Hornblende Diorite Research Articles

An Early Permian transpressional continental arc and arc-arc collision recorded in the Baidunzi Complex, southern Central Asian orogenic belt, NW China

Our multidisciplinary study of the southern Beishan orogen in NW China, situated between the Tienshan and Solonker suture zones, sheds light on the closure of the Paleo-Asian Ocean and termination of orogeneses in the Central Asian Orogenic Belt. We identify the Baidunzi Complex, an exotic Permian transpressional continental arc terrane, characterized by sheared and isoclinally folded metasedimentary and syn-kinematic plutonic rocks transformed into upper greenschist to amphibolite tectonites. The complex comprises basement rocks of thick-bedded orthoquartzite and orthogneiss, the Carboniferous to Permian meta-sedimentary strata including volcaniclastic sedimentary rocks, marble, and marl interlayered psammite, and the Baidunzi Intrusive Suite featuring 294–289 Ma hornblende diorite, tonalite, granodiorite, and granite. The Late intrusive rocks, including leuco-granite to trondhjemite sills and a stitching gabbro unit, suggest an amalgamation age of 281–280 Ma between the Baidunzi and Ganquan arcs. The spatial and temporal linkage between the Baidunzi continental arc and the Liuyuan backarc-Ganquan arc suggests they were either built on a coeval doubly vergent subduction system or from the same north-vergent subduction system and structurally juxtaposed by sinistral convergence. Geochronological and Hf isotope evidence, along with continental rock affinity, indicates a close association between the Baidunzi Complex and the northern margin of the Tarim Craton during the Gondwana breakup and Pangea assembly.

Lithological Control to the Gold Mineralization in the Main Ridge Complex of the Bakan Gold District, Northern Sulawesi, Indonesia

Abstract Main Ridge complex is characterized by prominent high sulfidation epithermal-style mineralization with windows of porphyry style mineralization in the deeper zone. Several lithologic features play important controlling role in the occurrences of those mineralization. The oldest rock observed in the Main Ridge complex is unmineralized hornblende diorite (IDO-1) which encompasses the whole Bakan district. Several late intrusive stocks were concealed below the current topographic as mentioned as the microdiorite (IDO-2) and quartz diorite (IDO-3) in which the latter exhibits windows of porphyry-style mineralization. Those intrusive units were then covered by two unconformable volcanic tuff sequences which are andesitic tuff and dacitic tuff in compositions. Dacitic tuff is more favourable unit for gold mineralization compared to andesitic tuff which shown by near-surface ‘mushroom-like’ deposit form. In more peripheral area, the mineralization is also controlled by the bedding features of dacitic tuff which underlain by the less-mineralized andesitic tuff. In the latest stage, diatreme breccia cut all the lithologic units and becomes the most favourable host rock in sub-vertical deposit form. The comprehensive understanding of the lithological features is important to understand the mineralization control for further exploration target.

On the provenance of stone artefacts from the excavation materials of the zaporozhian Nekhvoroshchansky monastery of the Dormition of the Mother of God

Purpose. To determine, according to the data of petrographic analysis, the probable provenance of the raw materials of stone artefacts, namely, the fragments of tools and building stone of the Cossack times and the Late Bronze Age that were found during the archaeological excavations in the territory of the former Zaporozhian Nekhvoroshchansky Monastery, as well as to connect the results obtained with historical context. Methods. The study of the raw material of stone artifacts was carried out by the method of petrographic analysis using a polarizing microscope. The provenance was determined by comparing the obtained petrographic characteristics of the studied samples with similar rocks according to the data of geological materials. Findings. The studied samples were represented by a part of a stone tool of the Early Modern period and two fragments of building material from the cultural horizon of the Late Bronze Age. According to the results of the petrographic analysis, it was established that the tools of the Cossack period were made of hornblende diorite, and the samples dated to the Bronze Age were represented by sandstones with siliceous and carbonate cement, respectively. Regarding the origin of the raw material of the Cossack period artefact, it was found that the diorite manifestations in the zone of the Middle Dnipro megablock of the Ukrainian Shield are geographically closest to the site of the excavation. The sandstone with siliceous cement is most similar to such rocks of the Oligocene-Miocene Poltavska series, which are widespread in the region. The sample of sandstone with carbonate cement was most likely formed in the zone of groundwater emergence, has a Quaternary age and could well be of local origin. The originality. For the first time, a petrographic study of stone artefacts from the territory of a Zaporozhian Nekhvoroshchansky Monastery was carried out. The facts of the supply of sandstones of the Poltavska series from the Samara River Valey during the Late Bronze Age and diorites from the Dnipro Valley during the Zaporozhian Cossack period to the Oril River Valey have been established. Practical implementation. The obtained data can be used when writing scientific works on the history and archeology, as well as educational literature.

Petrogenesis of Alkaline Complex of the Longbaoshan Rare Earth Element Deposit in the Luxi Block, North China Craton, China

The alkaline complex in the southwest region of Luxi Terrane of the North China Craton is spatially correlated with the newly discovered Longbaoshan REE deposit. Its petrogenesis, however, remains ambiguous. In this study, we present an integrated petrology, whole-rock geochemistry, sphene U-Pb and rare earth element data from the Longbaoshan alkaline complex to investigate the petrogenesis, magma source and tectonic evolution. The Longbaoshan alkaline complex consists of mafic to intermediate rocks of hornblende diorite and alkaline hornblende syenite porphyry, biotite monzonite porphyry and aegirine diorite porphyrite. The hornblende diorites show a composition of low SiO2, high MgO, Fe2O3 and moderate Na2O, CaO and are metaluminous and medium-to-high-K calc-alkaline. The hornblende syenite porphyries, biotite monzonites and argirine diorite porphyrites display a relatively higher content of SiO2, Na2O, K2O and Al2O3 and lower contents of MgO, Fe2O3 and CaO and are metaluminous, peralkaline, high-K calcic-alkaline and shoshonite. The sphene U-Pb data shows that the parent magma of the hornblende diorite was emplaced at ca. 120 Ma. All these samples show a common depletion in Th, Nb-Ta and Zr-Hf and enrichment in large ion lithophile elements (e.g., Pb, Ba, Sr) and Light Rare Earth Elements. The magma may have experienced fractionation of pyroxene, amphibole, sphene, apatite and zircon during its evolution. The variable La content, La/Sm, Rb/Sr and (Ta/Th) N ratios indicate that the parent magma may produce by partial melting of a mantle source that was interacted with sediment-derived melts in a subduction setting. Therefore, we propose that the parent magma of the Longbaoshan alkaline complex was derived from a lithospheric mantle which was metasomatized by sediment-derived melt in a prior subduction process. The enriched magma was emplaced through an extension process and experienced subsequent fractionation and assimilation with the continental crust during the rollback of the Paleo Pacific Ocean plate.

Petrogenesis and Geological Significance of the Quartz Monzonites in the Jinling Area, Western Shandong Province

Jinling complex pluton is a key part of the Mesozoic magmatic belt in the eastern North China Craton. However, its petrogenesis is still being debated. The Jinling complex, mainly composed of biotite diorites, hornblende diorite, augite diorites, (quartz) monzonites, and quartz diorites, is outcropped in Huashan and Heitieshan. This paper studies the zircon geochronology, zircon Hf isotope, and Sr-Nd-Pb isotope of quartz monzonites. The samples have high contents of Cr, Ni, V, Al2O3, Ba, Sr, and are enriched in LREEs, LILEs (K, Ba, Sr), depleted in HREEs, and HFSEs (Nb, Ta, Ti). The samples with captured zircons of ~2.5 Ga yield a weighted mean age of ~127 Ma, and the zircons have negative values of εHf(t) from −3.2 to −9.4 while the ratios of 208Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb 37.75~38.15, 15.41~15.43, 17.59~17.98, respectively. The ratios of Th/U are from 3.77 to 3.82, while the values of μ and ω are 9.18 and from 35.72 to 36.15. Meanwhile, the ratios of 87Sr/86Sr are higher than the ones of the mantle. Geochemical and isotopic features indicate that the quartz monzonites derived from the subcontinental lithospheric mantle that probably assimilated ancient NCC upper crust materials (~15–20%) during the magma ascent in a lithospheric extension setting.

Two magma fractionation paths for continental crust growth: Insights from the adakite-like and normal-arc granites in the Ailaoshan fold belt (SW Yunnan, China)

Abstract Geochemical similarities between the continental crust and arc magmas have led to the inference that subduction zones may be the primary sites of crustal growth. Thus, it is necessary to unravel the petrogenetic mechanism(s) of granitoid generation in subduction-related settings to understand crustal growth through magmatic differentiation processes. In this study, we focused on granitoid generation in oceanic-continental subduction zones. We analyzed the whole-rock geochemistry and Sr-Nd isotopes, together with zircon U-Pb-Hf-O isotopes, of the newly identified Middle Triassic granitoids in the Ailaoshan high-grade metamorphic complex (Yunnan, SW China). All the studied granite samples were characterized by large ion lithophile element (e.g., Rb, Sr, and Ba) enrichments and high field strength element (e.g., Nb, Ta, and Ti) depletions, similar to arc-type rocks. They also showed a range of whole-rock Sr-Nd, (87Sr/86Sr)i = 0.7020–0.7048, εNd(t) = +0.6 to +4.2, and zircon Hf-O, εHf(t) = +10.3 to +18.1, δ18Ozircon = 5.09‰–6.65‰, isotope compositions, which overlap with those of previously reported coeval (ca. 237–235 Ma) hornblende diorite and granodiorite, the formation of which was interpreted to have originated from a mantle wedge metasomatized by a sediment-derived melt. Furthermore, the fractionation trends of some of the granitic samples and diorite-granodiorite suite overlap. They can be divided into two geochemical groups: Group 1 has intermediate to high SiO2 (66.9–73.8 wt%) and K2O (3.40–5.42 wt%) and low MgO (0.19–1.09 wt%) contents and shows depletion in heavy rare earth elements (HREEs; e.g., Yb and Y), resulting in adakite-like high Sr/Y (61–183) and La/Yb (47–90) ratios. Group 1 shows positive SiO2 versus Sr/Y and La/Yb correlations and negative SiO2 versus HREE and Y correlations, implying fractionation of a garnet-bearing assemblage. The negative correlations between SiO2 and εNd(t) and Nb/La reveal a crustal assimilation trend. Group 2 has relatively high SiO2 (72.6–76.5 wt%) and low K2O (1.93–3.82 wt%) and MgO (0.05–0.83 wt%) contents and shows depletion in middle REEs (MREEs; e.g., Gd and Dy) with low Sr/Y (1–10) and La/Yb (4–11) ratios. Group 2 granites show negative Gd/Yb versus SiO2 correlation, which indicates significant fractionation of an amphibole-bearing assemblage. Our results suggest that both group 1 and 2 granites were formed in a subduction setting from a common mantle-derived parental dioritic magma, but they experienced two distinct fractionation processes. While group 1 granites were likely formed by crustal assimilation and high-pressure (lower-crustal) garnet-dominated fractionation, group 2 granites were generated through low-pressure (middle-/upper-crustal) amphibole-/plagioclase-dominated fractionation. We suggest that these two fractionation trends are critical to crustal growth and the development of a more fractionated (felsic) upper crust.

Geochemical anomaly definition using multifractal modeling, validated by geological field observations: Siah Jangal area, SE Iran

Recognition of geochemical anomalies is a pivotal assignment in exploration projects. This study aims to delineate different AuCu geochemical anomalies using number-size (N-S) and concentration-area (C-A) multifractal models in the Siah Jangal area, SE Iran. In this research, lithogeochemical datasets were applied for the exploration of Au and Cu. A comparison between geochemical anomaly maps based on the N-S and C-A fractal models shows the N-S fractal modeling is a powerful tool for separation of weak elemental geochemical anomalies in all of sampling zones. Based on a comparison between the results of these two methods and field studies, the geochemical anomaly zones, defined by the N-S fractal model, are more accurate than those recognized by the C-A fractal model. The obtained results of the N-S and C-A fractal models have been interpreted with the extensive set of information including structural interpretation, geological and alteration data. Au and Cu mineralization in the Siah Jangal area are hosted mainly by Oligocene-Miocene sub-volcanic rocks, especially strongly altered porphyric quartz diorite, hornblende diorite and diorite. Moreover, the positive dependence between various alteration zones and high concentrations of Au and Cu proves that strongly anomalous areas are correlated with these alteration zones. High grade Au (> 1000 ppb) and Cu (>150 ppm) are associated with the altered sub-volcanic rocks in the northern, eastern, and SW parts of the study area. Therewith, the strong anomaly populations are mostly occurred within the fault and fracture systems in the study area. This is a promising signal because quartz-sulfide veins and veinlets are associated with such structures.

Petrogenetic constraints on magma fertility in the Baguio Mineral District, Philippines: Probing the mineralization potential of the igneous host rocks in the Sangilo epithermal deposit

In the Baguio Mineral District (BMD), porphyry Cu ± Au deposition and associated epithermal mineralization are attributed to the highly evolved magmatism during the Pliocene. It has been well-documented that the interaction between silicic crustal melts and primitive mantle melts formed water-rich, oxidized magmas that resulted to hydrothermal mineralization. However, there are very few studies on the Early to Middle Miocene calc-alkaline magmatism which is considered to be barren of mineralization. This magmatic event is represented by phases of the Central Cordillera Diorite Complex (CCDC), which also serve as host rocks to the Sangilo epithermal deposit. The Sangilo quartz-carbonate veins in the BMD are hosted by an Early Miocene hornblende diorite (22.33 ± 0.63 Ma) intruded by a Middle Miocene quartz diorite (15.91 ± 0.6 Ma) which are, in turn, penetrated by Pliocene basaltic andesite dikes. The Miocene magmatic units with hybrid crust-mantle source affinity were formed from varying degrees of interaction within the MASH (mixing, assimilation, storage, and homogenization) zone during the formation of the CCDC. The basaltic andesite dikes, part of the Pliocene Mafic Dike Complex, represent direct differentiates of basaltic melts that experienced ponding at the base of the lower crust before ascending to shallow crustal levels. Based on the assessment of the physico-chemical conditions, three distinct magmatic events were identified: a barren Early Miocene event, a fertile Middle Miocene event and a fertile Pliocene event. The Middle Miocene fertile magmatism is attributed to further development of the MASH zone under the Luzon arc from the Early to Middle Miocene. On the other hand, the enhanced fertility during the Pliocene is associated with the subduction of the Scarborough Ridge.

Construction of an island arc and back-arc basin system in eastern Central Asian Orogenic belt: Insights from contrasting Late Carboniferous intermediate intrusions in Central Inner Mongolia, North China

Characterizing island arc and back arc basin system through detailed magmatic profile remains a pivotal task in any regional paleotectonic and supercontinental reconstructions along ancient convergent plate margins, as is of paramount importance for the northern China–southern Mongolia (NCSM) tract along the eastern Central Asian Orogenic belt (CAOB). Here we document episodic Late Carboniferous intermediate to felsic intrusive suites in the West Ujimqin region of central Inner Mongolia, spanning from ca. 315 Ma granodiorites through ca. 310 Ma hornblende diorites to ca. 305–301 Ma diorites. The earlier granodiorites exhibit dominant magnesian-tholeiitic affinity, enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), and possess whole-rock ISr(t) ratios of 0.707387–0.708799, negative ƐNd(t) of −2.3 to −2.4 and zircon ƐHf(t) values of −0.69 to +4.87 and δ18O of +6.33 to +8.40‰. These features are consistent with the magmatic derivation from dehydration melting of tholeiitic mafic to intermediate lower crustal underplates with obvious high δ18O supracrustal inputs. The ca. 310 Ma pod-like hornblende diorites display high-Ca boninite-like elemental traits and MORB-like isotopic compositions (ISr(t) = 0.704190 to 0.704203, ƐNd(t) = +7.9 to +8.3, zircon ƐHf(t) = +14.7 to +17.5 and δ18O = +4.88 to +6.16‰), indicative of a prior melt-depleted then fluid-enriched mantle source. By contrast, subsequent (301–305 Ma) diorites show arc-like elemental hallmarks, with notable enriched LILEs and depleted HFSEs as well as variable isotopic values (ISr(t) = 0.703690–0.704065, ƐNd(t) = +3.5 to +8.4, zircon ƐHf(t) = +7.10 to +16.4 and δ18O = +3.75 to +6.83‰). They are interpreted to represent the evolved products of hornblende-dominated fractional crystallization from depleted basaltic parental magmas. Given their coincidence with regional back-arc-basin type ophiolitic stratigraphy and detrital zircon age spectra, these intermediate to felsic intrusive suites tend to document the timeline of a back arc basin formation from initial rifting to maturation. In conjunction with Early Carboniferous forearc-type ophiolitic complexes and forearc magmatic records at West Ujimqin and the adjacent region, a Carboniferous island arc and back arc system can be reconstructed from Xilinhot to West Ujimqin. The characterization of this northward (present day coordinates) subduction system in terms of its temporal evolution not only provides overwhelming evidence that modern Pacific-like, archipelago-type arc configuration had prevailed in central Inner Mongolia during the Carboniferous, but also offers vivid insights into the diversity of crustal generation mechanism in a subduction factory that ushered in seminal continental crustal construction in the eastern CAOB.

Tectonic evolution of the northeastern North China Craton: Constraints from geochronology and Sr–Nd–Hf–O isotopic data from Late Triassic intrusive rocks on Liaodong Peninsula, NE China

This paper reports new whole-rock major and trace element, SrNd isotopic, and zircon UPb and HfO isotopic data for Late Triassic intrusive rocks on Liaodong Peninsula, NE China, aiming to constrain the petrogenesis of these rocks in terms of lithospheric thinning in the northeastern North China Craton (NCC). New zircon UPb data indicate that the rocks were emplaced during the Late Triassic (220–211 Ma). Based on petrology, petrography and geochemistry, we divided these rocks into three groups. The Group 1 rocks, including hornblende gabbro, hornblende diorite and mafic vein, show low SiO2, Na2O + K2O and high MgO contents, with (87Sr/86Sr)i (0.7066–0.7071), εNd(t) (−10.6 to −5.7), εHf(t) (−16.6 to +1.5), and δ18O (7.06‰–7.82‰) values, indicating that they were formed by partial melting of lithospheric mantle with addition of lower continent crust materials. The Group 2 rocks, including syenites, display high SiO2, Na2O + K2O and low MgO contents, with (87Sr/86Sr)i (0.7064–0.7073), εNd(t) (−14.8 to −14.5), εHf(t) (−11.7 to −5.7), and δ18O (5.49‰–6.37‰) values, indicating they were originated through partial melting of lithospheric mantle with continent crust materials added, coupled by fractional crystallization of mafic minerals. The Group 3 rocks, including monzogranite and syenogranite, show high SiO2, Na2O + K2O and low MgO contents, with different isotopic signatures: the former shows (87Sr/86Sr)i (0.7085–0.7087), εNd(t) (−17.0 to −16.8), εHf(t) (−17.9 to −4.4) and δ18O (6.10‰–6.95‰) values. The latter shows higher εNd(t) (−9.0 to −8.1), εHf(t) (−3.4 to +1.2) and lower δ18O (5.52‰–7.29‰) values. Combined with the adakitic features of monzogranite and different two model ages, we believe they were derived from partial melting of ancient thickened lower crust and juvenile crust, respectively. Together with the spatial and temporal distribution of the Late Triassic rocks, we believe that these rocks were related to the collision between NCC and Yangtze Craton, and the initiation of lithospheric thinning of NCC in eastern segment began in Late Triassic.

Remnants of a Middle Triassic island arc on western margin of South China Block: Evidence for bipolar subduction of the Paleotethyan Ailaoshan Ocean

Identifying ancient magmatic arcs in collisional orogens is important for regional tectonic reconstruction. Here, we first identified Middle Triassic (ca. 235–237 Ma) arc-type hornblende diorite and granodiorite in the Ailaoshan high-grade metamorphic belt in SW China, east of the Paleotethyan Ailaoshan-Song Ma suture. These rocks show arc-type geochemical affinities, such as large-ion lithophile element (LILE; e.g., Rb and Ba) enrichments and high-field-strength element (HFSE; e.g., Nb, Ta and Ti) depletions. They are characterized by positive εNd(t) (+2.90 to +4.63), εHf(t) (+9.9 to +14.1), and higher δ18O (mean 6.35–6.65‰) than typical mantle-derived magmas. Whole-rock geochemical and Sr-Nd-Hf-O isotope data demonstrate that the hornblende diorite was derived from depleted MORB-like mantle metasomatized by subducted oceanic sediments, and the granodiorite was formed by fractional crystallization from mafic magmas. These two rock types likely represent remnants of a continental arc, whose upper arc architecture may have been mostly-/completely-eroded by the subsequent Indosinian (Triassic) and Himalayan (Cenozoic) orogenic events. Tectonically, the presence of Triassic arc remnants east of the Paleotethyan Ailaoshan-Song Ma suture suggests the occurrence of east-dipping (present orientation) subduction of the Paleotethys beneath the South China Block, a hypothesis discarded by most previous studies due to the lack of arc magmatic evidence. Combined with previous studies in the Ailaoshan tectonic zone and its southern extension (Song Ma tectonic zone), we propose that the Paleotethyan Ailaoshan Ocean may have been bipolar-subducted westward and eastward beneath the Indochina and South China blocks, respectively.

New Occurrence of Gold Mineralization at the Eastern Part of Wadi El Gemal, South Eastern Desert, Egypt

The basement rock units of the study area are represented by metasediments-metavolcanics association, diorite rocks, granodiorites and syenogranites. The whole sequence is cut by basic dykes, pegmatites and different quartz (jasper) veins. Metasediments-metavolcanics association is represented by hornblende schist and metadacite respectively. There is visible sulfides mineralization in hornblende schist, metadacite and diorite rocks at their contacts at the wadi level. So, polished sections were prepared to study opaque minerals under the ore microscope, which reveals the presence of gold, pyrite and goethite. The radioactivity of this area is weak, where the average eU contents in metasediments-metavolcanics association, diorites, granodiorites, syenogranites, pegmatites and jasper veins are 2.8 ppm, 1.9 ppm, 3.26 ppm, 4.91 ppm, 4.32 ppm and 6.7 ppm respectively. The intrusion of granodiorite rocks may act as a source for hydrothermal solutions ascending along fractures of all older rocks. During this stage, a chemical reaction of hydrothermal fluids with metasediments-metavolcanics association and diorite rocks lead to forming sericite and enrichment of silica content. The enrichment of silica content led to the formation of quartz-diorite. Also, iron derived from the host rocks and sulfur from the hydrothermal fluids could have formed pyrite while the gold was precipitated at the contact between metasediments-metavolcanics association and diorite rocks.

Platinum-Group Element Geochemistry of the Escondida Igneous Suites, Northern Chile: Implications for Ore Formation

Platinum-group element (PGE) geochemistry may be used to constrain the timing of sulfide saturation in magmas, which influences the Cu and Au fertility of evolving magmatic systems. We report new geochronological and geochemical data, with emphasis on PGE geochemistry, for a suite of regional porphyritic hornblende–diorite intrusions and ore-bearing porphyries from the super-giant Escondida and smaller Zaldivar Cu deposits of Northern Chile. The regional dioritic intrusions have zircon U–Pb ages between 39·6 to 37·1 Ma, which overlap with the ages of the ore-bearing Escondida and Zaldivar porphyries (38·1 to 35·0 Ma). Whole-rock major and trace element, and Sr–Nd–Pb and zircon O–Hf isotope geochemistry indicate that the regional diorites and ore-bearing porphyries are co-magmatic and originated from the same mantle-derived magma by fractional crystallization, with minor contamination by Paleozoic crust (∼10%). The low concentrations of PGE in the regional diorites show that they reached sulfide saturation before the MgO content of the melt fell to 4·7 wt %, the MgO content of the most primitive sample analysed. The fraction of sulfide melt which separated from the melts that formed the regional diorites is estimated to be ∼0·12 wt %; this resulted in the partitioning of highly chalcophile elements (Au and PGE) into a sulfide phase that was retained in cumulus rocks at depth. However, the fraction of sulfide melt was too low to have a significant effect on the Cu content of the fractionating melt. As a consequence, when the evolving melt eventually reached volatile saturation, it contained enough Cu (40 ± 10 ppm) to form a super-giant Cu deposit. In contrast, Au was largely stripped from the melt by sulfide precipitation, with the result that the mineralization at Escondida is Cu dominant, with only minor Au. The Zaldivar deposit, on the other hand, contains even less Au, which is attributed to a longer fractionation interval between sulfide and volatile saturation. This study provides evidence to support previously proposed models which suggest that the timing of sulfide saturation, the amount of sulfide melt produced, the water content and oxidation state of the melt, and the magma volume are critical factors in determining the potential to form a porphyry Cu deposit. Plots of Pd/MgO against Y can be used as empirical indicators of magma fertility for porphyry mineralization, and to discriminate between Cu–Au and Cu-dominated systems, but cannot predict the size of the deposit. The super-giant status of the Escondida deposit is attributed to it being underlain by a large batholith with a calculated minimum mass of 10 12 tonnes (∼400 km 3 ).

Metamorphic evolution of Pan-African Wadi El Miyah Metasediments, Central Eastern Desert, Egypt: a distinctive LP/HT metapelitic sequence from the northern Arabian–Nubian Shield

The meta-volcanosedimentary sequence from the SW sector of Wadi El Miyah area constitutes an example of the Late Proterozoic Pan African supracrustal rocks exposed in the Central Eastern Desert of Egypt. This folded meta-volcanosedimentary sequence is juxtaposed along about 8-km-long WNW–ESE-trending thrust contacts with a highly foliated ophiolitic melange comprising serpentinites, metagabbros and quarztofeldspathic blocks and intruded by a gneissose hornblende diorite. Chaistolite-bearing mica schists crop out in three compositionally different metamorphic zones, marked by garnet, andalusite, and staurolite porphyroblasts, respectively. Microstructural data combined with careful mapping indicate that the studied metapelites were subjected to three deformational phases, D1–D3, each having its own impact on the development of planar and linear structures. These rocks witness the coexistence of a less ubiquitous dextral sense of shear and a later sinistral one that correspond to the D2-thrusting and D3-related phases, respectively. Geothermobarometry of the garnet-bearing assemblages combined with Th–U–Pb dating of metamorphic monazite by an electron microprobe indicates a counter-clockwise P–T–t path with a thermal peak assemblage of garnet + staurolite + chiastolite ± fibrolitic sillimanite at 622 °C/4.1 kbars during the D2 phase, with an age around 640–658 Ma. This is concordant with the ages assigned for the arc accretion phase in the Nubian Shield. The determined counter-clockwise P–T–t path of the studied rocks suggests that such a Buchan-type LP/HT sequence may be considered as a typical example of a regional–contact metamorphism which formed in areas that suffer from crustal thickening accompanied by emplacement of granitoids that affected the geothermal gradient of the crust.

Chapter 12 A 251 Ma diorite clast from a Maitai Group conglomerate and a c. 259 Ma microdiorite sill intruding the Productus Creek Group, Southland, New Zealand: note

Abstract We present the results of U–Pb dating of zircons of two Late Permian igneous rocks from the Dun Mountain–Maitai and Brook Street terranes of New Zealand's South Island. A conglomerate clast of hornblende diorite from the Early–Middle Triassic Stephens Subgroup, Maitai Group, gives an age of 251.1 ± 1.6 Ma. A sample of hypabyssal andesite–microdiorite (Weetwood Formation) that intrudes the Late Permian Productus Creek Group of the Brook Street Terrane gives an age of 258.8 ± 7.9 Ma. The intermediate and subalkaline geochemistry of the two samples is not distinctive of specific tectonic settings, but their ages match plutons of the I-type, subduction-related Longwood Suite of the nearby Median Batholith. Assuming that correlation with the Longwood Suite is correct, the Weetwood sample increases the known areal extent of intrusion of the Longwood Suite; and the Stephens Subgroup Formation clast is consistent with a detrital link between Gondwana and the Dun Mountain–Maitai Terrane by the Middle Triassic. However, because of uncertainties regarding the timing of magmatism along other parts of the Gondwana margin, the data in this paper do not establish any specific provenance links.

Petrogenesis of granitoids in the Wulan area: Magmatic activity and tectonic evolution in the North Qaidam, NW China

Numerous granitic intrusions crop out in the eastern segment of the North Qaidam block (NQ), NW China. To evaluate their ages, petrogenesis and genetic relationships to other granitoids in the NQ, we present geochemical and geochronologic data for six intrusive bodies and review regional data. Zircon U-Pb (SHRIMP) dating yielded ages of 413 ± 3 Ma for the Hadesengou granite; 254 ± 3 Ma for the Xugeigou granite; 251 ± 1 Ma for the Qiluoshan granite; 249 ± 1 and 248 ± 2 Ma for the Chahannuo hornblende diorite and granite, respectively; 240 ± 2 Ma for the Chahanhe granite; and 250 ± 1 and 244 ± 3 Ma for the Shailekegoulei granodiorite and granite, respectively. Consequently, the Wulan plutons can be divided into two petrologic groups: Early Devonian (D1) quartz monzonite and syenogranite, and Late Permian to Early Triassic (P3-T1) hornblende diorite, granodiorite, and granite. The D1 granitic intrusions have geochemical affinities with A-type granites (A2-type) characterized by low Ca, Sr, Ba and Nb, and high Fe, Ga, Y and Rb, consistent with derivation by partial melting of metapelitic source rocks containing a small amount of metagraywacke. The P3-T1 I-type granitic intrusions are geochemically typical of active continental margin rocks, consistent with derivation by partial melting of metabasalt and clay-poor metagraywacke. Combined with previous studies, we recognize five periods of granitic magmatism in the NQ: (1) 465–473 Ma; (2) 423–446 Ma; (3) 391–413 Ma; (4) 372–383 Ma; and (5) 240–271 Ma. Based on the temporal-spatial distribution of granitic intrusions in the NQ and the regional tectonic evolution, we interpret the first and second periods of granitic magmatism as related to normal plate subduction, and the third period to slab break-off and exhumation of the subducted plate. The fourth stage of granitic magmatism is attributed to large-scale lithospheric mantle delamination, involving the differential movement of orogenic blocks. The fifth period of granitic plutonism probably reflects northward subduction of the East Kunlun Paleotethys oceanic crust and southward subduction of Zongwulong oceanic crust beneath the Oulongbuluke continental block.

Ophiolitic Complex of the Matachingai River on Eastern Chukotka: Fragment of Lithosphere in Mesozoic Back-Arc Basin

The Matachingai River basin is known among the few ophiolitic complexes on eastern Chukotka as the southern boundary of the Chukotka Fold System (in terms of tectonics, the Chukotka microcontinent or a fragment of the Arctic Alaska–Chukotka microplate). This complex comprises tectonic blocks of residual spinel harzburgite with dunite bodies and pyroxenite, olivine gabbro, and leucogabbro veins; blocks of hornblende gabbro, diorite, and plagiogranite; and Upper Jurassic–Lower Cretaceous basaltic–cherty and cherty–carbonate rocks. The geological relationships of rocks within tectonic blocks, the compositions of primary minerals, the bulk geochemistry of rocks, as well as the strontium, neodymium, and lead isotopic compositions, make it possible to consider individual tectonic blocks of the complex as fragments of a disintegrated oceanic-type lithosphere that formed in a back-arc spreading center. The melts, crystallization products of which are represented by hornblende gabbro of blocks, olivine gabbro of veins, and basalts, separated from geochemically and isotopically heterogeneous mantle. Blocks composed of rocks with various modal composition are likely relicts of an oceanic lithosphere of different segments of a back-arc basin. The studied complex may be a lithosphere of one of the Middle–Late Jurassic back-arc basins. Fragments of these basins are retained in ophiolitic complexes on Great Lyakhovsky Island of the New Siberian Islands Archipelago, western Chukotka, and the Brooks Range in Alaska.

Модель мантийно-корового взаимодействия и сопряженного магматизма в надсубдукционном орогене (палеозой Урала)

A model of magma formation in the crust of the Urals mobile belt, which is the best example of epioceanic suprasubduction orogen, has been developed. Magma formation occurs both in the relic oceanic and in the newly formed orogen crust. In the first case small bodies of practically non-potassium plagiogranites are formed, in the second one can see large gabbro-tonalite-granodiorite-granite (GTGG) and essentially granite massifs. The main conclusion is that the formation of a new earth crust of the Urals mobile belt and magma generation in it was initiated by the replacement of low-water mantle magmatism with water-rich one. The latter accompanies crustal magma generation at all its stages. In the areas of intense water magmatism, centers of long-term (up to 100 million years or more) endogenous activity (CLEA) are formed, the products of which are GTGG and granite massifs. Two main stages are distinguished in the evolution of the CLEA: 1) spontaneous partial melting (automigmatization) of products of water-rich basic magmatism - hornblende gabbros and diorites, and formation of tonalite, granodiorite and plagiogranite melts; 2) partial melting of the tonalite and granodiorite which produces the melts of adamellitic and granite composition.

Phenocryst Zonation in Porphyry-Related Rocks of the Baguio District, Philippines: Evidence for Magmatic and Metallogenic Processes

Mantle-derived mafic magmas may be the source of ore-forming metals in large Cu porphyry systems, but evidence of primary petrogenetic and metallogenetic processes can be masked by hydrothermal alteration. The Baguio district of Northern Luzon, Philippines is a world-class mineral province containing approximately 40 Moz of gold and 5 Mt of copper, distributed across many porphyry Cu and epithermal Au deposit systems which are spatially related to Pliocene pre-ore Liw-Liw Creek (LCC) basaltic dikes (3·59–4·73 Ma) and a syn-ore plagioclase–hornblende diorite porphyry (2·81–2·98 Ma). A range of phenocryst phases (plagioclase, amphibole and clinopyroxene) are well preserved in both the basaltic dike suite and diorite porphyry suite, and provide an excellent natural laboratory to investigate magmatic processes and the relationship between metallogenesis and mafic magma. Widespread normally zoned clinopyroxene and amphibole grains in the basaltic dike suite show sharp decreases in Mg#, but increasing Mn, Sr, Cu, Y, Zr and REE from core to rim. Mixing of evolved felsic magma into the mafic magma is the most likely cause of these trends as fractional crystallization would typically result in gradual variations. In the diorite porphyry suite, reversely zoned amphibole is characterised by sharp increases in Mg#, Cu and Ni, but decreasing Mn, while oscillatory zoned plagioclase with distinct regions of patchy zonation shows repeated variations of An, Mg, Fe and Sr from core to rim. These signatures indicate the addition of basaltic magma to the diorite porphyry magma. The modeled Mg# of the melts (estimated assuming mineral-melt equilibrium) is also consistent with magma mixing. For example, the high Mg# of 66–72 and low Mg# of 38–44 estimated from the LLC clinopyroxene cores and rims, respectively, support the presence of basaltic magma, mixed with evolved felsic magma. All normally zoned clinopyroxene and amphibole in the basaltic dike suite and reversely zoned amphibole in the diorite porphyry suite show consistent increases in Cu from core to rim, suggesting relative enrichment of Cu in both types of injected magma. The pre-ore basaltic and ore-forming dioritic hybrid melts are estimated to have contained 45–96 ppm and 319–351 ppm Cu, respectively, based on Cu partition coefficients and zone area percentage. The Cu content of the basaltic hybrid melt is typical of arc magmas (Cu 50–100 ppm), whereas the Cu in the dioritic melt was 3–6 times higher than typical arcs magma. Both the clinopyroxene-melt thermobarometer and Al-in-hornblende geobarometer indicate similar crystallization pressures for both suites (8·6∼8·8, 4·4∼4·7, 2·1∼2·8 kbar), suggesting that both pre-ore basaltic and syn-ore dioritic suites formed in different magma chambers at similar depths. The addition of Cu enriched mafic magma may contribute ore-forming elements to the mineralising magmas and significantly increase the mineralization potential of coeval felsic rocks. Studies of phenocrysts have the potential to elucidate the role of magmatic process in the formation of porphyry systems and allow for the recognization of the key characteristics of fertile magmatic systems.

Mineralogical evidence for crystallization conditions and petrogenesis of ilmenite-series I-type granitoids at the Baogutu reduced porphyry Cu deposit (Western Junggar, NW China): Mössbauer spectroscopy, EPM and LA-(MC)-ICPMS analyses

Primary ore-forming minerals retain geochemical signatures of magmatic crystallization information and can reveal the petrochemical conditions prevalent at the time of their formation. The Baogutu deposit is a typical reduced porphyry Cu deposit. Amphibole and biotite Fe3+/ΣFe ratios, minerals (feldspar, biotite, amphibole, zircon and apatite), in situ elemental and apatite Nd isotopic compositions were determined by Mössbauer spectroscopy, electron probe microanalysis, and laser ablation multiple-collection inductively coupled plasma mass spectrometry, respectively, to investigate the magma oxidation state, petrogenesis, source features, and to constrain the carbon species at magmatic stages for the intrusive phases. The results show that the primary plagioclase and amphibole in the mineralized diorite to granodiorite porphyry and post ore hornblende diorite porphyry are distinct (An26-55 versus An60-69; Mg-hornblende versus tschermakite). In particular, the amphibole shows distinct major and trace element compositions with light rare earth element enrichments and negative Eu anomalies in Mg-hornblende and light rare earth element depletions and no Eu anomalies in tschermakite. All the analyzed biotites are primary igneous phases with a biotite phenocryst profile showing significant variations of Zn, Cr, Sc and Sr from core to rim. These results may indicate the occurrence of mixing between two distinct magmas during mineral formation. Titanium in zircon and Si∗ in amphibole thermometries indicate that magma crystallized at >900°C and continued to ∼650°C. In situ apatite Nd isotope (εNd(t)=5.6–7.6, TDM2=620–460Ma), indicate absence of significant reduced sedimentary contamination and the source of juvenile lower crust. Slightly decreasing Fe3+/ΣFe ratios from biotite and amphibole to whole rock indicate decreasing oxygen fugacity during magma crystallization. Recalculated biotite compositions according to Fe3+/ΣFe ratios indicate fO2 values of less than Ni-NiO buffer (NNO) which show slightly lower values than that estimated according to zircon/melt distribution coefficients Ce anomalies (∼ΔNNO+0.6). These values are consistent with the features of reduced porphyry Cu deposits. Crystallization of other mineral phases significantly affects the reliability of oxybarometer of zircon/melt distribution coefficients Eu anomalies and Mn contents in apatite. This oxidation state suggests that only CO2 was present at the magmatic stage, and implies that CH4 formed during CO2 reduction occurring later hydrothermal alteration. The alteration of primary amphibole to actinolite released Ti, Al, Fe, Mn, Na and K to the fluid with later precipitation of titanite, albite and minor ilmenite and magnetite during actinolite alteration.