Les Vosges sont caractérisées par la présence de nombreuses intrusions et extrusions magmatiques d’affinités variées. Elles constituent donc un excellent site d’étude pour contraindre, par la datation et la géochimie, l’évolution des évènements de ce segment de l’orogène Varisque. Ce travail révèle ainsi deux successions d’évènements magmatiques identiques, décalées dans le temps, caractérisent les domaines Moldanubien (360 à 320 Ma) et Saxothuringien (335 à 295 Ma). Ces successions d’évènements magmatiques résultent de deux processus majeurs. L’avancée des croûtes continentales subduites et sous-plaquées au niveau du Moho sous les blocs continentaux permet le passage du magmatisme calco-alcalin au magmatisme calco-alcalin riche en potassium. L’apport de chaleur par désintégration des éléments radiogéniques (K, U et Th) présents dans ces croûtes continentales subduites permet, dans un premier temps, la formation du magmatisme magnésio-potassique en profondeur. Dans un second temps, elle permet la formation du magmatisme d’origine crustale par l’intrusion du magmatisme magnésio-potassique, riche en K, U et Th, à la limite croûte moyenne - croûte supérieure. Ces successions d’évènements magmatiques et particulièrement, la présence des granites magnésio-potassiques, relient clairement les Vosges à la partie Est de l’orogène Varisque (Forêt Noire, Massif de Bohème, Alpes et Corse-Sardaigne). / The Vosges Mountains are characterized by the presence of numerous magmatic intrusions and extrusions of varied affinities. Accordingly, they constitute the best site to investigate, by dating and geochemistry, the evolution of the events affecting this segment of the Variscan orogeny. Two successions of identical magmatic events, shifted in the time, are identified, characterizing both Moldanubian (360 to 320 Ma) and the Saxothuringian (335 to 295 Ma) domains. These successions of magmatic events result of two major process. The progress of subducted and underplated continental crusts at Moho depth under continental blocks permits to shift from calc-alkaline to high potassic calc-alkaline magmatism. The radiogenic heat production from latter underplated continental crusts, in a first time, permits to generate magnesio-potassic magmas at depth. Then, this radiogenic heat permits to generate crustal magmas by intrusion of magnesio-potassic magmas rich in K, U and Th at mid-upper crust boundarie. These successions of magmatic events and particularly, the presence of the magnesio-potassic granites, imply a strong link between the Vosges Mts. and the eastern part of the Variscan orogeny (Black Forest, Bohemian Massif, the Alps and Corsica Batholith).

Submitted by Jordan (jordanbiblio@gmail.com) on 2016-10-20T13:16:35Z No. of bitstreams: 1 DISS_2015_Luzia Helena Siqueira.pdf: 3499968 bytes, checksum: 289d2c36c60121c189c7a71a65733fce (MD5) / Approved for entry into archive by Jordan (jordanbiblio@gmail.com) on 2016-10-20T13:16:58Z (GMT) No. of bitstreams: 1 DISS_2015_Luzia Helena Siqueira.pdf: 3499968 bytes, checksum: 289d2c36c60121c189c7a71a65733fce (MD5) / Made available in DSpace on 2016-10-20T13:16:59Z (GMT). No. of bitstreams: 1 DISS_2015_Luzia Helena Siqueira.pdf: 3499968 bytes, checksum: 289d2c36c60121c189c7a71a65733fce (MD5) Previous issue date: 2015-07-31 / O Granito São Domingos corresponde a um dos corpos da Suíte Intrusiva Guapé, localizado na Faixa Móvel Aguapeí, relacionado à Orogenia Sunsás, SW do Cráton Amazônico. Trata-se de um corpo com dimensões batolíticas de 150 Km² de área aflorante, levemente alongado segundo direção NE e localizado ao norte do distrito São Domingos, município de Jaurú, estado de Mato Grosso. Constitui-se de rochas holo a leucocráticas, de cor rosa-claro a cinza-rosado, isotrópicas, equi a inequigranulares, por vezes, porfiríticas e pegmatíticas, classificadas como Muscovita biotita monzo a sienogranitos tendo por vezes, granada e monazita como minerais acessórios primários e caracterizadas como granitos do tipo S ou Muscovite bearing Peraluminous Granitoids (MPG). Essas rochas apresentam restritos e elevados teores de sílica, caracterizando-as como muito evoluídas; formadas por magmatismo cálcio alcalino de alto K a shoshonítico, peraluminoso e ferroso. A idade U-Pb (SHRIMP) de 928 ± 5 Ma foi obtida em zircões ígneos, e coincide com idades U-Pb (TIMS) relatadas para este granito. A análise Sm-Nd indica uma idade modelo TDM de 1,58 Ga, e valor ɛND(0,93Ga) negativo (-2,90). Esses resultados indicam que o Granito São Domingos formou-se em um ambiente pós-tectônico, no final da Orogenia Sunsás, cuja origem magmática está associada ao retrabalhamento de crosta continental mesoproterozoica. Três padrões diferentes de ETR foram encontrados para esses litotipos, sugerindo a geração de magmas contemporâneos não cogenéticos, provenientes de fontes crustais distintas. / The São Domingos Granite is an intrusive body of the Guapé Intrusive Suite, located in the Aguapeí mobile belt, corresponding to a branch of the Sunsás Orogeny in SW Amazonian Craton. This body is considered as a batholith slightly elongated in the NE direction, which crops out over an area of ca. 150 km2. It is situated to the north of the São Domingos District, a municipality of the Jauru city, Mato Grosso State. It consists of hololeucocratic to leucocratic rocks ranging from pinky to pinky-gray. They are isotropic, ranging from equigranular to inequigranular grains, sometimes porphyritic and pegmatitic, classified as muscovite-biotite monzo to syenogranites. Sometimes they present garnet and monazite as primary accessory minerals. These features characterize them as S-type granites or Muscovite bearing Peraluminous Granitoides (MPG). The rocks contain high silica content, which characterizes them as very evolved, formed by high-K to shoshonitic, peraluminous, and ferrous calc-alkaline magmatism. A U-Pb age of 928 ± 5 Ma was obtained for one of the analyzed rocks, which agrees with previous U-Pb ages obtained for this granite. Sm-Nd analysis indicates a TDM model age of 1.58 Ga, and negative ND value (-2.90). These results demonstrate that the São Domingos intrusion corresponds to a post tectonic environment, related to the Sunsás orogeny, whose magmatic origin is associated to re-working of the ancient continental crust. Moreover, three different ETR patterns were found for these lithotipes, suggesting the generation of contemporaneous non-cogenetic magmas, involving distinct crustal sources.

From Introduction: The Canadian Cordilleran Orogenic Belt forms part of the circum-Pacific orogenic zone. It underlies an area of about 1,54 million sq. kilometres, is over 2400 kilometres long and 800 kilometres wide. The region is characteristically mountainous, much of it glaciated and alpine, containing plateaux, trenches, valleys, and fjords. The mountains, in general, rise to elevations between 2100 m and 3600 m above sea level, although Mount Logan in the St. Elias Mountains attains an altitude of 6000 m. The Canadian Cordillera is divided into two dominant orogenic belts: the eastern Columbian Orogenic Belt comprising defonned miogeosynclinal rocks and the western Pacific Orogenic Belt comprising allochthonous eugeosynclinal rocks. The Cordillera is further subdivided into five longitudinal tectonic belts within which rocks are broadly similar in type, age, and history. These belts are, from east to west: the Rocky Mountain Belt, the Omineca Crystalline Belt, the Intermontane Belt, the Coast Plutonic Complex, and the Insular Belt (Wheeler et al., 1972a). The Canadian Cordillera is important in that it contains: one of the world's largest lead-zinc-silver mine, Sullivan; the second-largest molybdenum mine, Endako; one of the most important concentrations of porphyry copper deposits, Highland Valley; Canada's largest tungsten mines, Cantung and Mactung; and Canada's second-largest silver district, Keno Hill (Sutherland Brown et a1., 1971). In addition, it contains several large massive sulphide and lead-zinc deposits.

Orientador: Elson Paiva de Oliveira / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Geociências / Made available in DSpace on 2018-08-26T10:50:20Z (GMT). No. of bitstreams: 1 Baldim_MauricioRigoni_M.pdf: 19163752 bytes, checksum: da3d83e0ed883c0984d79ed906f0528c (MD5) Previous issue date: 2014 / Resumo: Domos gnáissicos são estruturas que podem estar associadas tanto aos orógenos extensionais quanto aos colisionais. Em orógenos colisionais, normalmente balizam os distintos terrenos dispondo-se em corredores estruturais. Na região nordeste do Cráton São Francisco, Bloco Serrinha, localiza-se o Greenstone Belt Paleoproterozoico do Rio Itapicuru, interpretado como arco continental acrecionado a um Complexo de alto grau mesoarqueano. Mapeamento geológico realizado no segmento norte da transiçao, embasamento-greenstone, revelou a ocorrência de um domo gnáissico-migmatítico que limita dois terrenos, um arqueano e outro paleoproterozoico, que destoa litoestruturalmente de outros domos reconhecidos a sul do greenstone (e.g. domos do Ambrósio, Salgadália e Pedra Alta). Além disso, dados estruturais mostram que a evolução tectônica da área ocorreu a partir de tectônica compressiva em D1 com direção E-W, seguido de transcorrência N-S em D2, possivelmente associados a transpressão. O domo, denominado Alto Alegre, possui núcleo granito-diatexítico, sendo delineado por faixas anfibolíticas concêntricas e preserva paragênese de alto grau metamórfico. Análises de elementos maiores e traços revelam que as faixas de anfibolitos do referido domo possuem características geoquímicas semelhantes aos diques máficos que cortam o embasamento, e destoam dos basaltos toleíticos do greenstone belt. Dados geocronológicos e de campo revelam idades de ca. 3080 Ma para o embasamento arqueano e para gnaisses do domo Alto Alegre, e idades de ca. 2080 Ma para o granito que intrude a porção central do domo. Os dados mostram que o domo Alto Alegre representa o embasamento arqueano retrabalhado tectonicamente e influenciado por atividade granítica, durante colisão continente-continente em ca. 2080 Ma / Abstract: Gneiss domes are structures that may be associated with both extensional and collisional orogens. In collisional orogens typically delimit distinct land forming structural corridors. In northeastern of São Francisco craton, Serrinha Block, is located the Paleoproterozoic Rio Itapicuru Greenstone Belt which is interpreted as a continental arc acrecionado to a Mesoarqueano high degree Complex. Geological mapping carried out in the northern segment of the greenstone-basement transition, revealed the occurrence of a gneissic-migmatitic dome that limits two lands, one Archean and another Paleoproterozoic. This dome is different both on litology as structuraly when comparing with other domes recognized in a south of the greenstone (e.g., domes of Ambrose, Salgadália and Pedra Alta). Furthermore, structural data show that the tectonic evolution of the area occurred from compressive tectonics E-W in D1, followed by transcurrent N-S in D2, possibly associated with transpression. The dome, called Alto Alegre, has granite-diatexítico core being outlined by concentric amphibolitic bands that preserves high metamorphic grade paragenesis. Results of major and trace elements analyzes reveal that the amphibolites bands of dome has geochemical characteristics similar to mafic dikes that cut the basement, and differ from Rio Itapicuru greenstone belt basalts. Geochronological and field data reveal ages ca. 3080 Ma for the Archean basement and the dome Alto Alegre gneisses, and ages of ca 2080 Ma for the granite that intrude the central portion of the dome. The data show that the dome Alto Alegre represents the tectonically reworked Archean basement and influenced by granite activity during continent-continent collision at ca 2080 Ma / Mestrado / Geologia e Recursos Naturais / Mestre em Geociências

>Magister Scientiae - MSc / The Saldania Belt in southern Africa, a product of the Pan-African Saldanian Orogeny, forms part of a system of Neoproterozoic mobile belts that border and weld older cratons on the African continent. It is a low-grade orogenic belt situated along the southwestern margin of the Kalahari Craton and is composed of several inliers of greenschist facies metasedimentary and metavolcanic rocks (Malmesbury Group), unroofed in megaanticlinal hinges of the Permo-Triassic Cape Fold Belt. The Malmesbury Group rocks were syn- and post-tectonically intruded in a pervasive transpressive regime between 555 Ma and 515 Ma by Neoproterozoic to early Cambrian S-, I- and A-type granites, monzodiorites, gabbros and quartz syenites, which collectively constitute the rocks of the Cape Granite Suite (CGS). Along the south-western coastline of South Africa, the Saldanha Bay Volcanic Complex (which forms part of the CGS) is divided into two eruption centres both of which have been identified as “intra-caldera pyroclastic ignimbrites”. The Postberg eruption centre is situated to the south of the Saldanha Bay entrance and the Saldanha eruption centre is situated to the north of the entrance. Both eruption centres display distinct geochemical signatures, the most apparent being the greater TiO2 concentrations (> 0.25 wt. %) of the Saldanha centre ignimbrites when compared to its Postberg centre counterparts. The Postberg eruption centre consists of S-type rhyolitic ignimbrites which are subdivided into the two geochemically distinct Plankiesbaai and Tsaarsbank Ignimbrites. Small amounts of the Jacobs Bay and Saldanha Ignimbrites (less felsic tephra from the Saldanha eruption centre) are also present in the Postberg eruption centre. A robust geochemical analysis of both the Plankiesbaai and Tsaarsbank magma groups display high SiO2 content (>76 wt. %), a lack of variation in TiO2 and Zr, high Al2O3 and ASI (aluminium saturation index) values (> 1.0 and generally >1.1 which, on average, is higher than the Saldanha eruption centre ignimbrites), low CaO and Na2O, and a highly ferroan character. The Plankiesbaai ignimbrite also display lower #Mg concentration compared to the Tsaarsbank ignimbrite. Typical geochemical trends in the Postberg eruption centre include the lack of variation in Zr content, higher Rb content and lower Sr, Ba, V and Zn concentrations when compared to the tephra of the Saldanha eruption centre found in the Postberg area. The study’s main aim is not only to assess the geochemistry of the ignimbrites relative to the previous phases of magmatism originally proposed by Scheepers (1995) for the magmatism of the Cape Granite Suite, but also their age distribution. Previously defined phases of magmatism include Phase I (S-type granites subdivided into Sb, Sa1 and Sa2 all of which are dated to 555 - 540 Ma), Phase II (I-type granites subdivided into Ia and Ib both dated to 540 – 520 Ma), Phase III (A-type granites subdivided into Aa and Ab dated to ~ 520 Ma) and Phase IV (S-type volcanic and subvolcanic rocks dated to 515 Ma). Re-examination of the geochronology displays a U-Pb age for Postberg Centre Jacobs Bay Ignimbrite (tephra from the Saldanha eruption centre) of 538 ± 2.2 Ma: and for the Postberg Centre Tsaarsbank Ignimbrite between 536 ± 2 Ma – 540 ± 3.4 Ma. These new dates, in combination with the geochronological work done in the Saldanha Centre (particularly in light of the Clemens and Stevens (2016) and Clemens et al. (2017) studies that reclassify these rocks differing from the original and previous studies), place all the ignimbrites of the Saldanha Bay Volcanic Complex securely within the age bracket for the initial S-type magmatism of the CGS. This thesis presents a revised order for the phases of magmatism of the Saldania Belt, and by extension, of the Cape Granite Suite. All S-type magmatism, including that of the Saldanha Bay Volcanic Complex (Sv), forms part of the Phase I magmatism of the Saldania Belt (Sa1, Sa2, and Sb) emplaced between 555 – 540 Ma.

>Magister Scientiae - MSc / The Saldania Belt in southern Africa, a product of the Pan-African Saldanian Orogeny, forms part of a system of Neoproterozoic mobile belts that border and weld older cratons on the African continent. It is a low-grade orogenic belt situated along the southwestern margin of the Kalahari Craton and is composed of several inliers of greenschist facies metasedimentary and metavolcanic rocks (Malmesbury Group), unroofed in megaanticlinal hinges of the Permo-Triassic Cape Fold Belt. The Malmesbury Group rocks were syn- and post-tectonically intruded in a pervasive transpressive regime between 555 Ma and 515 Ma by Neoproterozoic to early Cambrian S-, I- and A-type granites, monzodiorites, gabbros and quartz syenites, which collectively constitute the rocks of the Cape Granite Suite (CGS). Along the south-western coastline of South Africa, the Saldanha Bay Volcanic Complex (which forms part of the CGS) is divided into two eruption centres both of which have been identified as “intra-caldera pyroclastic ignimbrites”. The Postberg eruption centre is situated to the south of the Saldanha Bay entrance and the Saldanha eruption centre is situated to the north of the entrance. Both eruption centres display distinct geochemical signatures, the most apparent being the greater TiO2 concentrations (> 0.25 wt. %) of the Saldanha centre ignimbrites when compared to its Postberg centre counterparts. The Postberg eruption centre consists of S-type rhyolitic ignimbrites which are subdivided into the two geochemically distinct Plankiesbaai and Tsaarsbank Ignimbrites. Small amounts of the Jacobs Bay and Saldanha Ignimbrites (less felsic tephra from the Saldanha eruption centre) are also present in the Postberg eruption centre. A robust geochemical analysis of both the Plankiesbaai and Tsaarsbank magma groups display high SiO2 content (>76 wt. %), a lack of variation in TiO2 and Zr, high Al2O3 and ASI (aluminium saturation index) values (> 1.0 and generally >1.1 which, on average, is higher than the Saldanha eruption centre ignimbrites), low CaO and Na2O, and a highly ferroan character. The Plankiesbaai ignimbrite also display lower #Mg concentration compared to the Tsaarsbank ignimbrite. Typical geochemical trends in the Postberg eruption centre include the lack of variation in Zr content, higher Rb content and lower Sr, Ba, V and Zn concentrations when compared to the tephra of the Saldanha eruption centre found in the Postberg area.

Deformation of the Eastern Cordillera, as a double-verging thrust belt that separates the Magdalena Valley from the Llanos Basin, is a defining moment in the history of the northern Andes in South America. Here we examine the age and depositional setting of the youngest stratigraphic unit in three sectors of the Eastern Cordillera: (i) the Santa Teresa Formation (western flank), (ii) the Usme Formation (southern central axis), and (iii) the Concentración Formation (northeastern central axis). These units were deposited prior to the main Neogene deformation events. They represent the last preserved record of lowland conditions in the Eastern Cordillera, and they are coeval with a thick syn-orogenic deposition reported in the Llanos Basin and Magdalena Valley. Based on palynological data, we conclude that the upper Usme Formation was deposited during the Bartonian-earliest Rupelian? (Late Eocene-earliest Oligocene?); the Concentración Formation was deposited during the Late Lutetian-Early Rupelian (Middle Eocene to Early Oligocene), and the upper Santa Teresa Formation was accumulated during the Burdigalian (Early Miocene). These ages, together with considerations on maximum post-depositional burial, provide important time differences for the age of initial uplift and exhumation along the axial zone and western foothills of the Eastern Cordillera. The switch from sediment accumulation to erosion in the southern axial zone of the Eastern Cordillera occurred during the Rupelian-Early Chattian (Oligocene, ca 30 to ca 26 Ma), and in the northeastern axial zone occurred prior to the latest Chattian-Aquitanian (latest Oligocene-Early Miocene ca 23 Ma). In contrast, in the western flank, the switch occurred during the Tortonian (Late Miocene, ca 10 Ma). In addition, we detected a marine transgression affecting the Usme and Concentración formations during the Late Eocene; coeval marine transgression has been also documented in the Central Llanos Foothills and Llanos Basin, as evidenced by the similarity in floras, but not in the western foothills. Our dataset supports previous sedimentological, geochemical and thermochronological works, which indicated that (i) deformation in the Eastern Cordillera was a diachronous process, (ii) the sedimentation along the axial zone stopped first in the south and then in the north during the Oligocene, (iii) depositional systems of the axial zone and central Llanos Foothills kept partly connected at least until the Late Eocene, and (iv) Miocene strata were only recorded in adjacent foothills as well as the Magdalena and Llanos basins.

Mountain building is the result of large compressional forces in the Earth’s crust where two tectonic plates collide. This is why mountains only form at plate boundaries, of which the Al Hajar Mountains in Oman and the United Arab Emirates is thought to be an example of. These mountains have formed near the Arabian–Eurasian convergent plate boundary where continental collision began by 30 Ma at the earliest. However, the time at which the Al Hajar Mountains developed is less well constrained. Therefore, the timing of both the growth of the mountains, and the Arabian–Eurasian collision, needs to be understood first to be able to identify a correlation. Following this a causal link can be determined. Here we show, using apatite fission track and apatite and zircon (U-Th)/He dating, as well as stratigraphic constraints, that the Al Hajar Mountains were uplifted from 45 Ma to 15 Ma. We found that the mountains developed 33 Myr to 10 Myr earlier than the Arabian–Eurasian plate collision. Furthermore, the plate collision is ongoing, but the Al Hajar Mountains are tectonically quiescent. Our results indicate that the uplift of the Al Hajar Mountains cannot be correlated in time to the Arabian–Eurasian collision. Therefore the Al Hajar Mountains are not the result of this converging plate boundary.

This thesis presents a 40Ar/39Ar geochronological analyses of biotite from thin ductile shear zones in Paleoproterozoic granulite-facies gneisses from the Ellesmere-Devon crystalline terrane, Nunavut, Canada. The gneisses are part of the Paleoproterozoic Thelon tectonic zone. U-Pb dates of zircon show that the gneisses have magmatic protolith ages ranging from 2007–1958 Ma. The quartzofeldspathic gneisses in southeast Ellesmere Island display centimeter-scale E- to NE-striking sinistral and dextral mylonite zones offsetting pegmatitic dikes that are the last stage of ductile deformation of the basement rocks. Samples were taken from nearshore outcrops at Hayes Fiord, Pim Island, NE of the Leffert Glacier and NW of Cape Isabella. Biotite clusters replace orthopyroxene as the result of post-granulite facies metamorphism in the gneisses. Biotite in mylonitic and ultramylonitic fabrics is found as flattened clusters and also as individual crystals defining shear bands related to mylonitization. Eight samples were dated, including biotite from five mylonites, one deformed pegmatite, one tonalite and muscovite from a pegmatite. Major element X-ray maps demonstrate that the biotite is chemically homogenous. Backscattered electron images and electron dispersive spectroscopy via scanning electron microscopy confirm that biotite lacks intercrystalline layering with other K phases. Step-heating analysis of mica at the University of Vermont yielded Paleoproterozoic 40Ar/39Ar ages. The apparent age spectra form plateau ages in all but one mylonite sample. Biotite from a protomylonite was 2051 ± 26 Ma, older than the protolith ages obtained from U-Pb zircon geochronology, and most likely indicates excess Ar. Pegmatitic muscovite was 1977 ± 35 Ma. Biotite dates range from 1874 ± 13 Ma to 1838 ± 14 Ma for the five mylonites without excess Ar. Biotite dated from ductile shear zones signals the latest deformation in the basement, which was active as early as 1887 Ma.

The mineral resources of the Tintic Mining District are influenced by three major events in its geologic history; the Mesozoic Sevier Orogeny, Paleogene volcanism and Late Neogene Basin and Range extension. In this paper a detailed analysis of each these geologic events is presented to help us understand the structural host, mineralization and exhumation of the Tintic Mining District ore. A kinematic analysis of the faults was completed to determine the origin of NE-trending faults, Sevier Orogeny or Basin and Range extension, in the northern part of the East Tintic Mountains in Allens Ranch 7.5' quadrangle, near the eastern margin of the Great Basin of central Utah. The structural history of the NE-trending faults found in the quadrangle was reconstructed to determine stress directions and fault kinematics. Maximum paleostress direction for the East Tintic fold and thrust system is between 80º–100º with fold axes oriented at ~350º. For example, the Gardison Ridge and Tintic Prince faults are NE-trending right-lateral transverse faults that formed at ~30º to paleostress directions similar to those of the Sevier Orogeny. The dominant NE-trending faults in the region are likely due to (1) differential shortening during progressive orocline development, (2) the pre-deformational Pennsylvanian-Permian Oquirrh basin geometry, and (3) the influence of the Leamington transverse zones of the Provo salient. Conversely, mixed paleostress directions for the north-trending Tintic Davis Canyon fault show it is a Basin and Range extension-related normal fault that may have originated as a Sevier related fault. Other N-trending faults within the quadrangle are only related to Basin and Range extension. However, large offset, range-bounding faults are buried by valley fill throughout the quadrangle and no young fault scarps are identified cutting Lake Bonneville deposits. An Oligocene to Miocene suite of extrusive volcanic units in the quadrangle correlates well with those of the East Tintic and Soldiers Pass volcanic fields. The Paleogene volcanic section is dominated by a suite of high-K calc-alkaline extrusive rocks (35 to 32 Ma). This intermediate to silicic sequence was followed by eruption of the mildly alkaline Mosida Basalt during the Miocene (19.5 Ma) marking the transition from subduction-related intermediate and silicic volcanism to extension-related mafic volcanism in the eastern Great Basin.