Host Diorite Research Articles

The Montecristo mining district, northern Chile: the relationship between vein-like magnetite-(apatite) and iron oxide-copper–gold deposits

The Montecristo district, northern Chile, is one of the few places worldwide where there is a direct relationship between magnetite-(apatite) (MtAp) mineralization and iron oxide-copper–gold (IOCG) mineralization. The MtAp mineralization includes Ti-poor magnetite, fluorapatite, and actinolite and is crosscut and partially replaced by a younger IOCG mineralization that includes a second generation of actinolite and magnetite with quartz, chalcopyrite, pyrite, and molybdenite. The MtAp stage at Montecristo is interpreted as the crystallized iron-rich melts that used the pre-existing structures of the Atacama Fault System as conduits. These rocks later acted as a trap for hydrothermal IOCG mineralization. Geochronology data at Montecristo indicate that the host diorite (U–Pb zircon 153.3 ± 1.8 Ma, 2-sigma), MtAp mineralization (40Ar-39Ar in actinolite, 154 ± 2 Ma and 153 ± 4 Ma, 2-sigma), and the IOCG event (Re-Os on molybdenite, 151.8 ± 0.6 Ma, 2-sigma) are coeval within error and took place in a time span of less than 3.4 Ma. The εHfi and εNdi values of the host diorite are + 8.0 to + 9.8 and + 4.3 to + 5.4, respectively. The whole-rock 87Sr/86Sri values of the IOCG mineralization (0.70425 to 0.70442) are in the lower end of those of the MtAp mineralization (0.70426–0.70629). In contrast, εNdi values for the IOCG mineralization (+ 5.4 and + 5.7) fall between those of the MtAp rocks (+ 6.6 to + 7.2) and the host diorite, which suggests that the IOCG event was related to fluids having a more crustal Nd (εNdi < + 5.7) composition than the MtAp mineralization. This likely reflects the mixing of Nd from the MtAp protolith and a deep magmatic-hydrothermal source, very likely an unexposed intrusion equivalent to the host diorite. Sulfur isotope compositions (δ34S, + 0.3 to + 3.4‰) are consistent with a magmatic source.

Age and genesis of the Jinshan gold deposit in the Chinese North Tianshan: A link to large-scale strike–slip shearing events

Large-scale transcrustal strike–slip shearing, orogenic gold deposits, and magmatic Ni–Cu sulfide deposits are prominent features in the Chinese North Tianshan in the northeastern segment of the Tianshan Orogenic Belt. Therelationshipbetween the shearing events and the different styles of mineralization, however, remains ambiguous. Here, structural data, zircon U–Pb–Hf isotopes, bulk-rock geochemistry, muscovite 40Ar/39Ar age dating, and S–Pb isotopes of pyrite from the Jinshan gold deposit are utilized in conjunction with the Pb isotope composition of mineralized mafic–ultramafic rocks of the Huangshandong Ni–Cu sulfide deposit to characterize thelinkbetweenstrike–slipshearing events, andorogenicAuandmagmaticNi–Cu sulfidemineralization. The new zircon U–Pb age results indicate that the host diorite of the gold orebodies formed at ca. 335 Ma. These diorites are enriched in light rare earth elements and large ion lithophile elements (e.g., Ba, U, K, and Pb), but depleted in high field strength elements (e.g., Nb, Ta, and Ti), and have low MgO (4.45–5.17 wt%) contents, Mg# (0.32–0.36), and (La/Yb)CN ratios (4.81–6.90). Based on the lack of ancient crustal basement rocks in the Qoltag Arc, the depleted Nd–Hf isotope composition (εHf(t) = 10.82–15.38; εNd(t) = 6.17–6.67), the juvenile Hf depleted-mantle model ages (350–530 Ma), and the enrichment in Na2O (4.45–5.17 wt%) relative to K2O (1.05–2.20 wt%) of the diorite, it is inferred that the primary magma from which the diorite crystallized was derived from partial melting of metasomatized mantle wedge in an oceanic arc environment. Two muscovite samples separated from an auriferous quartz lode yield 40Ar/39Ar plateau ages of 248.6 ± 0.8 Ma and 249.6 ± 0.9 Ma, representing the age of the gold mineralization event. The similarity in S–Pb isotope composition of pyrite in the Jinshan gold deposit and pyroxenite in the Huangshandong Ni–Cu sulfide deposit suggests that the auriferous fluids required for the formation of the Jinshan gold deposit could represent a mixture of deuteric fluids with limited upper crustal metamorphic fluids from the Kanggur–Huangshan Shear Zone.

Cumulate Mush Hybridization by Melt Invasion: Evidence from Compositionally Diverse Amphiboles in Ultramafic–Mafic Arc Cumulates within the Eastern Gangdese Batholith, Southern Tibet

Abstract Amphibole plays an important role in the petrogenesis and evolution of arc magmas, but its role is not completely understood yet. Here, a field, petrological, geochronological and geochemical study is carried out on ultramafic–mafic arc cumulates with textural and chemical heterogeneities and on associated host diorites from the eastern Gangdese Batholith, southern Tibet to explore the problem. The cumulates occur as a large body in diorite host-rocks. The core of the body consists of coarse-grained Cpx hornblendite with a porphyritic texture. Towards the contact with the host diorite, the coarse-grained Cpx hornblendite grades to relatively homogeneous fine-grained melagabbro. Zircon U–Pb dating indicates that they all crystallized at 200 ± 1 Ma. Textural features and whole-rock and mineral chemical data reveal that both the Cpx hornblendite and the melagabbro are mixtures of two different mineral assemblages that are not in equilibrium: (1) brown amphibole and its clinopyroxene inclusions; (2) matrix clinopyroxene + green amphibole + plagioclase + quartz + accessory phases. Clinopyroxene and brown amphibole from the first assemblage are enriched in middle rare earth elements (MREE) relative to light REE (LREE) and heavy REE (HREE), and are weakly depleted in Ti, whereas clinopyroxene and green amphibole from the second assemblage are characterized by LREE enrichment over MREE–HREE and more marked Sr and Ti depletion. The higher Mg#, MgO and Cr of the late-formed green amphibole than the early formed brown amphibole suggest that the two assemblages are not on the same liquid line of descent. Given the close relations of the three rock types in the exposed crustal section, the cumulates are interpreted to have formed in an open system, in which an ultramafic cumulate body consisting of the first assemblage reacted with the host dioritic melt to form new clinopyroxene and amphibole of the second assemblage. The melt calculated to be in equilibrium with the first mineral assemblage resembles an average continental arc basalt, which is less evolved than the host dioritic melt, responsible for the second mineral assemblage. On the basis of whole-rock Sr–Nd–Hf isotopic similarity of the cumulates and a host diorite sample, we argue that the host diorites were formed through crystal fractionation from the parent melt of the first assemblage. Results of least-squares mass-balance calculations suggest that the quantities of the host dioritic melts, involved in the generation of these modified cumulates, vary from ~25 to ~44 %. The presence of magmatic epidote in the host diorites and Al-in-Hb geobarometry indicate that the reaction that occurred when the dioritic melts percolated through the cumulate body was at ~6 kbar. Both the brown and green amphiboles are enriched in MREE relative to HREE, and can impart to residual melts a strong geochemical signature of amphibole fractionation (low Dy/Yb). Thus, we conclude that fractional crystallization and melt–rock reaction are two mechanisms by which amphibole controls arc magma petrogenesis and evolution.

Mafic microgranular enclaves from the Tieshan batholith, eastern China: Implications for petrogenesis of magma associated with Fe-Cu skarn mineralization

Mineralogy, geochemistry, and geochronology of the mafic microgranular enclaves (MMEs) and their host diorite from the Tieshan batholith, Edong district, eastern China, were studied to constrain their petrogenesis and relationship with Fe-Cu skarn mineralization. The MMEs are diorite in composition and characterized by mixed apatite morphologies (elongated thin needles and stubby prismatic form) and bladed biotite, as well as clots of host diorite and megacrystic alkali-feldspar xenocrysts. The MMEs have similar zircon U-Pb age as their host diorite (~139 Ma). Most of the major and trace elements of the MMEs show a linear trend on Harker diagrams and plot along the low-silica trend of the host diorite. They are also characterized by enrichment of LREE and large ion lithophile elements (LILE), but are depleted in high field strength elements (HFSE). The MMEs display similar initial Sr isotopes and εNd(t), and slightly lower εHf(t) compared to that of host diorite. These results indicate that the Tieshan batholith formed via partial melting of an enriched lithospheric mantle, which is believed to be in equilibrium with a garnet-bearing residue (>50 km), followed by contamination of low crustal materials and fractional crystallization, whereas the MMEs appear to be the products of magma mixing. The presence of Cu- and Ni-bearing minerals in the MMEs probably indicates upper mantle or lower crust accumulation of sulfide which may have played a key role in controlling the formation of the parental magma associated with the skarn-porphyry Cu deposit (148 to 135 Ma) from the Middle-Lower Yangtze River metallogenic belt (MLYRMB). In contrast, the thinner, normal crust (~30 km) at ~ 130 Ma, believed to be in equilibrium with a plagioclase-rich residue, was either unsuitable for producing Cu-bearing accumulations or failed to transfer the deep accumulations to shallower levels to form Cu-bearing skarn systems, resulting in Fe-only mineralization. This may imply that skarn-porphyry Cu deposits prefer to form under > 50 km crustal settings, whereas Fe skarns may preferentially occur under normal (~30 km) crustal thickness.

Mafic Microgranular Enclaves Formed by Gas-driven Filter Pressing During Rapid Cooling: an Example from the Gangdese Batholith in Southern Tibet

AbstractMafic microgranular enclaves (MMEs), widespread in intermediate to felsic arc plutons, carry significant information on the genesis and evolution of arc magmas, yet their origin remains debatable. Here, we examine MME-host diorite pairs from the c.200 Ma Cuijiu Igneous Complex in the eastern Gangdese Batholith, southern Tibet, to constrain the petrogenesis of MMEs and the evolution of arc magmas. Within the complex, MMEs are essentially similar to their host diorites with similar emplacement ages (∼200 Ma), mineral assemblages and mineral compositions, as well as whole-rock Sr–Nd–Hf and zircon Hf isotopic compositions. However, MMEs have higher modal contents of hornblende and biotite, and are enriched in compatible elements and depleted in incompatible elements. Zircons from some MME samples are characterized by dark cathodoluminescence (CL) cores overgrown by light-CL rims of varying thickness. The dark-CL cores show higher Th, U and rare earth elements (REE) abundances than the light-CL rims. Based on comparison with co-genetic mafic melts and mass-balance calculations, we propose that the MMEs were early-crystallized cumulates (autoliths) related to their host diorites. The chilled textures, flow microstructures and pillow shapes suggest that the MMEs experienced rapid cooling before being captured by the host magmas. The rapid cooling may result from contact between ascending diorite magmas and cooler wall rocks. As the magmas quickly crystallized, they reached second boiling and vesiculation, and separated into fine-grained crystal-rich margins and melt-rich centres. Gradients in crystallinity and pressure expelled interstitial melts from the crystal-rich margins to the crystal-poor centres, leading to crystal-liquid separation (gas-driven filter pressing). The dark-CL zircon cores with high Th and U abundances may crystallize from highly evolved interstitial melts within the crystal-rich margins. The fine-grained crystal-rich margins were subsequently captured and dragged as MMEs before their complete crystallization by later ascending host magmas. This differentiation process could have occurred over several kilometres of magma ascent, and have played an important role in the polybaric fractional crystallization of the Cuijiu Igneous Complex, feeding more differentiated andesitic magmas to upper crustal mushes.

Geochemical characteristics of the Nyemo intrusion and crust–mantle interactions in southern Gangdese, Tibet

AbstractThe Gangdese magmatic belt across the southern Tibetan Plateau is juxtaposed with the Indus–Yurlung Zangbo suture zone (IYS), and many mafic microgranular enclaves (MMEs) are exposed in the belt, thus providing a window for observing deep crust–mantle processes related to the Indo-Asian collision. The Nyemo intrusion is located in the middle part of the Gangdese magmatic belt and comprises host diorites with abundant MMEs. Compared with other parts of the Gangdese magmatic belt, the host rock of the Nyemo intrusion has a mineral composition similar to that of the MME, although differences are observed in chemical contents. To explore the genetic type of the MMEs and the deep processes of the Gangdese magmatic belt, the Nyemo intrusion is selected as the research object for this paper. Here, we report zircon U–Pb geochronological and whole-rock geochemical data for host diorites and MMEs, and electron probe data for hornblendes in diorites and MMEs, and combine mineralogy, petrology, petrogeochemistry and isotope geochemistry analyses. Research has shown that diorites in the Nyemo intrusion belong to the medium-K, metaluminous series. The light rare earth elements (LREEs) and heavy rare earth elements (HREEs) are significantly fractionated, and the LREE/HREE values are 5.77–7.71. The (87Sr/86Sr)i values of the diorites range from 0.704260 to 0.704287, and the εNd(t) values are from 3.73 to 4.17. The MMEs in the Nyemo intrusion have a limited range of SiO2 contents, are calc-alkaline with metaluminous affinity, and have relatively high contents of MgO (4.34–5.00 wt %) with Mg# (Mg2+/Mg2+ + Fe2+) values of 42.36–43.53, which is close to that of evolved basic magma. The contents of REEs vary from 108.87 to 120.59 ppm and show obvious Eu anomalies. The (87Sr/86Sr)i values of the MMEs range from 0.704680 to 0.704704, and the εNd(t) values are 0.35–3.74. The crystallization temperature of the hornblende in the diorite is 820 °C, the formation depth is 5.39 km, the oxygen fugacity is ΔNNO + 0.88 and the water content is 5.95 %. The crystallization temperature of the hornblende in the MMEs is 880 °C, the formation depth is 12.18 km, the oxygen fugacity is ΔNNO + 0.38 and the water content is 8.27 %. The Nyemo MMEs are formed by magma mingling, and originate from the partial melting of the depleted mantle, while the host diorite originates from partial melting of the juvenile crust with the addition of mantle material. The formation of the Gangdese magmatic belt is related to the Indo-Asian continental collision. The break-off of the subducted Neo-Tethyan oceanic plate triggered partial melting of the asthenosphere, which resulted in accumulation of the basaltic magma and then caused the partial melting of the juvenile crust with the addition of mantle material, thus forming a variety of granitic rocks and the large Gangdese magmatic belt.

In situ elemental and isotopic study of diorite intrusions: Implication for Jurassic arc magmatism and porphyry Cu-Au mineralisation in southern Tibet

The Gangdese porphyry Cu-Au belt (South Tibet, China) in the eastern Tethyan metallogenic belt hosts many deposits, which formed in subduction and collisional settings. Several porphyry Cu-Au deposits have been discovered in the Xietongmen region, including a large porphyry Cu-Au deposit at Xiongcun. However, the formation mechanism for the Xietongmen Cu-Au deposits and their igneous host rocks remain enigmatic. This study reports whole-rock major- and trace-element compositions, Sr-Nd isotopes, in situ zircon Hf- and plagioclase Sr-isotope compositions, and major- and trace-element compositions of amphiboles to constrain the magmatic emplacement and evolution related to the porphyry Cu-Au mineralisation in the Xietongmen region. New zircon U-Pb ages show that the Xietongmen diorites formed at 185Ma, consistent with the age of Cu-Au mineralisation. The Xietongmen diorites are characterised by enrichment in LILEs and LREE, and depletion in HFSEs. These features suggest that the diorites were produced by arc magmatism in an oceanic-slab subduction environment. In situ isotopic compositions exhibit radiogenic εHf(t) (+9.8 to +14.4) and low initial 87Sr/86Sr values (0.7039–0.7043); whole-rock results show positive εNd(t) (+5.26 to +5.31) and unradiogenic initial 87Sr/86Sr values (0.7042–0.7043). These isotopic results are similar to coeval mantle-derived hornblende gabbros and the Sangri volcanic rocks in the Gangdese belt. Amphibole clots in the Xietongmen diorites have higher MgO and Ni contents, with AlT>1.0, Mg#>0.5, Ni>143ppm, and REE patterns different from those of magmatic and idiomorphic amphiboles in the host diorites, suggesting that the clots are xenocrysts. All the petrological and geochemical evidence suggests that the Xietongmen mineralised diorites were derived from a Jurassic hydrous mantle wedge induced by slab dehydration. These magmas contain higher contents of water and other volatiles and have higher oxidation states. These conditions favored the transfer of metals (such as Cu, Au) to the upper crust, and then formed the Jurassic porphyry Cu-Au deposits.

On the significance of temperatures derived from major element and REE based two-pyroxene thermometers for mantle xenoliths from the North China Craton

Thermal evolution of the North China Craton (NCC) has been a subject of extensive geochemical and geophysical investigations. To better constrain the thermal state of lithospheric mantle beneath the NCC, we calculated and compared equilibrium or closure temperatures for peridotite and pyroxenite xenoliths from the NCC lithospheric mantle using major element-based two-pyroxene thermometers and a REE-based two-pyroxene thermometer. Samples included in this study are ancient refractory peridotites entrained by Early Cretaceous high-Mg diorites from Fushan within the central NCC, peridotites with varying chemical compositions and rhenium-depletion model ages entrained by younger than 100Ma alkali basalts from the central and eastern NCC, and pyroxenites entrained by Early Cretaceous alkali basalts from Feixian and Fangcheng within the eastern NCC. The Fushan peridotites have low major element-derived temperatures and slow cooling rates, owing to the shallow intrusion of their host diorites. The peridotites in younger than 100Ma alkali basalts have a more complicated thermal history. The Fexian and Fangcheng pyroxenites have relatively high temperatures and moderate to fast cooling rates. The present thermometric data, combined with the published petrologic, geochemical, and seismic observations, provide new insight into the thermal state and thermal evolution of the NCC lithospheric mantle in the Mesozoic and Cenozoic.

Zonation of primary haloes of Atud auriferous quartz vein deposit, Central Eastern Desert of Egypt: A potential exploration model targeting for hidden mesothermal gold deposits

The Atud gold mine located in the Neoproterozoic diorite and metagabbro of the Central Eastern Desert of Egypt has been initially excavated during Pharaonic times. Between 1953 and 1969, the Egyptian Geological Survey and Mining Authority performed underground prospection in the auriferous quartz vein and metasomatic alteration zones in the main Atud area, estimating a principal gold lode of 19,000tones (16.28g/ton), and 1600tons of damp (1.24g/ton). Yet the potentiality of the deposit has not been exhausted. However, for exploration of hidden ore, quantitative characterization using trace elements zoning of mineralization haloes with 280 samples from surface and three underground mining levels is applied. This was through multivariate statistical analysis (Factor analysis) of 11 selected trace elements. Axial (vertical) extents of primary haloes above and beneath gently dipping orebody are also visualized to interpret the level of erosion, determine the direction of mineralizing solutions as well as to examine whether the hidden orebody is promising at the Atud mine.Axial zones of primary dispersion aureoles of trace elements are: Ag, As, S and U around the auriferous quartz veins; Cu, and Pb in the surface horizons; and Zn, Ni, Co, and U along the lower margin of mineralization zone. Gold contents in bedrock and quartz vein samples from level-42M are the highest (5.7 and 40.3ppm, respectively). In the transverse (lateral) direction, the maximum relative accumulation of Au and Zn occurs at the Northern Shaft; Pb, Cu, As, and U at the Main Shaft; and Ag, S, Co, and Ni at the Southern Shaft. The estimated axial zonation sequence of indicator elements using the variability index is Pb→Cu→Ag→Au→As→S→Ni→Co→U→Zn. According to this zonation, an index such as (Pb×Cu)D/(U×Zn)D can be a significant for predicting the Au potentiality at a particular depth. In addition, the Pb/U zonality index is an appropriate indicator for the degree of erosion at the Atud gold mine. The degree of surficial zonality of the mineralization as deduced from geochemical maps and the level of erosion of the geochemical anomalies as well as the decreasing of gold content with depth recorded throughout the different underground mine workings make it necessary for the prospection model to evaluate the drainage patterns dissecting the mineralized zone.The application of R-mode factor analysis estimated seven statistical factors, and factor score maps are portrayed. Factors 1 (Ag, Au, As, Co, S, U and Zn) and 2 (Zn, U, Co and S) significantly reflect the Au-mineralization (ore-controlled), and their score maps enable a more precise delineation of auriferous quartz veins and the area which may contain primary gold mineralization. The other factors reveal the distribution of Cu- and Pb-bearing minerals (supergene alteration factors), and Ba and Ni in the host diorite (lithologically-controlled). These are consistent with the calculated maximum relative accumulation of trace elements, proposing a potential model of exploration based on integrating underground geochemical data from old gold mine workings with spatial information from R-mode factor score maps.

Basement nature and origin of the Junggar terrane: New zircon U–Pb–Hf isotope evidence from Paleozoic rocks and their enclaves

The question of whether the Junggar terrane in the central area of the Central Asian Orogenic Belt (CAOB) has a Precambrian basement is one of the hottest topics surrounding the tectonic evolution of the CAOB. Recent work shows that the Central Tianshan range has a Mesoproterozoic–Neoproterozoic basement. Here, we reported the SIMS U–Pb age and Hf isotopes of zircons from newly discovered dioritic gneiss and magnetite quartzite enclaves and their host diorite dike in the Taheir area, deformed Ordovician conglomerate in the Dazigou area, and mica gneiss relicts and their host granite in the Shuangchagou area in the Junggar terrane. The results reveal the following evidence: 1) the dioritic gneiss was metamorphosed from 2.52Ga diorite at approximately 1.88Ga, the magnetite quartzite interlayered with hornblende–plagioclase gneiss layers has an age of approximately 1.92Ga, and the inherited zircons from the diorite TH4-6 contain three zircon populations with upper intercept ages of approximately 3.06Ga, 1.98Ga and 1.89Ga; 2) the sienna and colorless zircon populations from the conglomerate reveal two magmatic rocks with ages of approximately 1.86Ga and 0.94Ga; and 3) the magmatic zircons that form mica gneiss have TDMC and TDM2 ages ranging from 1.2Ga to 2.08Ga, and the inherited zircons from the mica gneiss and granite in the Shuangchagou area contain five zircon populations with upper intercept ages of approximately 2.48Ga, 1.80Ga, 1.47Ga,1.14Ga and 0.96Ga.By combining the published U–Pb ages and Hf isotope data of zircons from the Junggar terrane, it is suggested that the Junggar terrane has three Precambrian basement units, including the 2.1–1.7Ga Taheir–Kalamaili–Dazigou reworked Archean crust, the 2.5–1.8Ga Shuangchagou–Luliang crust and the 1.86–1.7Ga Tianshan crust between the Aermantai subduction zone and south Tianshan suture. There is also the possibility of a 1.92Ga quartzite formation in the Taheir area, 1.9–1.8Ga orogen along the Taheir–Dazigou–Shuangchagou line, 1.47–1.40Ga granitoids in the Shuangchagou–Xingxingxia area, 1.2–1.0Ga magmatic rocks in the Dazigou–Shuangchagou area, a massive 1.0–0.9Ga magmatic–metamorphic rock belt along the Tianshan range and in the Shuangchagou–Dazigou area, and 0.8–0.6Ga magmatic rocks in the Taheir, Dazigou, Shuangchagou and Baluntai area. Regular arrangements of the three basement units and various sedimentary, magmatic and metamorphic belts indicate that the Precambrian Junggar terrane was possibly split from southern Laurentia near the Lake Superior region at 1.1Ga and adjacent to the Caborca region at 1.0–0.6Ga.

Ultramafic xenoliths in mesozoic diorite in west shandong province

Ultramafic xenoliths are common in Tietonggou intrusion, Laiwu, Shandong Province. Peridotite xenoliths develop two-stage metasomatism popularly: (i) The early metasomatism is characterized by intergranular clinopyroxene and phlogopite; (ii) The later metasomatism is characterized by orthopyroxene veins with a few plagioclases and amphiboles. These ultramafic xenoliths are thought to be mostly from the crust-mantle transitional zone on the basis of their combination, mineral chemistry, equilibrium temperature, and metasomatic characteristics. Major elements, rare earth elements, and Sr-Nd isotopic composition of the ultramafic xenoliths indicate that there is a petrogenetic relationship between the silica-enriched xenoliths and their host diorites. We propose that such silica enrichment might be an indicator of melt-rock interactions in the Mesozoic lithosphere beneath western Shandong.

Change from calc‐alkaline to adakitic magmatism recorded in the Early Cretaceous Darran Complex, Fiordland, New Zealand

The Early Cretaceous Darran Complex of the Median Tectonic Zone near Milford, South Island, New Zealand, consists dominantly of biotite ‐ two‐pyroxene diorites cut by biotite‐hornblende microdiorite and quartz monzodiorite dikes. The host diorite gives a 138 ± 2.9 Ma SHRIMP age on zircons, which is interpreted to be the age of igneous crystallisation. The calc‐alkaline geochemistry of the host diorites and the microdiorites is attributed to melting of a mantle wedge source fluxed by slab‐derived fluids. Ductile deformation (D1) of the host diorite and the microdiorite dikes took place at mid‐upper amphibolite facies conditions, with extension lineations indicating a top to the NNE sense of shear. A SHRIMP age of 136 ± 1.9 Ma on zircons from a quartz monzodiorite dike injected along D1 shears is statistically indistinguishable from that of the host diorite. This suggests that D1 was synmagmatic and that subduction, during or shortly after magma emplacement, was oblique to the Gondwana margin at c. 138 Ma. The quartz monzodiorite dikes are enriched in Na2O, Al2O3, and Sr, depleted in Y, and have a distinctly adakitic geochemistry. The change in chemistry from calc‐alkaline magmas to alkali‐calcic adakitic magmas reflects the melting of a mafic, garnet‐bearing, essentially plagioclase‐free source in the root of a volcanic arc system.

Rb-Sr dating and petrological characteristics of a granodiorite dike from the southern Hidaka metamorphic belt, Hokkaido, Japan.

Rb-Sr mineral isochron age was obtained for the unmetamorphozed granodiorite from a dike intruded into the Opirarukaomappu gabbroic complex, southern Hidaka metamorphic belt, Hokkaido, Japan. The isochron gives solidification age of 25.5 Ma with an initial Sr isotope ratio of 0.70362. The obtained age is obviously younger than inferred intensive metamorphic age of 56 Ma in the Hidaka metamorphic belt. The young solidification age is in accord with the other evidences. The hornblende geobarometer of the the granodiorite implied that it crystallized in a shallow crust. The granodiorite dike rocks have more evolved nature than the host diorite, but they are not co-magmatic.

Age, origin and geodynamic significance of plagiogranites in lherzolites and gabbros of the Piedmont-Ligurian ocean basin

U-Pb zircon dating, Sr-Nd isotope tracing and major/trace/RE element analyses were performed to constrain the age, origin and geodynamic significance of plagiogranites that intrude lherzolites and gabbros in the Ligurian Alps and the Northern Apennines. In addition, a host Fe-diorite was investigated. Samples from the Ligurian Alps were collected from the Voltri Group and the Sestri-Voltaggio Zone, whereas the plagiogranites from the Northern Apennines were taken in the Bracco unit. All these units have been affected by Alpine metamorphism reaching eclogite facies in the Voltri Group, blueschist degree in the Sestri Voltaggio samples, and prehnite-pumpellyite facies in the Bracco Unit, which has additionally been affected by rodingitization.U-Pb zircon ages of 150 ± 1, 153 ± 1 and ≈ 156 Ma were obtained, respectively, for two plagiogranites and the host Fe-diorite in the Ligurian Alps, and an age of 153 ± 1 Ma was determined for the plagiogranite in Northern Apennines. Inherited components in zircon and initial Pb in plagioclase indicate mixing of variously differentiated basaltic magmas with small amounts of roughly 1.7–2.1 Ga old continental crust material. REE patterns in both the plagiogranites and the host diorite are characterized by high REE abundance, and moderate LREE enrichment. Nd isotopic compositions lie in the range of N-MORB sources, yielding initial epsilon Nd values between + 8.8 and + 9.7, whereas Sr is isotopically heterogeneous. The geochemical pattern of the plagiogranites and the host Fe-diorite requires melting of a MORB-type mantle source that experienced LREE enrichment shortly before melting. The most likely explanation for such enrichment is the injection of melts derived by small degrees of melting from an adjacent mantle region. The basaltic, LREE-enriched parent magmas generated from this enriched domain have probably undergone up to about 72% of low-pressure fractional crystallization prior to their emplacement into the gabbro-peridotite complex.The 156–150 Ma magmatism occurred in close relation to normal faulting, sedimentation of breccias, and detachment of the mantle complex from its overlying continental crust, followed by exposure on the ocean floor. This tectono-magmatic event in the Ligurian Alps and the Northern Apennines reflects rifting of the Adriatic-Iberian continental plate segment, preceding wider opening of the Piedmont-Ligurian ocean basin and pillow basalt deposition.

Petrology of basic and intermediate orogenic granitoids from the Sila Massif (Calabria, southern Italy)

AbstractHercynian gabbroic, dioritic and tonalitic rocks crop out in the neighbourhood of Rovale (Sila Grande, Calabria). They make up a crude rectangular outcrop with the western part consisting of gabbroic rocks and the eastern of dioritic and tonalitic rocks. They come into contact with medium to high grade metapelites on the western side and with heterogeneous granodiorites on the other sides. In the gabbroic body both opx ± ol bearing cumulates and amphibole differentiates occur and are characterized by the widespread presence of brown pargasite. Sporadic magmatic to subsolidus corona textures between olivine and plagioclase or orthopyroxene and plagioclase can be observed and their preservation clearly suggests a post‐tectonic emplacement for the gabbroic magma. Diorites and tonalites display hypidiomorphic textures free of olivine and orthopyroxene and bearing green Mg‐hornblende. The granitoids, on the basis of chemical data, display orogenic features of the continent‐continent collision type. The gabbroic rocks have high Al tholeiitic composition and fractionation of orthopyroxene and plagioclase played an important part in their evolution.The Rb/Sr isochron method did not give a precise emplacement age for the granitoids as a whole. Initial 87Sr/86Sr ratios (at 290 Ma) are higher in the gabbroic body (0.7091–0.7095) than in diorites and tonalites (0.7083–0.7092). Thus gabbroic rocks appear more displaced than diorites and tonalites towards crustal isotopic composition. The eNd data seem to confirm this feature, thus suggesting that the gabbroic rocks and diorites derived from distinct mantle magma batches. Interestingly, small isotropic gabbroic masses occur within the diorites and show general features that allow them to be considered as possibly parental with respect to the host diorites. The evolution to the dioritic composition might have occurred through fractionation and minor mixing with a more acidic component such as the northern granodiorites.Geochemical, Sr and Nd isotopic data indicate a scenario of a composite plutonic body formed by distinct magma batches of mixed crust and mantle origin.

Magma-mingling and melt modification between granitic pipes and host diorite, Guernsey, Channel Islands

Within the Northern Igneous Complex of Guernsey cylindrical pipe intrusions, 20–40 cm in diameter, developed by buoyant emplacement of granite magma into overlying, denser diorite magma. The high viscosity of the partially crystallized magmas and lack of mechanical stirring inhibited physical mixing, and mesoscopically sharp, well-defined interfaces developed. The granitic pipes are surrounded by aureoles up to a metre in diameter which exhibit changes in bulk chemical composition and partial mineralogical re-equilibration. These formed in response to the modification of interstitial melt in the partially crystallized host resulting from residual melt migration and/or melt interdiffusion across the pipe/host interface. Such processes between partially crystallized magmas may, in part, provide an explanation for textural, mineralogical and chemical compositional features commonly exhibited by mafic enclaves within granites.

Nature and origin of meladiorite layers in northern Guernsey, Channel Islands

AbstractThe Cadomian igneous complex of N. Guernsey includes gabbro, diorite, granodiorite, and adamellite. The gabbro is layered, shows variations in colour index and texture, contains abundant amphibole, and is cut by the diorites. The diorites show little variation in mineral compositions despite considerable variation in modal proportions. They contain numerous thin and roughly parallel veins of leucodiorite and granodiorite, and layers of meladiorite occur in several places within the veined diorites. These meladiorites, previously interpreted as separate intrusions, form individual layers from a few centimetres to several metres thick which can be traced for some tens of metres along strike. The upper surfaces of the meladiorites are planar but the lower tend to be indented and irregular. The layers thin out laterally and usually pass into the host diorite by way of a zone containing dark spots and both the spots and the meladiorite layers are composed of actinolitic hornblende and Mg-biotite.Chemical and petrographic data are presented for most of the rocks and minerals especially for the meladiorites and their host, the veined diorites. The rock chemistry and particularly the compositions of the amphiboles and micas of the meladiorites and the mafic spots, taken together with the presence of unmixing lamellae in the amphibole and the details of the field relationships, indicate that the rocks tended to equilibrium under subsolidus conditions and are most probably of metasomatic origin.Acid pipes and pegmatites occur within and are restricted to the meladiorites. The forms of the pipes were previously taken to indicate that they were magmatic at the same time as the meladiorites. The metasomatic origin for the meladiorites proposed here calls into question this magmatic interpretation and it is suggested that the pipe formation is connected with the metasomatic process. It is envisaged that the alteration of the veined diorites took place under conditions in which high gas pressures allowed acidic melt to be produced at temperatures only a little above the solidus of the diorites. The thermal impulse for this recrystallization may have resulted from the polyphase intrusive history of the complex as a whole.

Discussion

The A uthors ' reply: We naturally agree with Mr Hutchison that the overall relationships between acid and basic rocks are very different in orogenic zones from what they are in non-orogenic regions. We never maintained that the process of magma-production was the same for both; nor did we intend our diagram showing a possible situation in which a net-veined intrusion, a composite dyke, and a composite lava flow might originate to have any relevance to the production of syntectonic granites or batholiths with satellitic basic bodies. By our remark that we ‘did not believe that the tectonic level was as important as Dr Bishop believed’ (Blake et al. 1965, p. 45) we meant that, in our opinion, certain detailed relationships at the level represented by the igneous complex of Guernsey should not have to be given a different significance from similar detailed relationships in complexes that formed at higher levels. Specifically, we are convinced that granitic pipes in basic diorite occurring in Guernsey formed in the same way as granitic pipes in dolerite at Slieve Gullion; and that a net-veined complex in Guernsey in which the host diorite shows a progressive reduction of its grain-size against the veins was formed in the same way as certain high-level net-veined complexes having this and other features in common with it. We think that the finer-grained margins mean the same thing at the two levels. It is not clear to us what are the too-broad generalizations that Mr Hutchison says we have been tempted