological Position, Mineralogy, and Formation Conditions

Geological and structural conditions of localization, hydrothermal metasomatic alteration, and mineralization of the Petropavlovskoe gold deposit (Novogodnenskoe ore field) situated in the northern part of the Lesser Ural volcanic–plutonic belt, which is a constituent of the Middle Paleozoic island-arc system of the Polar Urals, are discussed. The porphyritic diorite bodies pertaining to the late phase of the intrusive Sob Complex play an ore-controlling role. The large-volume orebodies are related to the upper parts of these intrusions. Two types of stringer–disseminated ores have been revealed: (1) predominant gold-sulfide and (2) superimposed low-sulfide–gold–quartz ore markedly enriched in Au. Taken together, they make up complicated flattened isometric orebodies transitory to linear stockworks. The gold potential of the deposit is controlled by pyrite–(chlorite)–albite metasomatic rock of the main productive stage, which mainly develops in a volcanic–sedimentary sequence especially close to the contacts with porphyritic diorite. The relationships between intrusive and subvolcanic bodies and dating of individual zircon crystals corroborate a multistage evolution of the ore field, which predetermines its complex hydrothermal history. Magmatic activity of mature island-arc plagiogranite of the Sob Complex and monzonite of the Kongor Complex initiated development of skarn and beresite alterations accompanied by crystallization of hydrothermal sulfides. In the Early Devonian, due to emplacement of the Sob Complex at a depth of approximately 2 km, skarn magnetite ore with subordinate sulfides was formed. At the onset of the Middle Devonian, the large-volume gold porphyry Au–Ag–Te–W ± Mo,Cu stockworks related to quartz diorite porphyry—the final phase of the Sob Complex— were formed. In the Late Devonian, a part of sulfide mineralization was redistributed with the formation of linear low-sulfide quartz vein zones. Isotopic geochemical study has shown that the ore is deposited from reduced, substantially magmatic fluid, which is characterized by close to mantle values δ34S = 0 ± 1‰, δ13C =–6 to–7‰, and δ18O = +5‰ as the temperature decreases from 420–300°C (gold–sulfide ore) to 250–130°C (gold–(sulfide)–quartz ore) and pressure decreases from 0.8 to 0.3 kbar. According to the data of microanalysis (EPMA and LA-ICP-MS), the main trace elements in pyrite of gold orebodies are represented by Co (up to 2.52 wt %), As (up to 0.70 wt %), and Ni (up to 0.38 wt %); Te, Se, Ag, Au, Bi, Sb, and Sn also occur. Pyrite of the early assemblages is characterized by high Co, Te, Au, and Bi contents, whereas the late pyrite is distinguished by elevated concentrations of As (up to 0.7 wt %), Ni (up to 0.38 wt %), Se (223 ppm), Ag (up to 111 ppm), and Sn (4.4 ppm). The minimal Au content in pyrite of the late quartz–carbonate assemblage is up to 1.7 ppm and geometric average is 0.3 ppm. The significant correlation between Au and As (furthermore, negative–0.6) in pyrite from ore of the Petropavlovskoe deposit is recorded only for the gold–sulfide assemblage, whereas it is not established for other assemblages. Pyrite with higher As concentration (up to 0.7 wt %) is distinguished only for the Au–Te mineral assemblage. Taking into account structural–morphological and mineralogical–geochemical features, the ore–magmatic system of the Petropavlovskoe deposit is referred to as gold porphyry style. Among the main criteria of such typification are the spatial association of orebodies with bodies of subvolcanic porphyry-like intrusive phases at the roof of large multiphase pluton; the stockwork-like morphology of gold orebodies; 3D character of ore–alteration zoning and distribution of ore components; geochemical association of gold with Ag, W, Mo, Cu, As, Te, and Bi; and predominant finely dispersed submicroscopic gold in ore.

The volcanic succession of Baoligaomiao, central Inner Mongolia: Evidence for Carboniferous continental arc in the central Asian orogenic belt

The Baoligaomiao Formation, within the Hegenshan ophiolite-arc-accretion complex is an important segment to understand the tectonic evolution of the Central Asian Orogenic Belt (CAOB), world's largest Phanerozoic orogenic belt. In this study, we present an integrated study of zircon U-Pb isotopic ages, whole rock major-trace elements, and Sr-Nd-Pb isotopic data from the volcanic succession in the Baoligaomiao Formation. The volcanic succession can be divided into the lower sequence with zircon U-Pb ages in the range of 326.3Ma–307.4Ma and the upper sequence of 305.3Ma. The succession belongs to two suites: calc-alkaline volcanics and high-Si rhyolites. The calc-alkaline volcanic rocks include basaltic andesite through andesite and dacite to rhyolite and their pyroclastic equivalents. These rocks exhibit a well-defined compositional trend from basaltic to rhyolitic magma, reflecting continuous fractional crystallization. These rocks show obvious enrichment in LILEs and LREEs and relative depletion of HFSEs, typical of subduction-related magma. The calc-alkaline rocks have low initial 87Sr/86Sr (0.7023–0.7052), positive ɛNd(t) values (2.75–4.80), and their initial Pb isotopic compositions are 17.875–18.485 of 206Pb/204Pb, 15.481–15.520 of 207Pb/204Pb and 37.467–37.764 of 208Pb/204Pb, respectively. Geochemical and isotopic results suggest that the volcanic succession represents Carboniferous subduction-related, mature, continental arc volcanism. The outcrops of high-Si rhyolites are restricted to the northern edge of the continental arc, marking a transition zone between volcanic arc and back-arc basin, where they are interbedded with the calc-alkaline rocks in the lower sequence, and the upper sequence is composed only of high-Si rhyolites. The high-Si rhyolites have high SiO2 (71.12–81.76wt%) and varied total alkali contents (K2O+Na2O=5.46–10.58wt%), low TiO2 (0.06–0.27wt%), MgO (0.09–0.89wt%) and CaO (0.08–0.72wt%). Based on the presence of mafic alkali phenocrysts, such as arfvedsonite and siderophyllite, high Zr/Nb ratios (>10) and peralkalinity index (PI) near unity, the high-Si rhyolites can be classified as peralkaline comendites. The high-Si rhyolites are characterized by unusually low Sr and Ba, and high abundance of Zr, Th, Nb, HREEs and Y. They show geochemical characteristics similar to those of typical A2-type granites including their high K2O+Na2O, Nb, Zr and Y, and high ratios of FeOT/MgO, Ga/Al and Y/Nb. Our study suggests that the high-Si rhyolites were derived from discrete trachytic parent magma with fractional crystallization within shallow magma reservoirs. Their Nd-Pb isotopic characteristics are similar to those of the calc-alkaline arc rocks and are compatible with partial melting of pre-existing juvenile continental arc crust. We observe that the widespread eruptions of A2-rhyolitic magmas (305.3Ma–303.4Ma) following a short period of magmatic quiescence was temporally and spatially associated with voluminous intrusion of the bimodal magmas (304.3Ma–299.3Ma) in the pre-existing arc volcanic-plutonic belt (329Ma–307Ma). We envisage northward subduction and slab breakoff process resulting in an obvious change of the regional stress field to extensional setting within the Carboniferous continental arc running E-W for thousands of kilometers. Therefore, we propose the existence of an east-west-trending Carboniferous continental arc in the Hegenshan ophiolite-arc-accretion complex, with the slab breakoff event suggesting that the age of the upper sequence (305.3±5.5Ma) likely indicates the maximum age for the cessation of the northward subduction of the Hegenshan oceanic lithosphere.

Sources of clastic material of Cretaceous–Paleogene deposits in the southwestern part of the Amur–Zeya Basin: results of dating U–Pb detrital zircons

The results of our study indicate that at the Late Cretaceous–Paleogene boundary in the southeastern part of the Amur–Zeya Basin, the sedimentation conditions changed drastically, namely, the change of provenance areas of debris. In the Maastrichtian, the clastic material was mainly transported from the Bureya–Jiamusi Superterrane and the volcanic–plutonic belts of Khingan–Okhotsk and East Sikhote–Alin located to the east: sedimentation occurred simultaneously with magmatic activity. During the Danian Stage, the major source of debris to the southern part of the basin was located to the south of the young mountain system of the Lesser Khingan (the uplifted part of the basement of the Songliao Block).

The Mongol-Okhotsk Ocean subduction-related Permian peraluminous granites in northeastern Mongolia: Constraints from zircon U-Pb ages, whole-rock elemental and Sr-Nd-Hf isotopic compositions

Northeastern Mongolia represents a unique area that experienced evolution of both the Mongol-Okhotsk Ocean (MOO) and the Central Asian Orogenic Belt (CAOB). In order to better understand the evolution of MOO and its effect on the CAOB, we performed zircon U-Pb dating and Hf isotope, and whole-rock elemental and Sr-Nd isotopic analyses on three Permian peraluminous granitic plutons in northeastern Mongolia. Zircon dating result revealed that granitoids of East Monhhaan, South Monhhaan and Tuvshinshiree plutons were emplaced at 254±3, 265±5, and 273±3Ma, respectively. The result provides Permian ages for the Permian-Triassic volcanic-plutonic belt in northeastern Mongolia. Geochemical analysis suggests that all the granitoids are of peraluminous S-type granites. Elemental and Sr-Nd-Hf isotopic data indicate that these S-type granites were generated from mainly clay-poor greywacke with some contribution of juvenile mantle component. The Permian S-type granites emplaced likely in an active margin related to the subduction of the Mongol-Okhotsk Ocean. The protracted intermittent extension during the long-lived compression was considered as a likely geodynamic mechanism for the upwelling of mantle-derived basaltic magma and melting of crustal materials.

Glass at endocontacts of granite massifs in Volcanic–Plutonic belts (Exemplified by granite of Central Kazakhstan)

Glass formation takes place under conditions of rapid cooling of a melt at a great temperature difference between the melt and the host rock, which must have high thermal conductivity. The most favorable conditions for glass formation exist when the melt intrudes in the form of thin apophyses. Glass has been found in granite massifs related to the volcanic–plutonic association.

U–Pb zircon geochronology, Sr–Nd geochemistry, petrogenesis and tectonic setting of Mahoor granitoid rocks (Lut Block, Eastern Iran)

The Mahoor Cu–Zn-bearing porphyritic granitoid rocks belong to the Lut Block volcanic–plutonic belt (central Eastern Iran). These granitoid rocks occur mainly as dykes and stocks that intrude into Eocene volcanics and pyroclastic rocks. Petrographically, all the studied intrusives display porphyritic textures with mm-sized phenocrysts, most commonly of plagioclase and hornblende, embedded in a fine-grained groundmass with variable amounts of plagioclase, hornblende, clinopyroxene, quartz and opaque minerals. Hydrothermal alteration affected these granitoid rocks, as revealed by the common occurrence of sericite, chlorite, titanite, epidote and calcite. Chemical classification criteria show that the intrusives may be named as gabbrodiorites, diorites, monzodiorites and tonalites. Major elements geochemistry reveals that all the studied lithologies are typically metaluminous (A/CNK⩽0.9). Magnetic susceptibility (1485×10−5 SI) together with mineralogical and geochemical features shows that they belong to magnetite granitoid series (I-type). Trace element patterns normalized to chondrite and primitive mantle are very similar to each other and show enrichments in LREE relative to HREE and in LILE relative to HFSE, as well as negative anomalies of Ta, Nb and Ti. Eu/Eu∗ ratios vary from 0.88 (in the most mafic composition) to 0.65, showing that plagioclase played a role in magma differentiation. LA-MC-ICP-MS U–Pb zircon data from a diorite, yielded similar concordia ages of ca. 31.88±0.2Ma (Error: 2σ), which corresponds to the Oligocene period. These granitoid rocks have (87Sr/86Sr)i values vary between 0.7055 and 0.7063. In terms of isotopic compositions, while εNdi is between −0.6 and −2.5, suggesting that magmas underwent contamination through being exposed to the continental crust. The whole set of geochemical data agree with the emplacement of the studied intrusions in a magmatic belt above a subduction zone. Primitive magmas should have formed by melting of mantle wedge peridotite, and during magma ascent to crustal levels, both magma differentiations took place by crystal fractionation and crustal contamination. Sulfide mineralizations (pyrite, chalcopyrite and sphalerite) related to these granitoid rocks is common and occurs as both disseminated and hydrothermal veins, indicating a high mineralization potential for this area.

Tectonic evolution of Kazakhstan and Tien Shan in Neoproterozoic and Early-Middle Paleozoic

Geological information on Kazakhstan and the Tien Shan obtained up to the present time has been considered and integrated in order to demonstrate the main features of continental massifs, basins with oceanic crust, island arcs, marginal volcanic-plutonic belts, and transform fault zones differing in type and age. We ascertained the character and probable causes of their evolution and transformations resulting in the origination and development of mosaic structural assembly at margin of the Paleoasian ocean that existed from Neoproterozoic. The main stages of the geodynamic history of Paleozoides in Kazakhstan and Tien Shan are characterized, and a model of the probable course of regional tectonic events has been proposed. This model is illustrated by published paleomagnetic data and a series of paleotectonic reconstructions for time intervals 950–900, 850–800, 750–700, 650–630, 570–550, 530–515, 500–470, 460–440, and 390–380 Ma.

Petrochemistry and tectonic setting of the Davarzan-Abbasabad Eocene Volcanic (DAEV) rocks, NE Iran

The Davarzan–Abbasabad Eocene Volcanics (DAEV) is located at the northeastern edge of the volcanic–plutonic belt of the Central Iran structural zone. DAEV start with continental to shallow marine sediments of the Paleocene–Eocene and then continue extensively with shallow submarine to sub-aerial basaltic and andesitic volcanics, related volcaniclastics and sedimentary rocks during the Middle–Upper Eocene. The volcanics are olivine basalts, basaltic andesites and andesites in composition and show porphyric, hyaloporphyritic, glomeroporphyric, fluidal, and seriate textures. The geochemistry of DAEV points to a fractional series from olivine basalt to andesite with transitional and medium to high K calc–alkaline nature. Their primitive mantle and chondrite-normalized trace element patterns show that these rocks have high light- and low heavy rare earth elements and are enriched in large-ion lithophile elements and depleted in high field strength elements (Nb, Ti, P, and Zr). Integration of geochemical and petrological data with regional studies indicates that the magma(s) forming the DAEV rocks were derived from 14 to 16 % partial melting of an enriched mantle source in 70–100 km depths during Middle–Upper Eocene time in an intra-arc extensional setting. This mantle source had been previously metasomatized by fluids derived from Sabzevar Neo–Tethyan-subducted slab during the Upper Cretaceous–Paleocene.

پترولوژی و سن سنجی زیرکن به روش U-Pb در توده های نفوذی مناطق A، C جنوبی و دردوی، معدن سنگ آهن سنگان خواف

معدن سنگ آهن سنگان خواف در 300 کیلومتری جنوب‌شرقی مشهد، در شرق کمربند آتشفشانی- نفوذی خواف- کاشمر- بردسکن قرار دارد. توده های گرانیتوئید در سه محدوده معدن سنگ آهن سنگان شامل نواحی A، C جنوبی و دردوی مورد مطالعه قرار گرفتند. سه توده نفوذی بیوتیت هورنبلند مونزونیت پورفیری، بیوتیت سینیت و سینوگرانیت در مناطق مورد مطالعه شناسایی شد. براساس روابط قطع‌شدگی صحرایی، عدم وجود گارنت اسکارن و مگنتیت در مرز این توده ها و آلتره‌شدن آنها توسط محلول کانه دار بعدی، این توده های گرانیتوئیدی از اسکارن مگنتیت دار قدیم‌ترند. سن سنجی زیرکن به روش U-Pb سن آنها را 42 میلیون سال قبل (ائوسن‌ میانی) تعیین می‌کند. مقدار پذیرفتاری مغناطیسی آنها بین SI5-10×310 تا SI5-10×900 است، بنابراین متعلق به گرانیتوئیدهای سری مگنتیت (اکسیدان) و از نوع I هستند. براساس بررسیهای ژئوشیمیایی، توده های نفوذی عمدتا از نوع متاآلومینوس، منیزیمی، آلکالی‌کلسیک تا آلکالیک و شوشونیتی تا اولتراپتاسیک می باشند. غنی‌شدگی نسبی عناصر LREE نسبت به HREE و عناصر LILE (Sr, Cs, Rb, K and Ba) نسبت به HFSE (Nb, Ta, Ti, Hf, Zr) مؤید تشکیل ماگما در زون فرورانش است. این ماگما از ذوب‌بخشی اندک (کمتر از 1) یک منشأ لرزولیتی گارنت- اسپینل دار (با مقدار کم گارنت) حاصل‌شده که با پوسته قاره ای آلودگی پیدا کرده است. وجود اندک گارنت به‌عنوان کانی باقی‌مانده در منشأ این توده ها با نسبت پایین (La/Yb)N (29/6 تا 73/34) توجیه‌پذیر است. مقدار بالای نسبت Rb/Sr و LILE/HFSE و مقدار بالای K2O، Th و Nb نقش اختلاط پوسته قاره ای را روشن می کند. بررسیهای پترولوژیکی و تعیین سن دقیق توده های نفوذی موجود در معدن سنگان به‌عنوان بخشی از کمربند آتشفشانی- نفوذی خواف- کاشمر- بردسکن، به شناخت هر‌چه بهتر جایگاه تکتونوماگمایی این کمربند و نیز چگونگی ارتباط توده های نفوذی با کانی سازی برای اکتشاف بیشتر در آینده کمک خواهد کرد.

Mesozoic and Cenozoic granitoid complexes in the stucture of the continental margin of northeast Asia

The integral data on structural position, age, and paleo-geodynamic setting of Mesozoic and Cenozoic granitoid complexes in northeast Asia make it possible to divide them into preaccretionary, accretionary, and postaccretionary groups participating in the structure of the accretionary-type continental margin. The preaccretionary granitoids are members of volcanic-plutonic associations of ensimatic island arcs or suprasubduction ophiolitic complexes, which mark the onset of growth of the granitic-metamorphic layer in the future continental crust. The accretionary granitoids emplaced during the accretion of diverse rock complexes to the continental margin and are localized in its frontal zone, where granitic-metamorphic layer grows further. The postaccretionary granitoid plutons of the marginal continental volcanic-plutonic belts seal up fold-nappe structures, determining the upper age limit of accretion and deformation. The origin of postaccretionary granitoids is related to remelting of older heterogeneous accretionary-island arc crust.

Sr–Nd isotope geochemistry and tectonomagmatic setting of the Dehsalm Cu–Mo porphyry mineralizing intrusives from Lut Block, eastern Iran

The Dehsalm Cu–Mo-bearing porphyritic granitoids belong to the Lut Block volcanic–plutonic belt (central eastern Iran). These rocks range in composition from gabbro-diorite to granite, with dominance of monzonites and quartz monzonites, and have geochemical features of high-K calc-alkaline to shoshonitic volcanic arc suites. Primitive mantle-normalized trace element spider diagrams display strong enrichment in large-ion lithophile elements such as Rb, Ba and Cs and depletions in some high-field strength elements, e.g., Nb, Ti, Y and HREE. Chondrite-normalized plots display significant LREE enrichments, high LaN/YbN and a lack of Eu anomaly. High Sr/Y and La/Yb ratios of Dehsalm intrusives reveal that, despite their K-rich composition, these granitoids show some resemblances with adakitic rocks. A Rb–Sr whole rock–feldspar–biotite age of 33 ± 1 Ma was obtained in a quartz monzonite sample and coincides, within error, with a previous geochronological result in Chah-Shaljami granitoids, further northwest within the Lut Block. (87Sr/86Sr)i and eNdi isotopic ratios range from 0.70481 to 0.70508 and from +1.5 to +2.5, respectively, which fits into a supra-subduction mantle wedge source for the parental melts and indicates that crustal contribution for magma diversification was of limited importance. Sr and Nd isotopic compositions together with major and trace element geochemistry point to an origin of the parental magmas by melting of a metasomatized mantle source, with phlogopite breakdown playing a significant role in the geochemical fingerprints of the parental magmas; small amounts of residual garnet in the mantle source also help to explain some trace element patterns. Geochemical features of Dehsalm porphyries and its association with Cu–Mo mineralization agree with a mature continental arc setting related to the convergence of Afghan and Lut plates during Oligocene.

Petrochemical Characteristics and Timing of Middle Eocene Granitic Magmatism in Kooh‐Shah, Lut Block, Eastern Iran

Abstract:The Kooh‐Shah region located in a Tertiary volcanic–plutonic belt of the Lut Block in eastern Iran comprises several subvolcanic intermediate to acidic intrusive rocks, diorite to syenite in composition, which have intruded into volcanic rocks. The Kooh‐Shah granitoid rocks are characterized by enrichment in large ion‐lithophile elements (LILE: e.g. Sr, Ba, Rb) and depletion in high field‐strength elements (HFSE: e.g. Nb, Ta, Ti). The chondrite‐normalized REE patterns are characterized by moderate LREE enrichment (La/Yb)N=6.01–10.01, medium‐heavy REE enrichment, and absence of Eu anomalies. The Kooh‐Shah intrusive rocks are metaluminous, shoshonitic with calc‐alkaline affinity and high values of magnetic susceptibility, and classified as the magnetite‐series of oxidant I‐type granitoids. The age of Kooh‐Shah granitoid rocks based on zircon U–Pb age dating is 39.7±0.7 Ma (=Middle Eocene) and the ranges of their initial 87Sr/86Sr and 143Nd/144Nd ratios are from 0.704812 to 0.704920 and 0.512579 to 0.512644, respectively, when recalculated to an age of 39 Ma. The initial cNd isotope values for the Kooh‐Shah intrusive rocks range from −0.18 to 1.09. This geochemical data indicates that the Kooh‐Shah granitoid rocks formed from depleted mantle in an island arc setting. The geochemical signature of the studied granitoid rocks represents a characteristic guide for future exploration of copper‐gold porphyry‐type deposits in the Lut block.

Structure formation in the zone of the northeastern fragment of the Kolumbe Fault in northern Sikhote-Alin

By the stress field’s reconstruction and the analysis of the distribution for the slickenside belts and the dike-hosting faults, the dominating left-lateral strike slip component is found for the Kolumbe Fault. This component is caused by the regional SES-NWN compression. The igneous rocks of the East Sikhote Alin volcanic-plutonic belt are considered as units synchronous with the regional displacement. Following the opinion of V.P. Utkin, the rift strike-slip-tension origin of these structures is suggested. Based on the structural-tectonic position of the volcanic rocks and the accompanying intrusive bodies and on the other geological data as well, several dislocation stages are distinguished in the Kolumbe Fault’s evolution. It is found that the dominating strike-slip motions along the main fault and its feathering faults changed to normal and reverse faulting deformations at the end of every stage.

Earliest Cretaceous Pacificward offset of the Klamath Mountains salient, NW California–SW Oregon

Although Late Triassic igneous rocks are present, the Sierra Nevada–Klamath calc-alkaline arc began massive construction along the continental margin at ca. 170 Ma during oblique underflow of paleo-Pacific oceanic lithosphere; intense activity continued throughout the volcanic-plutonic belt until at least ca. 140 Ma. This volcanic-plutonic arc supplied detritus to the Mariposa-Galice proximal clastic sequence starting by ca. 165–160 Ma. After onset of uppermost Jurassic Myrtle overlap sedimentation on the western flank of the Klamath Mountains, but before Hornbrook and Valanginian Great Valley Group overlap deposition on the eastern and southeastern sides, the Klamath Mountains salient was displaced ∼200 km westward relative to the igneous arc. The orogen thus moved off the deep-seated magmagenic zone underlying the arc and did not participate in the massive Sierra Nevada igneous flare-up between ca. 125 and ca. 85 Ma. I suggest that, beginning at ca. 140 Ma, underflow of a young, thin oceanic slab beneath the Klamath Mountains slid beneath the gently east-dipping stack of thrust sheets without disturbing their inclinations. Subduction and collision of much thicker oceanic lithosphere on both the north and south caused contraction, eastward relative displacement of the continental margin arc, and ductility-enhanced rotation of the superjacent stack of allochthons into near-vertical dips. After a magmatic lull, heightened igneous activity in the Sierra Nevada recommenced at ca. 125 Ma. The earliest Cretaceous oceanward plate junction rollback lay directly offshore the Klamath imbricate orogen, but to the south trapped the ca. 165 Ma Coast Range ophiolite on the North American side of the suture. After ca. 140 Ma, first-cycle arc detritus began to accumulate on the mafic igneous basement flooring the Great Valley forearc, and turbiditic clastic material also was carried oceanward across the forearc into the coeval Franciscan trench.

Geochronological, geochemical and Sr–Nd–Hf isotopic constraints on petrogenesis of Late Mesozoic gabbro–granite complexes on the southeast coast of Fujian, South China: insights into a depleted mantle source region and crust–mantle interactions

AbstractThe Quanzhou (QZ) and Huacuo (HC) gabbro–granite complexes on the southeast coast of Fujian, South China, are important components of a Late Mesozoic calc-alkaline volcanic–plutonic belt in the region. The complexes provide an excellent opportunity to investigate the genetic relationships between acid and basic magmas, and their interactions within the intrusive environment. The complexes are composed mainly of monzogranite and biotite granodiorite in the QZ complex, and biotite granite in the HC complex, with lesser amounts of hornblende gabbro. Zircon U–Pb dating provides consistent crystallization ages of 109 ± 1 Ma and 108 ± 1 Ma for the QZ gabbros and monzogranites, and an age of 111 ± 1 Ma for the HC gabbro, which is contemporaneous with the spatially associated HC granites. Both the mafic and felsic intrusions in these complexes are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and are depleted in high-field-strength elements (HFSEs; e.g. Nb and Ta). They show similarly homogeneous Sr–Nd isotopic compositions. All these factors indicate a close genetic relationship between the gabbroic and granitic rocks in the QZ and HC complexes. Although the enriched Sr–Nd isotopic signatures of the QZ and HC gabbros seemingly point to an enriched mantle source (EM-1), they have highly variable zircon Hf isotopic compositions, with εHf(t) values ranging from negative to positive (specifically –4.6 to +6.1 for the QZ gabbros and –4.8 to +11.6 for the HC gabbros). We interpret the parental basic magmas of these gabbros to have received contributions from a depleted mantle source and crustal components. Contributions from such a depleted mantle source resulted in the growth of juvenile basaltic lower crust, the partial melting of which generated the parental felsic magmas of the QZ and HC complexes. Furthermore, based on a synthesis of petrography, geochronology, elemental and isotopic geochemistry and tectonics, we propose that break-off and rollback of the Late Mesozoic subducted Palaeo-Pacific Plate triggered the upwelling of asthenospheric mantle below the coastal area of the South China Block, which induced extension of the overlying continental lithosphere, and finally initiated the large-scale Late Yanshanian magmatism in the study area.

Au and Ag metallogeny of the Kedon volcanic-plutonic belt (D2–3) (Northeast Russia)

Within the Omolon cratonic terrane the preaccre� tionary Kedon volcanicplutonic belt (KVB) is located, which was formed in the middle Paleozoic (D2-3) on the continental crust. The KVB has attracted particular attention due to the discovery of the Kubaka gold deposit, one of the richest in the world, within this belt. In the period 1998-2007, gold mining at this deposit reached 12-15 tons per year. However, by 2008, the gold reserves had been depleted; in total only 102 tons of gold and the same amount of silver were mined. We suggest that the ques� tions raised in this work on the metallogeny within the KVP are highly relevant for the prediction of new Kubakatype deposits.

Sr–Nd isotope geochemistry and petrogenesis of the Chah-Shaljami granitoids (Lut Block, Eastern Iran)

Chah-Shaljami porphyritic granitoids belong to the Lut Block volcanic–plutonic belt of central eastern Iran. These intrusive rocks are mostly quartz monzonites, granodiorites and diorites. Geochemical evidence reveals that they are co-genetic and that they have features typical of high-K calc-alkaline to shoshonitic rocks from volcanic arc setting. Primitive mantle-normalized trace element spider diagrams display strong enrichment in LILE, such as Rb, Ba, and Cs, and depletion in some HFSE, e.g. Nb, Ti, Y and HREE. Chondrite-normalized plots show a very marked REE fractionation, with significant LREE enrichment (23 ⩾ La N/Yb N ⩾ 14) and the lack of Eu anomaly. Sr/Y and La/Yb ratios of Chah-shaljami intrusives are respectively 20–67 and 21–34, which reveals that, despite their K-rich composition, these rocks also, have some adakitic affinity. Plots on the Sr/Y–Y and La/Yb–Yb diagrams show that the Chah-Shaljami intrusives may be subdivided into two distinct classes. A Rb–Sr age of 33.5 ± 1 Ma, mainly dependent on the Sr isotopic composition of biotite, was obtained in a quartz monzonite sample. Taking into account that this sample was almost unaffected by hydrothermal and meteoric alteration and that cooling was probably fast, the 33–34 Ma date is interpreted as the intrusion age. With the exception of two samples, initial 87Sr/ 86Sr ratios and C ̵ Nd values are clustered in the restricted ranges of 0.70470–0.70506 and +1.9–+2.7, which fits into a supra-subduction mantle wedge source for the parental melts and indicates that, in general, crustal contribution for magma diversification was not relevant; however, one sample shows higher 87Sr/ 86Sr and lower C ̵ Nd, revealing that, occasionally, crustal rock assimilation also contributed to geochemical variation; one other sample, strongly affected by hydrothermal alteration, departs from the main group only by higher 87Sr/ 86Sr, suggesting that its alteration involved crustal fluids. Sr and Nd isotope compositions together with major and trace element geochemistry points to the origin of the parental magmas by melting of a metasomatized mantle source, with garnet behaving as a residual phase, whilst phlogopite was an important contributor to the generated melts.