Quartz Monzonite Pluton Research Articles

The behavior of Fe isotopes in Fe skarns: A case study from the Yeshan Fe skarn deposit, Eastern China

To characterize the applicability of Fe isotopes to skarn petrogenesis and exploration, a robust understanding of their fractionation behavior during skarn alteration and mineralization is required. Here, we characterize the bulk-rock Fe isotope composition of endo- and exoskarn, magnetite ore, and the intrusive rocks that comprise the Yeshan Fe skarn deposit in Eastern China, aiming at better constraining the behavior of Fe isotopes during skarn formation and mineralization. The δ56Fe values of garnet and garnet–diopside skarn (–0.19 ‰ to 0.15 ‰, n = 15) are negatively correlated with bulk-rock MgO/Al2O3, suggesting that the variation in δ56Fe of these samples is mainly controlled by mineralogy. Garnet skarn is characterized by δ56Fe values (0.07 ‰ to 0.15 ‰, n = 11) that are similar to those of the spatially associated quartz monzonite pluton (0.12 ‰ to 0.16 ‰, n = 2), and it has systematically higher Fe2O3 contents. Based on petrological observations and bulk-rock geochemistry (e.g., REE and P2O5 contents), garnet, the main Fe-bearing mineral in the garnet skarn, is inferred to have achieved equilibrium (or near equilibrium) with the hydrothermal fluids that circulated throughout the mineralized system, implying that the δ56Fe values of garnet skarn can be used to trace the δ56Fe values of the fluids. Epidote skarn has a similar Fe2O3 content as garnet skarn, but lower δ56Fe values (–0.07 ‰ to 0.01 ‰, n = 2). Epidote skarn was genetically associated with low temperature, FeCl2(H2O)4-dominated fluids, which are in contrast with that (high-temperature, [FeCl4]2--dominated fluids) formed the garnet skarn. Iron in diopside skarn is mainly hosted by magnetite; the δ56Fe values of this lithology (0.00 ‰ to 0.12 ‰, n = 2) are, therefore, mainly controlled by magnetite. Taken together, the Fe isotopic signatures of various types of skarn at Yeshan could enhance our understanding of skarn deposits worldwide, especially those sharing geological features similar to the Yeshan Fe skarn deposit.

Genesis and Evolution of the Yolindi Cu-Fe Skarn Deposit in the Biga Peninsula (NW Turkey): Insights from Genetic Relationships with Calc-Alkaline Magmatic Activity

The current work investigates the impact of magmatic fluids and metasomatic processes on the Yolindi Cu-Fe skarn deposit in the Biga Peninsula, Turkey. It traces the stages of skarn evolution, from prograde to retrograde alterations, and investigates findings within a broader geological, mineralogical, and geochemical framework. Additionally, it assesses the evolutionary history of the Yolindi deposit in relation to calc-alkaline magmatic activity in an island-arc environment and compares its mineral compositions and genesis with other global and regional Cu-Fe skarn deposits. The Yolindi Cu-Fe skarn deposit in the Biga Peninsula was formed by the intrusion of Şaroluk quartz monzonite pluton into Upper Paleozoic Torasan Formation rocks such as phyllite, schists, hornfels, marble, and serpentinites. During skarnification, reactions between the magmatic fluids from the Şaroluk quartz monzonite pluton and the Torasan Formation produced skarn minerals associated with metals such as Fe and Cu. Initially, these reactions formed prograde skarn minerals such as augite-rich pyroxenes and andradite garnets with magnetite and pyrite. As the system cooled, these initial minerals underwent retrograde alteration, leading to the formation of minerals such as epidote, actinolite, and chlorite, as well as other copper and iron minerals including chalcopyrite, bornite, secondary magnetite, and specular hematite. Therefore, four main stages influenced the formation of the Yolindi Cu-Fe deposit: metamorphic bimetasomatic, prograde metasomatic, and retrograde metasomatic stages. Later, oxidation and weathering resulted in supergene minerals such as cerussite, malachite, and goethite, which serve as examples of the post-metamorphic stage. The mineralogical shifts, such as the andradite–grossular transition, reflect changing hydrothermal fluid compositions and characteristics due to the addition of meteoric fluids. Importantly, the formation of magnetite after garnet and clinopyroxene during the retrograde stage is evidenced by magnetite crystals within garnet. The mineral associations of the Yolindi Cu-Fe skarn deposit align with the global skarn deposits and specific Turkish skarns (e.g., Ayazmant Fe-Cu and Evciler Cu-Au skarn deposits). The Yolindi Cu-Fe skarn deposit, in association with ore-bearing solutions having magmatic origins, developed in an island-arc setting.

Geochronology and geochemistry of Meso- to Neoarchean magmatic epidote-bearing potassic granites, western Dharwar Craton (Bellur–Nagamangala–Pandavpura corridor), southern India: implications for the successive stages of crustal reworking and cratonization

Abstract We present field and petrographical characteristics, zircon U–Pb ages, Nd isotopes, and major and trace element data for the magmatic epidote-bearing granitic plutons in the Bellur–Nagamangala–Pandavpura corridor, and address successive reworking and cratonization events in the western Dharwar Craton (WDC). U–Pb zircon ages reveal three stages of plutonism including: (i) sparse 3.2 Ga granodiorite plutons intruding the TTG (tonalite–trondhjemite–granodiorite) basement away from the western boundary of the Nagamangala greenstone belt; (ii) 3.0 Ga monzogranite to quartz monzonite plutons adjoining the Nagamangala greenstone belt; and (iii) 2.6 Ga monzogranite plutons in the Pandavpura region. Elemental data of the 3.2 Ga granodiorite indicate their origin through the melting of mafic protoliths without any significant residual garnet. Moderate to poorly fractionated REE patterns of 3.0 Ga plutons with negative Eu anomalies and Nd isotope data with ε Nd (T) = 3.0 Ga ranging from −1.7 to +0.5 indicate the involvement of a major crustal source with minor mantle input. Melts derived from those two components interacted through mixing and mingling processes. Poorly fractionated REE patterns with negative Eu anomalies of 2.6 Ga plutons suggest plagioclase in residue. The presence of magmatic epidote in all of the plutons points to their rapid emplacement and crystallization at about 5 kbars. The 3.2 Ga intrusions could correspond to reworking associated with a major juvenile crust-forming episode, whilst 3.0 Ga potassic granites correspond to cratonization linked to melting of the deep crust. The 2.6 Ga Pandavpura granite could represent lower-crustal melting and final cratonization, as 2.5 Ga plutons are absent in the WDC.

Post-Mineralization, Cogenetic Magmatism at the Sungun Cu-Mo Porphyry Deposit (Northwest Iran): Protracted Melting and Extraction in an Arc System

The Sungun porphyry ore deposit is located in Eastern Azarbaijan province, Northwestern Iran. The oldest intrusive pulse in the region is a quartz-monzonite pluton, which hosts the porphyry copper-molybdenum mineralization. The Sungun Copper Mine includes the mineralized Sungun porphyry as well as six groups of cross-cutting and lithologically distinct post-mineralization dykes. The composition of these dykes ranges from quartz diorite, gabbro, diorite, dacite, lamprophyre, and microdiorite. Quartz diorite and dacite dykes are the oldest and youngest dykes, respectively. Based on their cross-cutting relationships, the composition of the dykes tend to become more primitive through time. The dykes strike Northwest–Southeast with Southwest dip, sub-parallel to the reverse faults within the deposit area. The lamprophyric dykes range from phonotephrite, to trachybasalt, tephrite, and basanite. The quartz-monzonite porphyry (SP) and the post-mineralization dykes (DK1-DK3) have clear and distinct negative anomalies of Ti, Zr, P, Pr, Ce, and Nb, as well as positive anomalies of Cs, U, K, Pb, and Nd with respect to primitive mantle. Microdioritic dykes (MDI) show depletion of Ti, Nb, P, Ta, Th, Yb, and Zr, and enrichment of Cs, Ba, U, Pb, Nd. The similarities in trace element abundances and patterns in the porphyry and post-mineralization calc-alkaline dykes implies a single source and fractional crystallization as the main mechanism controlling magmatic evolution in a collisional environment. Lamprophyric dykes have enrichment of LREE and LILE and depletion of HREE and HFSE such as Ti, Nb, and Ta. The parent magma of the lamprophyric dykes (LAM) was likely derived by low degrees of melting of a garnet lherzolite mantle peridotite. The 87Sr/86Sr and 143Nd/144Nd ratios range from 0.704617 to 0.706464 and from 0.512648 to 0.512773 for the dykes suggesting that the parental magmas came from a progressively more enriched mantle. Isotope ratios of 87Sr/86Sr and 143Nd/144Nd support a cogenetic relationship of porphyry and calc-alkaline dykes, except for the microdiorite ones. A common primary melt underwent gravity differentiation in a deep magmatic chamber to form a dioritic magma. This subsequently migrated to shallower levels to evolve further and feed individual dyke groups into the Sungun porphyry.

Geochemical constraints on the link between volcanism and plutonism at the Yunshan caldera complex, SE China

The Yunshan caldera complex is part of a larger scale, ca. 2000-km-long volcanic–plutonic complex belt in the coastal region of SE China. The volcanic rocks in the caldera complex are characterized by high-silica peraluminous and peralkaline rhyolites associated with an intracaldera porphyritic quartz monzonite pluton. In this study, we present zircon U–Pb, Hf and stable O isotopes along with geochemical data of both volcanic and plutonic rocks to evaluate the potential petrogenetic link between volcanism and plutonism in the Yunshan caldera complex. SHRIMP zircon U–Pb geochronology of both volcanic and plutonic rocks yields almost identical ages ranging from 95.6 to 93.1 Ma. The peraluminous and peralkaline rhyolites show negative anomalies of Sr, P, Ti and Ba and to a lesser extent negative Nb and Ta anomalies, along with positive Rb anomalies and ‘seagull-like’ rare earth element (REE) patterns with negative Eu anomalies and low (La/Yb)N ratios. The intracaldera porphyritic quartz monzonite displays minor negative Rb, Nb, Ta, Sr, P and Ti anomalies and a positive Ba anomaly with REE patterns characterized by relatively high (La/Yb)N ratios and lack significant Eu anomalies. The peraluminous and peralkaline rhyolites and the porphyritic quartz monzonite exhibit consistent e Nd(t) of − 3.7 to − 2.2 and display zircon e Hf(t) values of − 2.1 to 3.7. They further have similar, mantle-like, zircon oxygen isotopic compositions (δ18OVSMOW mainly = 4.63 to 5.76‰). We interpret these observations to be in agreement with a crystal mush model in which the parental magma of the volcanic and plutonic rocks of the Yunshan caldera complex was likely produced by interaction of asthenosphere melts with subduction-influenced enriched mantle wedge. The peralkaline rhyolites are interpreted to represent the most differentiated magma that has subsequently experienced significant fluid–melt interactions, whereas the porphyritic quartz monzonite may be representative of the residual crystal mush. The Yunshan rhyolites typically match the geochemical characteristics of ‘hot-dry-reduced’ rhyolites indicating that, during the late Cretaceous, the tectonic setting of SE China changed from a compressional environment to an extensional environment, i.e., from an arc into a back-arc setting. Our results imply that volcanic and plutonic rocks in caldera systems may provide unique constraints on the evolution of the magmatic system in which both the erupting melt and the residual crystalline material are being preserved.

Crust–mantle interaction beneath the Luxi Block, eastern North China Craton: Evidence from coexisting mantle- and crust-derived enclaves in a quartz monzonite pluton

The Laiwu quartz monzonite in the Luxi Block of eastern North China Craton (NCC) is characterized by the presence of abundant plagioclase amphibolite and gabbro–diorite enclaves. Here we present LA-ICPMS zircon U–Pb ages which show that the host quartz monzonite was emplaced at 129.8±1.0Ma, whereas the protolith of the plagioclase amphibolite enclaves formed during early Paleoproterozoic. The gabbro–diorite enclaves were produced simultaneously with or slightly earlier than the formation of the host quartz monzonite. Combined with the Archean and Paleoproterozoic zircons as well as the low εNd(0) values (−18.4 to −18.0) in the plagioclase amphibolite enclaves, the equilibrium temperature and pressure conditions (645–670°C and 4.8–6.5 Kb) suggest that the plagioclase amphibolite enclaves are fragments of the middle crust. The gabbro–diorite enclaves mainly originated from an enriched lithospheric mantle metasomatized by melts/fluids derived from the continental crust, as indicated by their low SiO2 (54.4–54.7wt.%) and high MgO (10.9–11.1wt.%) contents as well as the negative εNd(t) values (−13.5 to −10.7) and enrichment of LILEs (e.g., Ba and Sr) and depletion of HFSEs (e.g., Nb, Ta, P and Ti). Compared with the ancient crustal rocks and the mafic plutons considered to have been derived from lithospheric mantle in the Luxi Block, the moderate εNd(t) (−15.7 to −15.1) and εHf(t) (−20.7 to −13.0) values of the quartz monzonite in our study suggest that both mantle- and crust-derived melts were involved in the magma generation. Thus we propose a model involving magma mixing between mantle- and crust-derived melts for the formation of the quartz monzonite. Since significant crust–mantle interaction is recorded not only in the quartz monzonite and its enclaves in the Luxi Block but also in the other granitoids widespread in the NCC, it is considered that large-scale crust–mantle interaction and magmatic underplating were associated with the Mesozoic lithospheric mantle thinning and crustal reactivation in the eastern NCC.

Geochemical investigation on quartz-monzonite pluton of Tuye-darvar, in eastern Alborz structural Zone, Damghan, north-east of Iran

Geochemical investigation on quartz-monzonite pluton of Tuye-darvar, in eastern Alborz structural Zone, Damghan, north-east of Iran

Fluid Inclusion Investigations of the Raigan Porphyry Copper System, Kerman-Bam, Iran

The Raigan porphyry copper system is associated with diorite/granodiorite to quartz-monzonite plutons of Miocene age that intruded Eocene volcano-sedimentary rocks. Several stages of hydrothermal alteration and associated mineralization occurred within the system. The main intense hydrothermal alterations are of potassic, phyllic, and propylitic types, especially within and adjacent to the diorite/granodiorite intrusive stock. Copper mineralization was accompanied by mainly phyllic and less potassic alteration. Early hydrothermal alteration produced a potassic assemblage (orthoclase-biotite) in the central part of the stock, propylitic alteration occurred contemporaneously with potassic alteration in peripheral parts, and phyllic alteration occurred later, overprinting all the earlier alterations. The early hydrothermal fluids are represented by high-temperature (487°C to 598°C), high-salinity (up to 61.1 wt% NaCl equiv.) liquid-rich fluid inclusions, and high-temperature (397°C to 401°C), low-salinity, vapor-rich inclusions. These fluids are interpreted to represent a very hot orthomagmatic fluid that boiled episodically; the brines evidently caused potassic alteration, producing the first generation of chalcopyrite. Propylitic alteration is attributed to a liquid-rich, lower-temperature (523°C to 298°C), Ca-rich, evolved meteoric fluid. Based on this research, the temperature of mineralizing fluids was too high to produce an economic porphyry copper deposit at Raigan.

The Shahewan rapakivi-textured granite-quartz monzonite pluton, Qinling orogen, central China: Mineral composition and petrogenetic significance

WANG, XIAOXIA, WANG, TAO, HAAPALA, ILMARI and LU, XINXIANG 2002. The Shahewan rapakivi-textured granite – quartz monzonite pluton, Qinling orogen, central China: mineral composition and petrogenetic significance. Bulletin of the Geological Society of Finland 74, Parts 1–2, 133–146. The Mesozoic Shahewan pluton consists of four texturally different types of biotite-hornblende quartz monzonite. In the porphyritic types alkali feldspar occurs as euhedral or ovoidal megacrysts that are often mantled by one or more plagioclase shells, and as smaller grains in the groundmass. Quartz, plagioclase (An20–28), biotite, and hornblende occur as inclusions in the alkali feldspar megacrysts and, more abundantly, in the groundmass. Euhedral quartz crystals in the groundmass are not as common and well developed as in typical rapakivi granite. Compared to typical rapakivi granites, the mafic minerals (biotite and hornblende) are rich in Mg and poor in Fe, and the whole rock is low in Si, K, F, Ga, Zr, LREE, Fe/Mg, and K/Na. The rocks of the Shahewan pluton are thus regarded as rapakivi-textured quartz monzonites and granites but not true rapakivi granites.

The Donoso copper-rich, tourmaline-bearing breccia pipe in central Chile: petrologic, fluid inclusion and stable isotope evidence for an origin from magmatic fluids

The copper-rich, tourmaline-bearing Donoso breccia pipe is one among more than 15 different mineralized breccias in the giant (>50 million metric tonnes of copper) Miocene and Pliocene Rio Blanco-Los Bronces copper deposit in the high Andes of central Chile. This breccia pipe, bracketed in age between 5.2 and 4.9 Ma, has dimensions of 500 by 700 m at the current surface 3,670 m above sea level. Its roots have yet to be encountered, and it is >300 m in diameter at the depth of the deepest drill holes. The Donoso breccia is, for the most part, monolithic, containing clasts of the equigranular quartz monzonite pluton which hosts the pipe. It is matrix supported, with between 5 and 25% of the total rock volume consisting of breccia-matrix minerals, which include tourmaline, quartz, chalcopyrite, pyrite, specularite, and lesser amounts of bornite and anhydrite. An open pit mine, centered on this breccia pipe, has a current production of 50,000 tonnes of ore per day at an average grade of 1.2% copper, and copper grade in the breccia matrix is significantly higher. Measured δ18O for tourmaline and quartz from the matrix of the Donoso breccia at different levels of the pipe range from +6.9 to +12.0‰, and measured δD in tourmaline ranges from –73 to –95‰. Temperatures of crystallization of these minerals, as determined by the highest homogenization temperatures of highly saline fluid inclusions, range from 400 to >690°C. When corrected for these temperatures, the stable isotope data indicate that fluids from which these breccia-matrix minerals precipitated were magmatic, with δ18O between +5.6 to +9.1‰ and δD between –51 to –80‰. These isotopic data preclude participation of a significant amount of meteoric water in the formation of the Donoso breccia. They support a model in which brecciation is caused by expansion of magmatic fluids exsolved from a cooling pluton, and breccia-matrix minerals, including copper sulfides, precipitated from the same magmatic fluids responsible for brecciation. Sericitic alteration of clasts in the breccia was also caused by these magmatic fluids. Different types of fluid inclusions imply that several different magmatic fluids were involved in formation of the Donoso breccia. These include high-temperature, highly saline, non-boiling fluids, trapped in inclusions that homogenize by halite dissolution, which probably exsolved from a magma cooling under relatively high (>1 kbar) lithostatic pressure conditions, consistent with geologic constraints. Other high-temperature, highly saline fluids are trapped in inclusions that homogenize by vapor-bubble disappearance and are spatially associated with vapor-rich inclusions, suggesting either phase separation (boiling) or simultaneous separation of immiscible brine and vapor from a magma cooling at lower hydrostatic pressure conditions. Both types of high-temperature, highly saline fluids circulated intermittently, as pressure fluctuated between lithostatic and hydrostatic conditions because of episodes of sealing and rebrecciation.

Pan-African, post-collisional, ferro-potassic granite and quartz–monzonite plutons of Eastern Nigeria

Three Pan-African hypersthene-bearing monzogranitic and quartz–monzonitic plutons from the Eastern terrane of Nigeria have been investigated in detail. New major, trace and REE data, used to constrain their origin and nature, indicate that they display chemical features of ferro-potassic trans-alkaline affinity. Further trace element discrimination suggests (i) production of calc-alkaline medium-K diorite magmas by partial melting of fluid-metasomatised mantle wedge possibly combined with melts from the dehydration partial melting of altered oceanic crust; (ii) simultaneously production of the granite–quartz–monzonite ferro-potassic magmas from partial melting of hornblende-bearing granodioritic crustal sources; (iii) mixing of the two magmas. Sr initial ratios of 0.707 to 0.711 witness that the source of the granite magmas is the lower crust. Ages of the lower crustal granulitic protoliths is bracketed by Nd model ages between 1.9 and 2.2 Ga. Pb evaporation ages on single zircons constrain the emplacement of the three plutons around 580 Ma. 40 Ar/ 39 Ar ages of amphiboles at about 560 Ma suggest cooling rates around 15°C/Ma. Extensive field work has established that pluton emplacement occurred during a regional north–south dextral strike-slip tectonics following the 630–610 Ma stage of oblique continent–continent collision in this part of west Africa.

Jurassic magmatism in the central Mojave Desert: Implications for arc paleogeography and preservation of continental volcanic sequences

Research Article| June 01, 1994 Jurassic magmatism in the central Mojave Desert: Implications for arc paleogeography and preservation of continental volcanic sequences ELIZABETH R. SCHERMER; ELIZABETH R. SCHERMER 1Department of Geology, Western Washington University, Bellingham, Washington 98225 Search for other works by this author on: GSW Google Scholar CATHY BUSBY CATHY BUSBY 2Department of Geological Sciences, University of California, Santa Barbara, California, 93106 Search for other works by this author on: GSW Google Scholar Author and Article Information ELIZABETH R. SCHERMER 1Department of Geology, Western Washington University, Bellingham, Washington 98225 CATHY BUSBY 2Department of Geological Sciences, University of California, Santa Barbara, California, 93106 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1994) 106 (6): 767–790. https://doi.org/10.1130/0016-7606(1994)106<0767:JMITCM>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ELIZABETH R. SCHERMER, CATHY BUSBY; Jurassic magmatism in the central Mojave Desert: Implications for arc paleogeography and preservation of continental volcanic sequences. GSA Bulletin 1994;; 106 (6): 767–790. doi: https://doi.org/10.1130/0016-7606(1994)106<0767:JMITCM>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Exposures of Jurassic magmatic rocks in the west-central Mojave Desert provide insight into the changing character of volcanism throughout Jurassic time and the paleogeography of the continental arc. Middle Jurassic explosive volcanism (Lower Sidewinder volcanic series) resulted in collapse of multiple calderas. This was followed by north-south extension broadly coeval with batholith emplacement. Late Jurassic effusive volcanism (Upper Sidewinder volcanic series) appears to reflect transtension over a larger region during intrusion of the Independence dike swarm.The Lower Sidewinder volcanic series consists of a subaerial, nested caldera complex with an aggregate thickness of >4 km. The first caldera formed during the eruption of crystal-poor rhyolite ignimbrite. Outflow and intracaldera facies are intercalated with quartzose sandstone of probable cratonal provenance. The second caldera formed during the eruption of crystal- rich rhyolite to dacite ignimbrite. This ignimbrite exhibits complex mineralogical zoning and contains two units of probable collapse-related mesobreccia within the 1,400-m-thick intracaldera sequence. A third caldera, nested within the second, is filled with crystal-rich biotite-dacite ignimbrite and tuff breccia. A fourth caldera, largely coincident with the second, formed during eruption of lithic- and pumice-lapilli dacite ignimbrite. Eruption and collapse appear to have been multistage, as several units of reworked tuff and fallout tuff occur within the 1,750-m-thick intracaldera sequence, and caldera collapse breccias occur at the base and near the middle of the sequence. The top of this sequence contains reworked tuffs and epiclastic rocks, suggesting that the fourth caldera provided a posteruptive depocenter for accumulation of sediment.Normal faulting, tilting, and erosion followed explosive volcanism and was broadly contemporaneous with intrusion of Middle Jurassic porphyritic quartz monzonite plutons. Plutons and tilted Lower Sidewinder volcanic series are intruded and unconformably over-lain by latest Jurassic volcanic rocks (Upper Sidewinder volcanic series). The Upper Sidewinder volcanic series is characterized by alkalic basalt to basaltic andesite and rhyolite lavas, hypabyssal intrusions, and dikes, including the 148 Ma Independence dike swarm. Regional northeast-southwest extension is suggested by the presence of the northwest-striking dike swarm and by bimodal volcanism. The absence of debris-flow deposits and epiclastic rocks suggests an intra-arc region of low relief.Although local caldera-related subsidence was the principal control on accumulation of Lower Sidewinder volcanic rocks, preservation was likely enhanced by Middle Jurassic extension. Late Jurassic extension was of broader regional extent but lesser magnitude because it did not create basins or cause significant tilting of strata. The geometry, timing,and regional setting of Middle and Late Jurassic extension and magmatism suggest a sinistral oblique subduction regime for the Jurassic arc of the southern U.S. Cordillera. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Isotopic and chemical constraints on the petrogenesis of Blackburn Hills volcanic field, western Alaska

The Blackburn Hills volcanic field is one of several Late Cretaceous and early Tertiary (75–50 Ma) volcanic fields in western Alaska that comprise a vast magmatic province extending from the Arctic Circle to Bristol Bay. It consists of andesite flows, rhyolite domes, a central granodiorite to quartz monzonite pluton, and small intrusive rhyolite porphyries, overlain by basalt and alkali-rhyolites. Most of the field consists of andesite flows which can be divided into two groups on the basis of elemental and isotopic composition: a group having lower (87Sr86Sr)i, higher (143Nd144Nd)i, and moderate LREE and HREE contents (group 1), and a group having higher (87Sr86Sr)i, lower (143Sr144Sr)i, and lower HREE contents. Basalts are restricted to the top of the stratigraphic section, comprise the most primitive part of group 1 [(87Sr86Sr)i = 0.7033; (143Nd144Nd)i = 0.5129], and have trace-element ratios that are similar to those of oceanic island basalts (OIBs). In contrast to the basalts, group 1 andesites have higher (87Sr86Sr)i and lower (143Nd144Nd)i, and represent interaction of mantle-derived magmas with the lower crust of Koyukuk terrane. Group 2 andesites have (87Sr86Sr)i and (143Nd144Nd)i that are near bulk-earth values and probably formed by partial melting of the lower crust of Koyukuk terrane. The central pluton and rhyolite porphyries are isotopically uniform (87Sr86Sr)i ≈ 0.704, (143Nd144Nd)i ≈ 0.51275, and are interpreted to have formed by melting of young mafic to intermediate crustal rocks or by fractionation of group 1 andesites. The rhyolite domes have an isotopic range similar to that of the basalts and andesites [(87Sr86Sr)i = 0.70355-0.70499; (143Nd144Nd)i = 0.51263–0.51292], which suggests they formed by fractionation of the and site and basalt magmas. Although some workers have suggested that the volcanic field is underlain by old continental crust, none of the data require the presence of Paleozoic or Precambrian continental middle or upper crust under this part of the volcanic field. However, the ultimate source of some of the rocks in the Yukon-Koyukuk province that have high 87Sr86Sr and low 143Nd144Nd ratios may be old sub-continental mantle and/or lower crust, which was previously subducted beneath the Yukon-Koyukuk province during Early Cretaceous arc-continent collision.

Zoning and genesis of the Darwin Pb-Zn-Ag skarn deposit, California; a reinterpretation based on new data

The > 1-million-metric-ton Darwin Pb-Zn-Ag-W skarn deposit has been previously described as a group of sulfide replacement bodies zoned away from the Darwin quartz monzonite pluton and formed from magmatic fluids at 325oC. Detailed surface mapping and available radiometric data, however, indicate that the Pb-Zn skarn sulfide bodies are appreciably (>20 Ma) younger than the Darwin pluton, and underground mapping and core logging indicate there are several skarn sulfide pipes with strong concentric zoning. One of the pipes is zoned around a deep granite porphyry plug. The pipes exhibit outward zoning in wt percent Pb/Zn and oz/ton Ag/wt percent Pb (both ratios 1.0 margin). The pipes show mineralogical zoning, with a core defined by higher sphalerite-galena, higher chalcopyrite, darker sphalerite, more abundant pyrite inclusions in sphalerite, and evidence for multiple sulfide depositional events. In contrast, both graphite in marble and pyrrhotite in sulfide ores are zoned around the Darwin pluton, which suggests that pyrrhotite stability is influenced by prePb-Zn skarn (Darwin pluton related?) bleaching of marble beds. Garnet zoning is highly complex, with four generations identified by petrographic and compositional relations; younger garnet types are more abundant in upper and lateral parts of the pipe. Retrograde alteration of garnet is concentrated in the upper and laterally distal parts of the skarn, but garnet in apparent equilibrium with sulfide is present throughout the vertical extent of skarn. Systematic mineral compositional patterns include outward increase in hedenbergite + johannsenite components in clinopyroxene ( 20 outward), increase in Sb -I- Bi contents of galena, initial increase followed by decrease in Mn contents of sphalerite (range from 1% Mn), and an initial increase followed by outward variable increase and decrease in FeS contents of sphalerite (range of 20% FeS). Previously published sulfur isotope data are compatible with a decrease in sulfur isotope ratios outward around the pipe core. Published isotopic data combined with temperature estimates from phase homogenization and arsenopyrite-sphalerite geothermometry show a systematic decrease in temperature from the skarn sulfide pipe center (>425oC) to the margin (<300oC). Comparison of stope maps to isotherm cross sections indicates that the bulk of mined sulfides were from areas surrounding the pipe core, in which temperatures declined from approximately 375 o to 300oC (gradient of loC/m). Combined mineral composition and assemblage and sulfur isotope systematics indicate that as the ore fluids flowed outward they underwent progressive decrease in oxidation state (''1 log unit) and increase in pH (2-3 units); upwardmoving fluids underwent initial decrease in oxidation state and increase in pH followed by a reversal to higher oxidation state and lower pH. The process of ore deposition was chemically complex and may have involved remobilization of earlier deposited sulfides. Realistic ore depositional models at Darwin require simultaneous changes in (at least) temperature, pH, and oxidation state.

Tectonic evolution of the Meatiq infrastructure, Central Eastern Desert, Egypt

The Pan‐African basement exposed in the Meatiq area, Central Eastern Desert, Egypt, was found to possess tectonic styles belonging to infrastructure, transition and super‐structure levels. Tectonic style of the infrastructure was developed during an old orogeny in the Lower Pan‐African, and overprinted by those of the transition and superstructure levels of a younger orogeny in the Middle and Upper Pan‐African, respectively. In the Lower Pan‐African, old sequences of sediments and volcanics, together with ophiolitic rocks, were intensely deformed and regionally highly metamorphosed into gneisses and amphibolites. In the infrastructure level such rocks were migmatized, plastically deformed and intruded by synkinematic granitoid (diorite to alkali granite) lenses, sheets and plutons which were probably crystallized as amphibolite‐facies gneisses. Later on, these infrastructural rocks were cut by mafic to intermediate dykes and sills, isostically uplifted and cratonized. Parts were exposed to erosion. In the Middle Pan‐African the cratonized rocks sank into a transition tectonic level where they underwent large‐scale thrusting, varying degrees of cataclasis and mylonitization, and low‐ to medium‐grade regional metamorphism. An episode of plutonism guided by thrusts followed, mainly in the form of synkinematic intrusion of tonalite, granodiorite and quartz monzodiorite. In the Upper Pan‐African, isostatic uplift occurred and the mylonitic rocks attained the superstructure level. There they were subjected to two main phases of folding resulting in the major Meatiq domal structure. Subsequently, the folded rocks were intruded by a postkinematic quartz monzonite pluton.

Paleozoic plutonism in southeastern British Columbia

U–Pb dates from zircons indicate that plutonic events occurred during the Paleozoic in the Omineca Crystalline Belt in southeastern British Columbia. In the Kootenay Arc, granitoid cobbles in conglomerate of the Carboniferous Milford Group were derived from quartz monzonite and diorite plutons of probable Ordovician age. Near Little Shuswap Lake, gneissic granitoid units have yielded Cambro-Ordovician ages. At least one episode of deformation affected country rocks of unknown age before intrusion. In the Monashee Complex south of Thor–Odin Nappe in South Fosthall Creek, lineated quartz monzonite is of probable Ordovician age. Comparison of fabrics suggests that at least one episode of metamorphism and deformation occurred prior to intrusion. No clear relationship between the cobbles and these plutons can be demonstrated because major faults lie between them, but substantial revision to accepted models of Paleozoic paleogeography of this region will have to be made. In the Clachnaeudainn tectonic slice east of the Monashee Complex, granitic gneiss is of Paleozoic, possibly Siluro-Devonian, age. This pluton appears to be involved in all phases of deformation that affected its country rocks. Near Quesnel Lake, parts of a composite gneissic granitoid pluton appear to be of Devonian or earliest Carboniferous age.

High- 18O granitic plutons from the Frontenac Axis, Grenville province of ontario, Canada

18O 16O ratios have been determined for whole rock as well as coexisting minerals from nine syn- to post-tectonic monzonite and quartz monzonite plutons in the Westport-Gananoque area covering approximately 2500 km 2. The plutons fall in two distinct isotopic groups: 1. (I) Low- 18O group ( δ = 8.0–11.2): Rideau Lake, Westport and Wolfe Lake plutons and 2. (II) High- 18O group ( δ = 12.7–16.2): Lyndhurst, Perth Road, Battersea, Gananoque, South Lake and Crow Lake plutons. The 18O 16O ratios of granitic rocks from the latter group are among the highest ever reported in the literature. The isotopic compositions of samples within each pluton bear no relationship to the sample distance from the intrusive contacts nor are they affected by the types of country rocks (marbles vs. pelitic gneisses) into which they are emplaced. Oxygen isotope fractionations among coexisting quartz, feldspar, biotite and magnetite all show normal plutonic values. Thus, all lines of evidence suggest that the monzonite and quartz monzonite plutons did not acquire their characteristic 18O 16O ratios at the present level of exposure, either by isotopic exchange with the surrounding metasedimentary country rocks or by any post-crystallization low-temperature alteration processes. The distinct 18 O/ 16 O ratios observed in the high- and low- 18O groups probably reflect the difference in the isotopic compositions of the source materials. The most plausible source for the high- 18O group plutons is the Grenville Supergroup metasedimentary rocks and that for the low- 18O group plutons the Grenville basement gneisses mixed with a small amount of Grenville Supergroup metasedimentary rocks.

Folded cauldrons of the Early Proterozoic Labine Group, northwestern Canadian Shield

Great Bear Magmatic Zone (1.875–1.84 Ga) is a linear belt, 100 km wide by 800 km long, of little metamorphosed volcanic rocks and allied plutons of calc‐alkaline affinity. The entire zone was folded shortly after magmatism, and sections many kilometers thick through several cauldron complexes are exposed. Eruptions of rhyolite led to collapse of Black Bear Cauldron in which 1.5 km of tuff ponded. The cauldron then became the locus of fluvial and lacustrine sedimentation followed by augite‐plagioclase porphyritic andesite volcanism, both of which also accumulated within the cauldron. Ash flow eruptions of dacite caused collapse of Clut Cauldron which was accompanied by landsliding and avalanching of the cauldron walls. Abundant andesitic and intrusive debris is intercalated with the propylitized intracauldron facies tuff adjacent to the walls and some blocks are as large as 1 km across. Clut Cauldron also became the site for fluvio‐lacustrine sedimentation after collapse and was likely resurgent. Resurgence was probably related to the emplacement of a quartz monzonite pluton which occupies the core of the cauldron. Cornwall Cauldron, which is exposed entirely in cross section, contains 1–2 km of intracauldron facies tuff overlain by a complex of dacite lava flows. The cauldron was intruded by a granodiorite‐monzogranite pluton probably responsible for resurgence. Data from the area suggest the following: (1) the similarity of early Proterozoic cauldrons to Cenozoic examples indicates that cauldron collapse is a process that has occurred since 1.9 Ga; (2) batholiths beneath ash flow tuff fields are probably composite bodies made up of many individual plutons; (3) most of the plutons are sheetlike in cross section; and (4) cauldron collapse begins as soon as ash flow eruptions, and relief on the cauldron margin becomes large early in the subsidence history.

Geology of the area of induced seismic activity at Monticello Reservoir, South Carolina

This study provides geological background information necessary for an evaluation of the earthquake hazard in an area of induced seismic activity at Monticello Reservoir, South Carolina. This region contains a thick stratified sequence of Proterozoic Z and Cambrian metasedimentary and metavolcanic rocks. In the early to middle Paleozoic, this sequence was recrystallized and deformed under metamorphic conditions that ranged from greenschist to amphibolite facies and experienced at least two episodes of folding. The region has been intruded by late kinematic to postkinematic granitoid plutons of Silurian and Carboniferous ages and by numerous northwest trending diabase dikes of Late Triassic and Early Jurassic age. The region south of Monticello Reservoir in the Carolina slate belt experienced two episodes of faulting in the late Paleozoic and/or early to middle Mesozoic. The older group of faults trends approximately east, has only small displacements, and is characterized by extensive silicification of the fault zones. The younger group of faults trends approximately north, has experienced dip slip displacements up to 1700 m, and is characterized by carbonate mineralization in the fault zones. Both sets of faults are cut by an undeformed diabase dike of Late Triassic or Early Jurassic age. The induced seismic activity around Monticello Reservoir is occurring in a heterogeneous quartz monzonite pluton of Carboniferous age. Although laterally extensive faults have not been found in the vicinity of the reservoir, the pluton contains large enclaves of country rock and is cut by numerous, diversely oriented small faults and joints. These local inhomogeneities in the pluton, together with an irregular stress field, are interpreted to control the diffuse seismic activity around the reservoir. In view of the apparent absence of lengthy faults, it is unlikely that a large‐magnitude earthquake will occur in response to the stress and pore pressure changes related to the impoundment of Monticello Reservoir.

Tungsten-bearing skarns of the Sierra Nevada; I, The Pine Creek Mine, California

The Pine Creek mine, located 27 km west of Bishop, California, contains scheelite-bearing skarns found at and near the contacts between a septum of Paleozoic metasedimentary rocks and a mid-Cretaceous quartz monzonite pluton. Underground and surface mapping and drill core logging have suggested that skarn formation and accompanying metal deposition should be characterized as a series of events over a range in temperatures overprinting several different rock types.Metamorphism caused the formation of barren, iron-poor calc-silicate marbles and hornfelses, which were subsequently replaced by iron-rich calc-silicate and ore minerals during metasomatism. High-temperature metasomatism was characterized by the formation of garnet-quartz, garnet-pyroxene, and idocrase-wollastonite-pyroxene zones from marble; by pyroxene-plagioclase + or - garnet skarns from biotite and calc-silicate hornfels; and by pyroxene-plagioclase-quartz rock from quartz monzonite. These high-temperature assemblages were locally over-printed by hydrosilicate alteration characterized by biotiteoplagioclase-magnetite replacement of garnet, amphibole-quartz-chalcopyrite + or - calcite (epidote) replacement of garnet-pyroxene (plagioclase) skarn, and calcite-quartz-epidote-chlorite-fiuorite replacement of wollastonite-idocrase. Contemporaneous alteration of quartz monzonite comprised quartz-biotite-chalco-pyrite veins and locally pervasive biotitization. The latest, lowest temperature alteration con-sisted of vertical pipes of quartz-calcite-zeolite, which cut and altered all other rock types.Patterns of ore deposition were intimately tied to original lithologies and alteration types. Initial scheelite deposition accompanied high-temperature metasomatism of marble but not of hornfels or quartz monzonite. In the early-formed skarn, scheelite is concentrated toward the marble replacement front and depleted away from marble. Molybdenite is concentrated in quartz-rich masses near intrusive contacts and in quartz-garnet veins cutting across metaso-matized hornfels. Scheelite was remobilized and deposited in association with hydrosilicate alteration of skarn and hornfels. Copper was deposited with hydrosilicate alteration, particularly in association with amphibole-quartz calcite replacement of garnet-pyroxene skarn. The sequence of events inferred at Pine Creek appears to be similar to those found in other tungsten skarns from around the world.