Volcanic stratigraphy and structure in the Hunter Creek Fault area, Rouyn–Noranda, Quebec

1990 ◽  
Vol 27 (10) ◽  
pp. 1348-1358 ◽  
Author(s):  
G. Camiré ◽  
D. H. Watkinson
Keyword(s):  
Volcanic Rocks ◽  
Drill Hole ◽  
Volcanic Complex ◽  
Calc Alkaline ◽  
Volcanic Stratigraphy ◽  
The Past ◽  
The North ◽  
Northern Side ◽  
Volcanic Facies ◽  
Geochemical Studies

The Hunter Creek Fault (HCF) has been considered in the past as a major synvolcanic fault marked by a change in volcanic facies and separating two geologically distinct blocks within the Noranda volcanic complex: the North Mine and the Central Mine sequences. Detailed mapping in the HCF area, in conjunction with petrographic and geochemical studies, has revealed that most of the volcanic units defined south of the fault zone also occur on its northern side. Field and drill-hole correlations indicate that the rocks are cut by a N240–N250 reverse-slip fault dipping approximately 70° toward the northwest. Major- and trace-element data suggest that least-altered volcanic rocks are of tholeiitic affinity and that there is no calc-alkaline unit in the HCF area.

scholarly journals Structural and Chemical Controllers of the North and Northwest of Torud Based on Involved Fluid Studies, Structural and Geochemical Analyses

2021 ◽  
Vol 15 (3) ◽  
pp. 339-349
Author(s):  
Fatemeh Baseri ◽  
Arash Gourabjeri Pour ◽  
Nima Nezafati
Keyword(s):  
Volcanic Rocks ◽  
Mining Area ◽  
Hydrothermal Solutions ◽  
Base Metals ◽  
Laboratory Studies ◽  
Scattered Space ◽  
The North ◽  
Geochemical Analyses ◽  
Geochemical Studies ◽  
Semnan Province

Chah Mura mining area in Semnan province is located 30 km southwest of Shahroud and 20 km north of Torud village with an area of 35 km2 and includes a part of 1:250,000 Torud plate. Structurally, this area is located in the northeastern part of Central Iran and in the center of the volcanic-intrusive arc of Torud-Chah Shirin. Rock units of the area are volcanic and pyroclastic, depending on the Eocene age. Exposed assemblages in the Chah Mura area, based on field and laboratory studies, can be divided into basalt, andesite, andesite-basalt, trachyandesite, trachyandesibasalt and small outcrops of pyroclastic units in the form of agglomerates and sediments of sandstone and conglomerate. Volcanic rocks are influenced by sub-volcanic masses younger than Eocene with an intermediate to basic composition, and their predominant textures are granular, porphyroid with microcrystalline to microintragranular background. Finally, the units are cut by dikes. In this area, mineralization is mainly in the control of sub-faults and subvolcanic massifs. Mineralization is in the form of vein-veinlet, filling empty and scattered space in the oxidation-supergen stage. Mineral sequences include pyrite, chalcopyrite, chalcocite, digenite and covellite, cuprite, tenorite, natural copper, malachite, azurite, and iron oxides and hydroxides. Geochemical studies indicate that copper does not correlate well with any of the base metals and depositing elements. Copper shows only a relative correlation with silver. Micrometric studies of fluid inclusions in samples from this area indicate dilution as a result of mixing hydrothermal solutions with atmospheric fluids in formation of this reserve.

Lower Palaeozoic and Precambrian igneous rocks from eastern England, and their bearing on late Ordovician closure of the Tornquist Sea: constraints from U-Pb and Nd isotopes

1993 ◽  
Vol 130 (6) ◽  
pp. 835-846 ◽  
Author(s):  
S. R. Noble ◽  
R. D. Tucker ◽  
T. C. Pharaoh
Keyword(s):  
Volcanic Rocks ◽  
Igneous Rocks ◽  
Calc Alkaline ◽  
Nd Isotopes ◽  
Nd Isotope ◽  
Magmatic Arc ◽  
The North ◽  
Basement Rocks ◽  
Continental Arc ◽  
Lower Palaeozoic

AbstractThe U-Pb isotope ages and Nd isotope characteristics of asuite of igneous rocks from the basement of eastern England show that Ordovician calc-alkaline igneous rocks are tectonically interleaved with late Precambrian volcanic rocks distinct from Precambrian rocks exposed in southern Britain. New U-Pb ages for the North Creake tuff (zircon, 449±13 Ma), Moorby Microgranite (zircon, 457 ± 20 Ma), and the Nuneaton lamprophyre (zircon and baddeleyite, 442 ± 3 Ma) confirm the presence ofan Ordovician magmatic arc. Tectonically interleaved Precambrian volcanic rocks within this arc are verified by new U-Pb zircon ages for tuffs at Glinton (612 ± 21 Ma) and Orton (616 ± 6 Ma). Initial εNd values for these basement rocks range from +4 to - 6, consistent with generation of both c. 615 Ma and c. 450 Ma groups of rocksin continental arc settings. The U-Pb and Sm-Nd isotope data support arguments for an Ordovician fold/thrust belt extending from England to Belgium, and that the Ordovician calc-alkaline rocks formed in response to subductionof Tornquist Sea oceanic crust beneath Avalonia.

New FTIR data of calc-alkaline volcanic rocks from the Oas-Gutai Mts. and post eruption effects on the water content of phenocrysts

2020 ◽  
Author(s):  
Ákos Kővágó ◽  
Marinel Kovacs ◽  
Dóra Kesjár ◽  
Csaba Szabó ◽  
István Kovács
Keyword(s):  
Water Content ◽  
Volcanic Rocks ◽  
Parental Magma ◽  
Volcanic Eruptions ◽  
Ftir Spectra ◽  
Future Research ◽  
Calc Alkaline ◽  
Structural Hydroxyl ◽  
Volcanic Facies ◽  
Water Contents

<p>We studied volcanic rocks from the Oas-Gutai Mts. (Transylvania, Romania) to measure the ‘structural hydroxyl’ content of the nominally anhydrous minerals (NAMs such as clinopyroxene, plagioclase, quartz), from which water content of the parental magma can be estimated.  The Neogene volcanic chain of the Carpathian-Pannonian region (CPR), due to petrologic variability, is an excellent area for such investigation.</p><p>Recent FTIR studies on the calc-alkaline rocks from CPR, showed that the ‘structural hydroxyl’ content of NAMs could be modified during and after volcanic eruptions [1], [2], [3]. However, transmission FTIR-microscopy is an adequate technique for recognizing this these changes because FTIR spectra of the NAMs indicate signs in the case of hydroxyl loss [4].</p><p>For studying the pre-eruptive water contents clinopyroxenes are the most promising mineral because it has one of the lowest diffusion rates for hydroxyl in NAMs [5]. With the detailed study of the clinopyroxenes FTIR spectra, conclusions can be drawn concerning the potential post-eruptive loss of hydroxyl [4].</p><p>We have examined 8 volcanic rock samples, four dacite samples from Oas and one basalt two andesite and one rhyolite sample from the Gutai Mts. The samples show diverse volcanic facies such as lava, ignimbrite and debris avalanche. The diversity of samples is important for future research because it will help to choose the most adequate volcanic facies to estimate the magmatic equilibrium water contents.</p><p>The studied clinopyroxenes contain 83-371 ppm ‘structural hydroxyl’ content,which can be considered as normal values compared to the work of [6] where ‘structural hydroxyl’ content in clinopyroxenes show a range from 75 to 390 ppm in the mafic calc-alkaline lavas from Salina, Italy.</p><p>[1] Lloyd, A.S., Ferriss, E., Ruprecht, P., Hauri, E.H., Jicha, B.R., & Plank, T. (2016): Journal of Petrology, 57, pp. 1865-1886</p><p>[2] Biró, T., I. Kovács, D. Karátson, R. Stalder, E. Király, G. Falus, T. Fancsik, J. & Sándorné Kovács (2017): American Mineralogist, 102, pp.</p><p>[3] Pálos, Z., Kovács, I. J., Karátson, D., Biró, T., Sándorné Kovács, J., Bertalan, É., & Wesztergom, V. (2019): Central European Geology, 62(1)</p><p>[4] Patkó, L., Liptai, N., Kovács, I., Aradi, L., Xia, Q.K., Ingrin, J., Mihály, J., O'Reilly, S.Y., Griffin, W.L., Wesztergom, V., & Szabó, C. (2019): Chemical Geology, 507, pp. 23-41.</p><p>[5] Farver, J.R. (2010): Reviews in Mineralogy and Geochemistry, 72 (1), pp. 447–507.</p><p>[6] Nazzareni, S., Skogby H., & Zanazzi, P.F. (2011): Contributions to Mineralogy and Petrology, 162, pp. 275–288.</p>

scholarly journals Early Tertiary volcanic rocks from eastern Disko and south-eastern Nûgssuaq

1987 ◽  
Vol 135 ◽  
pp. 11-17
Author(s):  
A.K Pedersen ◽  
L.M Larsen
Keyword(s):  
Volcanic Rocks ◽  
Volcanic Stratigraphy ◽  
South Eastern ◽  
Early Tertiary ◽  
Clastic Sediments ◽  
Greenland Basin ◽  
Geological Map ◽  
Volcanic Facies ◽  
Volcanic Succession ◽  
Basalt Plateau

As part of an integrated study of the interaction between sedimentary and volcanic facies in the Cretaceous-Tertiary West Greenland basin, early Tertiary extrusive and intrusive basic igneous rocks have been investigated in eastern Disko and south-eastern Nûgssuaq (fig. 1). The volcanic activity started in the early Tertiary. It was centred in the western regions of Disko and Nûgssuaq and west of the present land areas, and consequently western Disko and western Nûgssuaq have a thick and complete volcanic succession, the Vaigat and Maligât Formations (Hald & Pedersen, 1975; Pedersen, 1975a, 1985). The coeval deposits in the eastern part of the basin are clastic sediments of the Upper Atanikerdluk Formation (Koch, 1959). With time, the volcanic deposits prograded eastwards, and eventually the whole area was covered by a coherent basalt plateau. The present study attempts to extend the established volcanic stratigraphy from western Disko and the geological map sheets 1:100 000 Qutdligssat (published in 1976) and Mellemfjord (published in 1987) into eastern Disko and eastern Nûgssuaq where volcanic rocks and sediments interfinger.

Geology, geochronology, and fluid inclusion studies of the Xiaorequanzi volcanogenic massive sulphide Cu–Zn deposit in the East Tianshan Terrane, China

2020 ◽  
Vol 57 (12) ◽  
pp. 1392-1410 ◽  
Author(s):  
Xi-Heng He ◽  
Xiao-Hua Deng ◽  
Leon Bagas ◽  
Jing Zhang ◽  
Chao Li ◽  
...  
Keyword(s):  
Fluid Inclusion ◽  
Fluid Inclusions ◽  
Tectonic Setting ◽  
Volcanic Rocks ◽  
Massive Sulphide ◽  
Calc Alkaline ◽  
The North ◽  
Volcanogenic Massive Sulphide ◽  
Secondary Fluid ◽  
East Tianshan

The Xiaorequanzi Cu–Zn deposit is in the westernmost part of East Tianshan Terrane in northwestern China. The deposit is unique in the region being a volcanogenic massive sulphide (VMS) deposit located near a zone (or belt) containing giant late Paleozoic porphyry Cu deposits. Aiming to better understand the genesis of the mineral deposits in the terrane and their tectonic setting, we report our findings of detailed studies on fluid inclusion microthermometry, Re–Os dating of chalcopyrite from the massive ore, and U–Pb dating of zircons from the host volcanic rocks. There are two sulphide stages with early pyrite succeeded by chalcopyrite–sphalerite, which are hydrothermally overprinted and supergene enriched. The hydrothermal overprinting is characterised by quartz–sulphide veins crossed by carbonate-rich quartz veins. Quartz from the chalcopyrite–sphalerite stage is characterised by primary fluid inclusions containing H2O–NaCl(–CO2) and homogenise at 228–392 °C with a salinity of 2.2–13.3 wt.% NaCl equiv. Secondary fluid inclusions related to the hydrothermal overprinting homogenise at 170–205 °C with a salinity of 2.7–12.1 wt.% NaCl equiv. Fluid inclusions in the quartz–sulphide stage of the hydrothermal overprinting contain H2O–NaCl with homogenisation temperatures of 164–281 °C and salinities in ranging from 2.9 to 12.4 wt.% NaCl equiv. Fluid inclusion in the quartz–calcite stage contain H2O–NaCl with homogenisation temperatures of 122–204 °C with salinities of 1.4–12.4 wt.% NaCl equiv. These characteristics are like those of the secondary fluid inclusions in the VMS mineralisation. Combining these findings with H–O isotopic data from previous studies, we propose that the primary mineralising fluid is magmatic in origin. Tuff hosting the mineralisation yields a SHRIMP U–Pb zircon age of 352 ± 5 Ma, which is interpreted as the age of the tuff, and a porphyritic felsite dyke intruding the tuff yields a SHRIMP U–Pb zircon date of 345 ± 6 Ma, interpreted as the emplacement age of the dyke. Chalcopyrite from the main orebody at Xiaorequanzi yields a Re–Os isochron age of 336 ± 13 Ma with an initial 187Os/188Os ratio of 0.25 ± 0.55 (MSWD = 12). Given that the VMS deposit is a syngenetic deposit, we regard the upper ca. 349 Ma limit of the Re–Os date as the approximate age of the chalcopyrite. The three dates are the same within error, and the upper limit of the Re–Os date of ca. 349 is taken as the age of the volcanic, dyke, and mineralisation. The volcanic rocks around the Xiaorequanzi deposit have been previously classified as calc–alkaline to high-K calc–alkaline enriched in large-ion lithophile elements and depleted in high-field-strength elements, which are characteristics indicative of a forearc setting. It is suggested that VMS mineralisation formed in a forearc setting related to the north-directed subduction of the Palaeo-Kangguer or North Tianshan oceanic plates.

Oxygen isotope evolution of volcanic rocks at the Sturgeon Lake volcanic complex, Ontario

2000 ◽  
Vol 37 (1) ◽  
pp. 39-50 ◽  
Author(s):  
E M King ◽  
J W Valley ◽  
D W Davis
Keyword(s):  
Oxygen Isotope ◽  
Volcanic Rocks ◽  
Isotope Ratios ◽  
Hydrothermal Systems ◽  
Superior Province ◽  
Volcanic Complex ◽  
Volcanic Stratigraphy ◽  
Oxygen Isotope Ratios ◽  
Ore Body ◽  
The Many

Igneous zircons from the Sturgeon Lake volcanic complex, host to several massive sulphide deposits in the Superior Province, Canada, have an average δ18O(zircon) of 5.4 ± 0.3‰ VSMOW (n = 9 rocks). These zircons are from units differing in age by 18 million years in the 2.7 Ga complex. There is no detectable interaction of high δ18O, supracrustal lithologies in the magma. Quartz from volcanic units beneath the largest ore body, the Mattabi deposit, has an average δ18O of 9.3 ± 0.6‰. Quartz phenocrysts from the Mattabi unit and overlying volcanics have elevated and heterogeneous δ18O values averaging 13.8 ± 0.9‰ and are not in magmatic equilibrium with zircons. The δ18O values of whole-rock powders range from 5.6‰ to 14.3‰ and follow the trend observed in the δ18O values of quartz. Healed microcracks are visible in cathodoluminescence images (but are not obvious optically) of quartz phenocrysts from units with high δ18O values and disequilibrium Δ(quartz-zircon) suggesting that recrystallization facilitates the elevation of δ18O. Quartz phenocrysts from volcanic units with Δ(quartz-zircon) values near equilibrium at magmatic temperatures do not display healed microcracks in cathodoluminescence. The elevated δ18O(quartz) values are not restricted to units hosting orebodies, but are seen in all rocks in the volcanic stratigraphy that postdate eruption of the Mattabi unit. Oxygen isotope ratios combined with physical volcanology studies suggest that impermeable volcanic layers control the size and location of the many hydrothermal systems that may have occurred in the Sturgeon Lake complex.

scholarly journals Understanding the offshore flood basalt sequence using onshore volcanic facies analogues: an example from the Faroe–Shetland basin

2009 ◽  
Vol 146 (3) ◽  
pp. 353-367 ◽  
Author(s):  
DOUGAL A. JERRAM ◽  
RICHARD T. SINGLE ◽  
RICHARD W. HOBBS ◽  
CATHERINE E. NELSON
Keyword(s):  
Volcanic Rocks ◽  
Flood Basalt ◽  
Reflection Seismology ◽  
Flood Basalts ◽  
Geological Interpretation ◽  
Seismic Modelling ◽  
The North ◽  
Volcanic Facies ◽  
Volcanic Succession ◽  
Simple Flows

AbstractFlood basalts in associated volcanic rifted margins, such as the North Atlantic Igneous Province, have a significant component of lavas which are preserved in the present day in an offshore setting. A close inspection of the internal facies architecture of flood basalts onshore provides a framework to interpret the offshore sequences imaged by remote techniques such as reflection seismology. A geological interpretation of the offshore lava sequences in the Faroe–Shetland Basin, using constraints from onshore analogues such as the Faroe Islands, allows for the identification of a series of lava sequences which have characteristic properties so that they can be grouped. These are tabular simple flows, compound-braided flows, and sub-aqueously deposited hyaloclastite facies. The succession of volcanic rocks calculated in this study has a maximum thickness in excess of 6800 m. Down to the top of the sub-volcanic sediments, the offshore volcanic succession has a thickness of about 2700 m where it can be clearly identified across much of the area, with a further 2700 m or more of volcanic rock estimated from the combined gravity and seismic modelling to the north and west of the region. A large palaeo-waterbody is identified on the basis of a hyaloclastite front/apron consisting of a series of clinoforms prograding towards the eastern part of the basin. This body was > 500 m deep, must have been present at the onset of volcanism into this region, and parts of the water body would have been present during the continued stages of volcanism as indicated by the distribution of the hyaloclastite apron.

scholarly journals Flood basalts, rhyolites, and subsequent volcanism of the Columbia River magmatic province in eastern Oregon, USA

2021 ◽  
pp. 301-352
Author(s):  
Emily B. Cahoon† ◽  
Martin J. Streck† ◽  
Mark Ferns†
Keyword(s):  
Large Volume ◽  
Late Miocene ◽  
Middle Miocene ◽  
Volcanic Rocks ◽  
Columbia River ◽  
Flood Basalt ◽  
Lava Flows ◽  
Calc Alkaline ◽  
The North ◽  
High Lava Plains

ABSTRACT The Miocene Columbia River Basalt Group (CRBG) is the youngest and smallest continental flood basalt province on Earth. This flood basalt province is a succession of compositionally diverse volcanic rocks that record the passage of the Yellowstone plume beneath eastern Oregon. The compositionally and texturally varied suite of volcanic rocks are considered part of the La Grande–Owyhee eruptive axis (LOEA), an ~300-km-long, north-northwest–trending, Middle Miocene to Pliocene volcanic belt that extends along the eastern margin of the Columbia River flood basalt province. Volcanic rocks erupted from and preserved within the LOEA form an important regional stratigraphic link between the flood basalt–dominated Columbia Plateau to the north, the north and bimodal basalt-rhyolite volcanic fields of the Snake River Plain to the east, the Owyhee Plateau to the south, and the High Lava Plains to the south and east; the latter two have time transgressive rhyolite centers that young to the east and west, respectively. This field-trip guide details a four-day geologic excursion that will explore the stratigraphic and geochemical relationships among mafic rocks of the CRBG and coeval and compositionally diverse silicic rocks associated with the early trace of the Yellowstone plume and High Lava Plains in eastern Oregon. The trip on Day 1 begins in Portland then traverses across the western axis of the Blue Mountains, highlighting exposures of the widespread, Middle Miocene Dinner Creek Welded Tuff and aspects of the Picture Gorge Basalt lava flows and northwest-striking feeder dikes situated in the central part of the CRBG province. Travel on Day 2 progresses eastward toward the eastern margin of the LOEA, examining a transition linking the Columbia River Basalt province with a northwestward-younging magmatic trend of silicic volcanism of the High Lava Plains in eastern Oregon. Initial field stops on Day 2 focus on the volcanic stratigraphy northeast of the town of Burns, which includes regionally extensive Middle to Late Miocene ash-flow tuffs and lava flows assigned to the Strawberry Volcanics. Subsequent stops on Day 2 examine key outcrops demonstrating the intercalated nature of Middle Miocene tholeiitic CRBG flood basalts, temporally coeval prominent ash-flow tuffs, and “Snake River–type” large-volume rhyolite lava flows cropping out along the Malheur River. The Day 3 field route navigates to southern parts of the LOEA, where CRBG rocks are associated in space and time with lesser known and more complex silicic volcanic stratigraphy forming Middle Miocene, large-volume, bimodal basalt-rhyolite vent complexes. Key stops will provide a broad overview of the structure and stratigraphy of the Middle Miocene Mahogany Mountain caldera and of the significance of intercalated sedimentary beds and Middle to Late Miocene calc-alkaline lava flows of the Owyhee basalt. Initial stops on Day 4 will highlight exposures of Middle to Late Miocene silicic ash-flow tuffs, rhyolite domes, and calc-alkaline lava flows overlying the CRBG across the northern and central parts of the LOEA. The later stops on Day 4 examine more silicic lava flows and breccias that are overlain by early CRBG-related rhyolite eruptions. The return route to Portland on Day 4 traverses the Columbia River gorge westward from Baker City. The return route between Baker and Portland on Day 4 follows the Columbia River gorge and passes prominent basalt outcrops of large volume tholeiitic flood lavas of the Grande Ronde, Wanapum, and Saddle Mountains Formations of the CRBG. These sequences of basaltic and basaltic andesite lavas are typical of the well-studied flood basalt dominated Columbia Plateau, and interbedded silicic and calc-alkaline lavas are conspicuously absent. Correlation between the far-traveled CRBG lavas and calcalkaline and silicic lavas considered during the excursion relies on geochemical fingerprinting and dating of the mafic flows and dating of sparse intercalated ashes.

scholarly journals Mineralogical, Petrological and Geochemical Study of the Agios Ioannis Volcanic Rocks, Kamena Vourla Area, Greece

2019 ◽  
Vol 55 (1) ◽  
pp. 274 ◽  
Author(s):  
Christos Kanellopoulos ◽  
George Vougioukalakis ◽  
Constantinos Mavrogonatos ◽  
Ifigeneia Megremi ◽  
Ioannis Iliopoulos
Keyword(s):  
Volcanic Rocks ◽  
Chemical Study ◽  
Small Islands ◽  
Calc Alkaline ◽  
Plagioclase Composition ◽  
Volcanic Arc ◽  
The North ◽  
Geochemical Study ◽  
High K ◽  
Tectonically Active

The Plio-Pleistocene volcanic center of Lichades is located in the Northern Euboean Gulf, at the western extremity of the North Anatolian Fault and it is one of the most neo-tectonically active areas in Greece. Volcanic rocks are exposed in the form of lava flows and/or domes mostly in the small islands (Lichades) offshore Kamena Vourla, as well as in a small outcrop in mainland, namely the Agios Ioannis area. Based on the results of the present study, the Agios Ioannis volcanic rocks are characterized as trachyandesites with high-K calc-alkaline affinities, similar to several volcanic rocks from the South Aegean Volcanic Arc. The petrological and mineral-chemical study reveal that the studied volcanic rocks are characterized by vitrophyric texture and a matrix dominated by glass, numerous randomly-oriented microlites of plagioclase and minor sanidine, clinopyroxene and amphibole. Phenocrysts comprise of plagioclase, olivine, quartz, clinopyroxene and amphibole. Plagioclase composition ranges from andesine to bytownite (An30-An73). Olivine cores are typically Mg-rich, and the rims display elevated FeO content. Clinopyroxenes display a narrow compositional range between augite and diopside, with the latter being more common. Amphiboles, are calcic and their composition ranges from tschermakite to tschermakitic hornblende. Mineralogical and geochemical similarities with other volcanic rocks in Greece such as Lesvos lamproites, may be helpful in understanding the genesis of the studied Agios Ioannis volcanics.

Neogene volcanism on the eastside of Mount Diablo, Contra Costa County, California

2021 ◽  
Author(s):  
Raymond Sullivan ◽  
Ryan P. Fay ◽  
Carl Schaefer ◽  
Alan Deino ◽  
Stephen W. Edwards
Keyword(s):  
Middle Eocene ◽  
Volcanic Rocks ◽  
Calc Alkaline ◽  
Stream Channels ◽  
California Coast ◽  
Late Pliocene ◽  
The North ◽  
Great Valley Group ◽  
California Coast Ranges ◽  
Great Valley

ABSTRACT Two spatially separated areas of Neogene volcanic rocks are located on the northeast limb of the Mount Diablo anticline. The southernmost outcrops of volcanics are 6 km east of the summit of Mount Diablo in the Marsh Creek area and consist of ~12 hypabyssal dacite intrusions dated at ca. 7.8–7.5 Ma, which were intruded into the Great Valley Group of Late Cretaceous age. The intrusions occur in the vicinity of the Clayton and Diablo faults. The rocks are predominantly calc-alkaline plagioclase biotite dacites, but one is a tholeiitic plagioclase andesite. Mercury mineralization was likely concomitant with emplacement of these late Miocene intrusions. The northern most outcrops of Neogene volcanic rocks occur ~15 km to the north of Mount Diablo in the Concord Naval Weapons Station and the Los Medanos Hills and are probably parts of a single andesite flow. A magnetometer survey indicates that the flow originated from a feeder dike along the Clayton fault. The lava flow is flat-lying and occu pies ancient stream channels across an erosional surface of tilted Markley Sandstone of middle Eocene age. New radiometric dates of the flow yield an age of 5.8–5.5 Ma, but due to alteration the age should be used with caution. The flow is a calc-alkaline andesite rich in clinopyroxene and plagioclase. What appear to be uplifted erosional remnants of the flow can be traced northeastward in the Los Medanos Hills across a surface of tilted Cenozoic rocks that eventually rest on formations as young as the Lawlor Tuff dated at 4.865 ± 0.011 Ma. This stratigraphic relationship suggests that the andesite flow is probably late Pliocene in age and was impacted by the more recent uplift of the Los Medanos Hills but postdates the regional folding and faulting of the rocks of Mount Diablo. In terms of timing, location, and composition, the evidence suggests these two areas of dacitic and andesitic volcanics fit into a series of migrating volcanic centers in the California Coast Ranges that erupted following the northward passage of the Mendocino Triple Junction.

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