Syenogranite Porphyry Research Articles

Silver Behavior During Magmatic and Magmatic-Hydrothermal Evolution of a Highly Evolved Reduced Granitic System Related to the Giant Shuangjianzishan Ag-Pb-Zn-(Sn) Epithermal Deposit, Northeast China

Abstract Magmatic-related epithermal silver-rich polymetallic deposits are among the most important sources of Ag in the world, and they are found associated with magmatic systems with striking differences. Most of the time, they are associated either with I-type oxidized (magnetite-series granite) intermediate to evolved intrusions or with S-type/A-type reduced (ilmenite-series granite) highly evolved intrusions. To better understand these associations, the Ag evolution has been tracked during the magmatic differentiation and the magmatic-hydrothermal transition stage of A-type highly evolved porphyritic granites associated with the giant Shuangjianzishan Ag-Pb-Zn-(Sn) epithermal deposit, the largest known Ag deposit of Asia (145 million tonnes at 128.5 g/t Ag and 2.2 wt % Pb + Zn) located in the largest known metallogenic province for Ag in China (the southern Great Xing’an Range). At the Shuangjianzishan deposit, the porphyritic granite complex consists of three temporally distinct intrusions—a coarse-grained monzogranite porphyry, a fine-grained syenogranite porphyry, and a fine-grained syenogranite—having crystallized at ~2 kbar and ~750°C and recording a continuous magmatic differentiation trend. The silicate melt that generated the last highly differentiated intrusion (fine-grained syenogranite) is interpreted as the source of the mineralizing fluids forming the Shuangjianzishan Ag-Pb-Zn-(Sn) epithermal deposit, as it is the only intrusive unit that reached fluid saturation, as indicated by cotrapped fluid and melt inclusions in quartz phenocrysts and by the occurrences of unidirectional solidification textures (USTs). Silver evolution in the different porphyritic granite facies was reconstructed with laser ablation-inductively coupled plasma-mass spectrometry analyses of quartz-hosted silicate melt inclusions, amphibole-hosted magmatic sulfide inclusions, and chemical modeling. The silicate melt forming the porphyritic granite complex was sulfide saturated during the first crystallization stage, as shown by the occurrence of Ag-rich monosulfide solid solution (MSS) inclusions hosted in amphibole phenocrysts from the coarse-grained monzogranite porphyry and from mafic microgranular enclaves hosted in the coarse-grained monzogranite porphyry. However, these Ag-rich MSSs had only a minimal impact on the Ag budget of the magmatic system, as shown by the increase of the Ag concentration (~100–1,000 ppb) in quartz-hosted silicate melt inclusions during the further evolution of the system until fluid exsolution was reached. These results combined with mass balance modeling suggest that Ag and Sn are efficiently transferred to the evolving residual melt during crystallization and crystal-melt segregation. The results of this study indicate that highly Ag endowed epithermal polymetallic deposits can be formed from the exsolution of Ag-rich mineralizing fluids from relatively low volume, highly evolved, reduced melts, similar to those responsible for the formation of Sn-rich greisen deposits.

Paleoproterozoic gold deposits at Alta Floresta Mineral Province, Brazil: two overprinted mineralizing events?

Abstract Large gold provinces commonly show complicated mineralization histories, and the Paleoproterozoic Alta Floresta, one of Brazil's most exciting Au–Cu mineral provinces, is a good example. The current models defined four deposit types, all connected to a single (1.88–1.75 Ga) magmatic–hydrothermal event. However, long Province history, diverse geodynamic environment and older ages of Type-1mineralization weaken the single metallogenic event and enable the hypothesis of overprinted mineral events. By scale-integrated analyses, we revise the tectonic–geological context, structural–hydrothermal alterations and chlorite–white mica geothermobarometry and propose the Type-1 as an older, granitoid-hosted orogenic mineralization, with subsequent overprinting by the magmatic–hydrothermal event. The older orogenic gold event developed orogenic gold deposits on WNW-trending shear zones in the Peixoto de Azevedo domain granitic–gneiss rocks. Phengite, biotite and chlorite–carbonate phyllonites (3.3–6.1 kbar, 300–420°C) host fault-fill quartz veins (pyrite–chalcopyrite–magnetite–pyrrhotite–gold–Bi–Ag tellurides). Mg-rich chlorite–phengite is the main alteration footprint for this mineralization type. A younger magmatic–hydrothermal event in the Juruena magmatic-arc rocks produced Fe-rich chlorite–white mica alteration zones (0.6–4.6 kbar, 120–380°C) and disseminated and stockwork–breccia ore (pyrite–chalcopyrite–gold–molybdenite–Ti minerals–allanite) in porphyry–epithermal deposits. Where the younger mineralization overprints the older, phyllic alteration destroyed the phengite orogenic gold phyllonite S n +1 foliation. The ages of two pyrite populations (1979 and 1841 Ma) in the older fault-fill veins and molybdenite in late fractures (1805–1782 Ma) or disseminated in the c. 1.79 Ga syenogranite porphyry suggest more than two episodes of mineralization. These two events differ in their alteration styles, P–T conditions and structural, mineralogical and textural ore styles. The multiscale approach sheds light on the relationships between the various mineralization events, allowing a new explorational potential within the province. Supplementary material: Chlorite and white mica complete chemical composition data and other characteristics of the studied deposits are available at https://doi.org/10.6084/m9.figshare.c.6056324

Petrogenesis of the Alubaogeshan intrusion in the Maodeng–Xiaogushan area, southern great xing'an range, NE China: Implications for magma evolution and tin–polymetallic mineralization

• The Alubaogeshan intrusion formed at ca. 134 Ma and belongs to transitional I–A 2 -type granite . • The Alubaogeshan intrusion was formed by partial melting of a Neoproterozoic juvenile crust. • The syenogranite porphyry evolved from the porphyritic monzogranite. • Fluid–melt interaction is crucial for W mineralization. • The magma evolutionary degree influences the size of the Sn–polymetallic deposits in the SGXR. The Alubaogeshan intrusion, consisting of porphyritic monzogranite in the central part and syenogranite porphyry in the edge, is genetically related to the Maodeng–Xiaogushan Sn–polymetallic deposit. This paper aims to investigate the genetic link between these two granite phases and the relationship between magma evolution and Sn mineralization. LA-ICP-MS zircon U–Pb dating of the porphyritic monzogranite and syenogranite porphyry yielded ages of 134 ± 1.0 Ma and 133 ± 0.5 Ma, respectively. The Alubaogeshan intrusion has high SiO 2 and total alkali (Na 2 O + K 2 O) contents, with low Al 2 O 3 , Fe 2 O 3 , FeO, and MgO contents, belonging to high-K calc-alkaline metaluminous–weakly peraluminous granites. Chondrite-normalized rare earth element (REE) patterns are characterized by light REE enrichment and heavy REE depletion, with extremely negative Eu anomalies. In addition, the intrusion was also enriched in Rb, Th, U, K, Zr, Hf, and light REE and strongly depleted in Ba, Sr, P, and Ti. Mineralogical and geochemical characteristics, together with high zircon saturation temperature, indicate that the Alubaogeshan intrusion belongs to transitional I–A 2 -type granites. Compared with the porphyritic monzogranite, the syenogranite porphyry reveals higher SiO 2 content and lower TiO 2 , Al 2 O 3 , Fe 2 O 3 , FeO, CaO, MgO, and P 2 O 5 contents, with more obvious negative anomalies of Eu, Ba, Sr, P, and Ti. The temporal and spatial relationship and geochemical characteristics of the two rocks indicate that the syenogranite porphyry evolved from the porphyritic monzogranite. The modeling results show that the fractional crystallization of amphibole, plagioclase, K-feldspar, biotite, apatite, monazite, and allanite occurred during the magma evolution. The granitic magma differentiation plays a key role in the formation of the Maodeng–Xiaogushan Sn–polymetallic deposit. The ε Hf ( t ) values of zircon range from 2.58 to 7.08, and the two-stage Hf model ages range from 619 Ma to 850 Ma, implying that the Alubaogeshan intrusion was predominantly derived from partial melting of a Neoproterozoic juvenile crust. The TE 1,3 values of the ore-related granites from W–(Sn) deposits are generally higher than 1.1, suggesting that fluid–melt interaction is crucial for W mineralization. The degree of magmatic evolution is one of the controlling factors for the size of Sn–polymetallic deposits in the southern Great Xing′an Range.

The relationship between magma and mineralization in Chaobuleng iron polymetallic deposit, Inner Mongolia

The Chaobuleng iron polymetallic deposit, an important ore system in China that is genetically related to the early Cretaceous Chaobuleng pluton, is located in the eastern part of Lizi Shan-Dong Ujimqin Banner metallogenic belt of Inner Mongolia. To understand the relationship of magmatism and mineralization in this iron polymetallic deposit, we have conducted a detailed geologic field study in conjunction with systematic investigation of U-Pb zircon and Re-Os isochron geochronology, mineralogy, petrology, major-and trace-element geochemistry, and synthesis of existing datasets across the Chaobuleng region. We use these observations to identify the origins and petrogenesis of mafic enclaves and the host granitoid, and to place new constraints on the tectonic setting at the time of magmatism. New U-Pb geochronology of magmatic zircons and Re–Os isochron ages of hydrothermal molybdenite from the iron polymetallic deposit allow us to constrain the sources of the hydrothermal components and the relationship between iron polymetallic mineralization and regional geodynamic evolution. The geology, paragenesis, and estimated P-T conditions suggest that the Chaobuleng iron polymetallic deposit was formed as a shallow, proximal skarn deposit. The ore-forming early Cretaceous Chaobuleng pluton can be divided into three distinct units based on crystallization age, texture, and composition: (1) a 138.1–140.6Ma syenogranite porphyry, (2) a 137.4–138.6Ma enclave bearing porphyritic syenogranite, and (3) a 133.9–135.98Ma coarse-grained porphyritic syenogranite. We suggest that the A-type Chaobuleng pluton was formed in a post-orogenic extensional setting with significant magma mixing from high-temperature melts (i.e., a Zr saturation temperature of 800–900°C). During the magmatic process, the Chaobuleng pluton crystallized under temperatures as low as 720–770°C and pressures of 0.5–1.0kbar. The chemical composition of biotite shows that the Chaobuleng magma was enriched in F (1.5–3.5%). We attribute the observed embayed texture of quartz to a three coexisting-phase equilibrium model that operated during the magmatic-hydrothermal transition and can be used to constrain the mineralization process and physico-chemical conditions. Due to a loss of volatiles, the residual melt was quickly quenched and crystallized into a fine-grained groundmass. The Re–Os model ages of samples from the inner pluton-mineralization contact belt are 135.0±2.1Ma and 131.2±4.1Ma, and a sample from the outer-contact belt yields an isochron age of 140.7±1.8Ma. The Chaobuleng deposit formed during protracted activity of the magmatic-hydrothermal system, which is similar to many mineralization systems around the world. However, the major mineralization stage (iron oxide stage) of the Chaobuleng deposit occurred during the early stage of magmatism, consistent with the emplacement time of syenogranite porphyry at ~139Ma.

Molybdenite Re–Os age, H–O–C–S–Pb isotopes, and fluid inclusion study of the Caosiyao porphyry Mo deposit in Inner Mongolia, China

The giant Caosiyao porphyry Mo deposit, located in the northern margin of the North China Craton, is a newly discovered Mo deposit in the Yan–Liao molybdenum belt. Orebodies of the deposit are mainly hosted in metamorphic rocks of the Mesoarchean Jining Group with a small amount of orebodies in the Late Jurassic syenogranite porphyry. The ore-forming process of the deposit can be divided into three stages: an early quartz–molybdenite±magnetite±pyrite stage, a middle quartz–molybdenite–pyrite±pyrrhotite±chalcopyrite stage, and a late quartz–carbonate–pyrite±fluorite±pyrrhotite stage. Molybdenite Re–Os dating indicates that the Caosiyao deposit formed at ca. 150Ma. Four types of fluid inclusions (FIs) have been distinguished in quartz phenocryst and various quartz veins including liquid-rich, gas-rich, H2O–CO2, and daughter mineral-bearing inclusions. The FIs in the quartz phenocrysts of the syenogranite porphyry contain liquid-rich, gas-rich, H2O–CO2, and daughter mineral-bearing types; their homogenization temperatures and salinities vary from 385°C to >550°C and 6.5wt% to 65.0wt% NaCl eqv., respectively. The FIs in the quartz of the early stage are also mainly liquid-rich, gas-rich, H2O–CO2, and daughter mineral-bearing types; their homogenization temperatures and salinities vary from 243°C to 401°C and 6.0wt% to 39.8wt% NaCl eqv., respectively. The FIs in the quartz of the middle stage are mainly liquid-rich, H2O–CO2, and daughter mineral-bearing types; their homogenization temperatures and salinities vary from 208°C to 336°C and 1.2wt% to 34.1wt% NaCl eqv., respectively. The FIs in the quartz of the late stage are only liquid-rich type; their homogenization temperatures and salinities vary from 124°C to 196°C and 3.9wt% to 13.2wt% NaCl eqv., respectively. The ore-forming fluids of the Caosiyao deposit are characterized by moderate–high temperature and great fluctuating salinity, belonging to an H2O–NaCl–KCl–MgCl2–CO2±CH4 system. The δ18OH2O and δDH2O values of the early and middle stages vary from 3.5‰ to 7.4‰ and −96‰ to −77‰, respectively; the δ18OH2O and δDH2O values of the late stage vary from −2.9‰ to −0.6‰ and −112‰ to −77‰, respectively; indicating that the ore-forming fluids of the early and middle stages mainly consist of magmatic water and that the fluid in late stage is a mixture of magmatic and meteoric water. The δ13C values vary from −16.5‰ to −8.8‰ for the early and middle stages and −11.4‰ to −5.3‰ for the late stage, suggesting that the carbon of the early and middle stages consists of a mixture of organic and magmatic sources and that the carbon in the late stage was mainly from a magmatic source. The δ34S values range from 4.0‰ to 6.1‰ with an average of 4.8‰. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of the metallic minerals are in the ranges of 15.962–17.832, 15.200–15.511, and 35.918–37.927, respectively. Both S and Pb isotopic systems indicate that the ore-forming metals came primarily from Mesozoic intermediate–felsic magma. The multi-stage boiling or immiscibility of the ore-forming fluids was the dominant mechanism for the deposition of ore-forming materials.

Age, geochemistry, and Sr–Nd–Hf–Pb isotopes of the Caosiyao porphyry Mo deposit in Inner Mongolia, China

The giant Caosiyao porphyry Mo deposit (Yinshan area, central Inner Mongolia) is recently discovered in the northern margin of the North China Craton. The mineralization is newly molybdenite Re–Os dated to be Late Jurassic (ca. 148Ma). LA-ICP-MS zircon U–Pb dating of the two syenogranite porphyry intrusions in the Caosiyao deposit yielded ca. 148.5Ma and ca. 140.9–140.1Ma, respectively, and the Caosiyao monzogranite porphyry yielded ca. 145.0Ma. These Late Jurassic to Early Cretaceous granitic porphyries are high-K calc-alkaline and featured by high Rb, Th, U, and Zr contents and low TiO2, P2O5, Nb, and La contents, with high K2O/Na2O and medium to strong negative Eu anomalies, all resembling post-orogenic granitoids. The Caosiyao granitic porphyries have high whole-rock initial 87Sr/86Sr (0.710417–0.718568), low zircon εHf(t) (−18.9 to −12.5) and whole-rock εNd(t) (−17.8 to −13.7), old single-stage and two-stage model ages for zircon Hf (1282–1552Ma and 1989–2393Ma, respectively) and for whole-rock Nd (1766–2431Ma and 2046–2375Ma, respectively), as well as low K-feldspar 206Pb/204Pb (16.603–17.177), 207Pb/204Pb (15.319–15.374), and 208Pb/204Pb (36.841–37.225) values. We suggest that the Late Jurassic and Early Cretaceous syenogranite porphyry may have been sourced from a highly and moderately mature lower crust, respectively, whereas the Early Cretaceous monzogranite porphyry may have been sourced from an immature lower crust. The Caosiyao porphyries may have formed during the post-orogenic extension after the Mongol–Okhotsk Ocean closure and coeval with the Paleo-Pacific plate subduction.

Age, igneous petrogenesis, and tectonic setting of the Bilihe gold deposit, China, and implications for regional metallogeny

The Bilihe gold deposit, along the northern margin of the North China Craton (NCC), is hosted in Permian intrusions that were emplaced into Bainaimiao Group greenschist- to amphibolite-facies Mesoproterozoic basement rocks, the early Paleozoic Bainaimiao Arc, and late Paleozoic sedimentary and volcanic cover rock sequences. The Middle Ordovician to Early Silurian calc-alkaline arc comprises volcanic sequences of basalt, andesite, and felsic lavas, and an intrusive complex ranging from gabbro through granite. The deposit is genetically related to ilmenite-series and highly fractionated quartz diorite porphyry host rocks. The most fractionated variety of the quartz diorite porphyry is granitic, which contains dendritic quartz phenocrysts and occurs at the top part of the intrusions. Gold mostly occurs as Au grains (>990 fineness) in trails in dendritic quartz, and the gold trails typically occur along certain crystallographic elements of dendritic quartz. The gold is believed to have crystallized directly from magma at temperatures of at least 750–800°C. The Fe2O3/(Fe2O3+FeO) ratio of the quartz diorite ranges from 0.21 to 0.29, and there is significantly more ilmenite than magnetite, suggesting that the magma was moderately reduced. There are also post-ore syenogranite porphyries in the deposit area. Zircon U-Pb LA-ICP-MS dating in this study shows that two samples of the volcanic wallrock have ages of 274±3Ma and 270±4Ma, two samples of gold-forming quartz diorite porphyries have ages of 261±2Ma and 259±3Ma, with 440–400Ma and 1344–1316Ma cores, and one syenogranite porphyry sample is dated at 253±3Ma.The mineralized quartz diorite porphyries exhibit a high-K calc-alkaline composition, high Mg# (46–51), and a high content of compatible elements (V: 114–164ppm; Cr: 80–93ppm; Ni: 37–42ppm) and Rb, Th, and U, as well as depletions in Ba, Sr, and HFSE (e.g., Ti, Nb, and Ta), suggesting melt from a slab fluid-metasomatized enriched mantle. The LREE enrichment, the intermediate oxidation state, and the presence of the Silurian and Mesoproterozoic zircon cores indicate that crustal materials also played a significant role in porphyry formation. In addition, the rocks have slightly negative εNd(t) (−2.5–0.1), but variable (87Sr/86Sr)i (0.70328–0.70573). Based on the above features, we propose that quartz diorite porphyries were produced by partial melting of the metasomatized mantle coupled with the assimilation of crustal materials of the Bainaimiao Group, Late Ordovician-Silurian subduction-related plutons, and Late Carboniferous-Early Permian marine sedimentary rocks. The assimilation of rocks of the Bainaimiao Group, which have an elevated gold content of 26.3ppb, is speculated to have played important roles in the formation of a rarely documented magmatic gold deposit. In contrast, syenogranite porphyries are characterized by high contents of K2O+Na2O (10.3–10.6wt%), Al2O3 (16.3–16.5wt%), K, Hf, and Zr; low CaO (0.6–1.2wt%), Mg# values (17–21), compatible elements (V: 16–21ppm; Cr: 4.9–7.9ppm; Ni: 2.8–4.5ppm), and Eu, Sr, and HFSE; and positive εNd(t) (1.4–1.5) with low (87Sr/86Sr)i (<0.70431), indicating that the porphyries were formed by partial melting of the subduction-modified lower crust with a minor input of asthenospheric materials.It is concluded that the quartz diorite porphyries formed during the southward subduction of the Paleo-Asian ocean underneath the northern margin of the NCC. This confirms that the northern margin was not a passive continental margin, but an active margin with subduction and development of a continental arc. In addition to Bilihe, there are at least four more gold deposits (Hadamiao, Changshanhao, Saiyinwusu, and Chaihulanzi) that formed at the northern margin of the NCC from Middle Permian to Early Triassic. Such a timing indicates that they may also be ultimately related to the tectonic regime imposed by southward subduction of the Paleo-Asian ocean. Based on the new understanding of the tectonic setting, with significant known mineral occurrences along the length of the margin, it is inferred that the >1000-km-long northern margin of NCC may have a large potential for subduction-related mineralization.

Alteration-mineralization, and radiometric ages of the source pluton at the Sangan iron skarn deposit, northeastern Iran

The world-class Sangan iron skarn deposit with a proven reserve of>1000Mt iron ore @ 53% Fe, is located in the Khaf-Kashmar-Bardaskan Volcano-Plutonic Metallogenic Belt (KKB-VPMB) of northeastern Iran along the E–W trending regional Doruneh Fault, north of the Lut Block. Skarn mineralization occurs at the contact of the 39.1±0.6Ma to 38.3±0.5Ma Middle Eocene syenite to syenogranite porphyry source pluton with Cretaceous carbonate rocks. The source pluton is part of an I-type, calcalkaline granitoid series with potassium to shoshonitic affinity, metaluminous to slightly peraluminous character. The granitoids are enriched in LILE (Cs, Rb, Ba, K, Th, and U) and LREE (La and Ce), are depleted in HFSE (Nb, Y, Ta, and Ti), HREE (Yb and Lu) and Eu, which together with the low Sr (96 to 362ppm) and (La/Yb)N contents suggest a metasomatized, slab-derived mantel source within the Cenozoic volcanoplutonic continental arc of northeaster Iran. The location of the skarn orebodies is controlled by the regional E–W Doruneh Fault structure, and the spatial distribution and the two, calcic and magnesian, skarns is controlled by the composition of the carbonate protolith that consists of limestone in the west and dolomite in the east. The chemistry of the skarn minerals reflects the composition of the protoliths. The western skarn is dominated by ca-rich calcsilicates that consist of prograde Ca-rich garnet (Adr50–97Grs0–45Sps-Alm2-7), Ca–Fe-rich pyroxene (Hd75.6Di20 Jo4.5), K–Cl–F-bearing Fe–Ca-rich amphibole ferrohastingsite (Fe4.4Ca2Mg 0.5K0.4Na0.3)(Si6.5Al1.8 O22)(Cl, F, OH)2 in endoskarns, and retrograde ferroactinolite (Fe4.2Ca2MgMn0.04Na0.1K0.06)(Si7.8Al0.2O22)(F, Cl, OH)2 and Fe-rich chlorite ripidolite (Fe4.3Mg1.1 Ca0.06Mn0.01)(Al2.8Si3O10)(OH)8 in the exoskarns. Wollasonite, plagioclase, and K-feldspar have been additionally formed together with ferrohastingsite in the endoskarns. The eastern magnesian skarn is typically phlogopite-rich (K0.9Mg2.7Fe0.2 Na0.02)(Si2.9Al1.2O10) (OH, F)2, and consists of prograde forsterite (Fo97.0Fa2.7), diopsidic pyroxene (Hd0.2Di0.88), and retrograde Mg-rich actinolite, Mg-rich chlorite clinochlore (Mg4.5Fe0.1K0.05)(Al1.8Si2.7O10)(OH)8, serpentine and talc. Iron mineralization overprints the calcsilicate skarns in both the western and the eastern skarns and is magnesian in the eastern orebodies (3.65wt.% MgO). The iron ore consists of a number of high grade, replacement, and magnetite orebodies with minor amounts of pyrrhotite, chalcopyrite, and pyrite indicating a large scale metasomatic transfer of iron bearing fluids during Middle Eocene magmatic activity at the KKB-VPMB.

Genesis of the Chehugou Mo-bearing granitic complex on the northern margin of the North China Craton: geochemistry, zircon U–Pb age and Sr–Nd–Pb isotopes

AbstractThe Chehugou granite-hosted molybdenum deposit is typical of the Xilamulun metallogenic belt, which is an important Mo–Ag–Pb–Zn producer in China. A combination of major and trace element, Sr and Nd isotope, and zircon U–Pb isotopic data are reported for the Chehugou batholith to constrain its petrogenesis and Mo mineralization. The zircon SIMS U–Pb dating yields mean ages of 384.7 ± 4.0 Ma and 373.1 ± 5.9 Ma for monzogranite and syenogranite and 265.6 ± 3.5 Ma and 245.1 ± 4.4 Ma for syenogranite porphyry and granite porphyry, respectively. The Devonian granites are calc-alkaline with K2O/Na2O ratios of 0.44–0.52, the Permian granites are alkali-calcic with K2O/Na2O ratios of 1.13–1.25, and the Triassic granites are calc-alkaline and alkali-calcic rocks with K2O/Na2O ratios of 0.78–1.63. They are all enriched in large-ion lithophile elements (LILEs) and depleted in high-field-strength elements (HFSEs) with negative Nb and Ta anomalies in primitive mantle-normalized trace element diagrams. They have relatively high Sr (189–1256 ppm) and low Y (3.87–5.43 ppm) concentrations. The Devonian granites have relatively high initial Sr isotope ratios of 0.7100–0.7126, negative ɛNd(t) values of −12.3 to −12.4 and 206Pb/204Pb ratios of 16.46–17.50. In contrast, the Permian and Triassic granitoids have relatively low initial 87Sr/86Sr ratios (0.7048–0.7074), negative ɛNd(t) values of −10.1 to −13.1 and 206Pb/204Pb ratios of 17.23–17.51. These geochemical features suggest that the Devonian, Permian and Triassic Chehugou granitoids were derived from ancient, garnet-bearing crustal rocks related to subduction of the Palaeo-Asian Ocean and subsequent continent–continent collision between the North China and Siberian plates.

Variations in magnetic anisotropy and opaque mineralogy along a kilometer deep profile within a vertical dyke of the syenogranite porphyry at Cı́novec (Czech Republic)

Vertical variations in magnetic fabric and paragenesis of the ferrimagnetic minerals for a virtually upright dyke of the Altenberg syenogranite porphyry were investigated using profile data of borehole E-16 to a depth of 922.7 m (Eastern Krušné hory Mts., NW Bohemia). It was revealed that this dyke likely consists of two magma pulses indicated both by magnetic fabric and by opaque mineralogy. In most of the profile, the magnetite grains are oriented parallel to the sub-vertical dyke. Only at the base of the upper magma pulse, in a depth interval of 200–400 m, are these grains oriented sub-horizontally being thus perpendicular to the dyke walls. This pattern can be interpreted as a consequence of a static vertical compaction of the magma of the basal portions of the upper pulse due to the pressure of the ascending lower pulse. The large planes of the ferrimagnetic minerals are oriented perpendicular to the shortening direction.