Oligocene Volcanic Rocks Research Articles

Granulitic xenoliths from the Eocene and Oligocene volcanic rocks in NW of Central-East Iranian microcontinent: Evidence from petrology of deep crust

Granulitic xenoliths from the Eocene and Oligocene volcanic rocks in NW of Central-East Iranian microcontinent: Evidence from petrology of deep crust

Structural architecture of the Mapimí Biosphere Reserve and its surroundings, north-central Mexico: New insights from U–Pb geochronology and interpreted structural data

The Mapimí Biosphere Reserve is in northeastern México, near the boundary between the Mesozoic Coahuila platform and the Mexican central basin. A large portion of the Reserve is a broad bolson where isolated mountains are surrounded by flat terrain. Two slightly elevated areas (benches) are subtle topographic features in the Reserve. There, in rolling hills occur folded, Upper Cretaceous – Paleogene, poorly-lithified and -exposed transitional and continental sediments and tilted Paleogene volcanic and subvolcanic rocks. Each bench occupies a band that is ∼75 × 20 km. A land strip where playa lakes and salt-flats occur separates the benches. Deformation style within the benches differ from those in regions where platform carbonates or Mexican central basin rocks dominate. Structures within the benches are gentle, open folds and medium sized (15 - 3 km in diameter) structural domes and basins. Furthermore, orientation of the fold axes shows a wide scatter. There is at least one folded structure in the Reserve that displays an interference pattern that indicates refolding. Post-Oligocene normal faults bound the benches and overprinted on the folded structures. However, the most important normal fault, which cuts in half two domes and one structural basin, lacks a clear topographic scarp. The occurrence of broad exposures of the Upper Cretaceous Indidura Formation in flat areas, interpreted as the footwall block of a major normal fault, is consistent with the interpretation that the benches are rocky pediments partially covered by alluvium and/or eolian sediments. Thus, it appears that Cenozoic extension has been inactive for a long period of time in this portion of the Mexican Basin and Range province. The best evidence of NNW, down to the SW normal faults within the Reserve is in the Sierra de Mohóvano. There, an array of domino-style normal faults cut Oligocene volcanic rocks and gravel deposits. The master fault of the domino array is adjacent and parallel to a fold in Mesozoic marine rocks.We attribute the unusual characteristic of the folds in the Reserve to 1) the region is near the boundary among the large calcareous platform and the basin, and the boundary had a significant curvature, concave toward the W. This geometry caused complex deformation during crustal shortening, as tectonic transport associated with the Mexican orogen is toward the NE, 2) a significant difference in competence between the poorly consolidated transitional and continental deposits and the more competent rocks in the carbonate platform also played a role in the formation of the structures, as folds in the benches formed during the waning stage of the Mexican orogen, as suggested by the detrital zircon U–Pb ages, which are as young as ∼30 Ma, 3) Oligocene magmatic activity caused the emplacement of laccoliths, which locally modified older folds forming domes, 4) finally, Basin and Range extension (<27 Ma) cut across the folded sediments and overlying volcanic rocks. Evidence of extension in the Reserve is subtle as normal faults have remained inactive for a long time, while isostatic uplift and erosion developed the pediment, effectively masked most fault scarps.

Petrogenesis of Oligocene volcanic rocks of the Lake Tana area, Ethiopian large Igneous Province

Petrogenesis of Oligocene volcanic rocks of the Lake Tana area, Ethiopian large Igneous Province

Subvolcanic Rock Petroleum System Potential in the South Malang Region, East Java, Indonesia

The South Malang Region is located in the south-eastern part of the Southern Mountain Volcanic Arc; it presents different opportunities for hydrocarbon exploration. The stratigraphy of the study area from old to young consists of Oligocene Volcanic rocks (Mandalika, Watupatok, and Arjosari Formation), Early Miocene Carbonates (Campurdarat and Jaten Formation), Middle Miocene Volcanic (Wuni Formation), Late Miocene-Pliocene Carbonates (Nampol, Oyo, and Wonosari Formation), and Holocene alluvial deposits (Kalipucang Formation). The dominance of volcanic rocks makes this area considered an area without hydrocarbon play prospects. Petroleum system potential is revealed by evaluating and analyzing potential source rock and reservoir rock outcrop samples. The study shows that the Nampol Formation can be considered as a gas-prone source rock, with type III kerogen, total organic content ranging from 3.48 - 26.18 wt%, and possess the potential to produce good to very good hydrocarbons and a hydrogen index ranging from 43 to 86 mgHC/g TOC. Furthermore, rock core analysis and petrographic studies were carried out on the Nampol sandstone where the rock samples showed good reservoir properties. However, the Nampol and Wonosari limestone that was considered as the secondary target for reservoir possesses a low quality of reservoir properties. This study shows that there is a potential for petroleum system existence in the Southern Mountain subvolcanic arc, which is indicated by the presence of source rock and potential reservoir rock as one of the various elements and processes present in a petroleum system.

Exhumation history of the La Caridad and Suaqui Verde porphyry copper deposits in the eastern Basin and Range province of Sonora: Insights from thermobarometry and apatite thermochronology

The Arizona-Sonora porphyry copper cluster is one of the most productive provinces of this type on Earth. There, upper-crustal deposits emplaced between Late Cretaceous and Early Eocene, subsequently exhumed and developed supergene enrichment profiles. Despite the Basin and Range regional extension, these shallow intrusions and associated mineralization were preserved in eastern Sonora in the exhumed footwalls of normal faults. In order to better constrain their exhumation history, we performed a thermobarometrical (Al-in-hornblende) and thermochronological (apatite U–Pb and fission-track) study on the La Caridad and Suaqui Verde porphyry copper deposits. On the one hand, thermobarometry allows us to determine that the Suaqui Verde porphyry copper emplaced at 4.8 ± 0.9 km-depth, which implies that Cu–Mo mineralization may occur deeper than the ~2–4 km depth classically proposed for porphyry copper. On the other hand, using thermobarometry and inverse modeling of thermochronologic data obtained for individual and drill core samples, we reconstruct the thermal history of these porphyry copper. After a very fast post-emplacement cooling, numerical models suggest that cooling of rocks was slow and mainly monotonous during exhumation. Indeed, our data do not record an acceleration of the exhumation in eastern Sonora during the Basin and Range regional extension (between ~25 and 12 Ma). However, inverse modeling of fission-track data obtained along a drill core in La Caridad allows us to reconstruct the evolution of the geothermal gradient through time, which is in agreement with the regional geodynamics. Overall, it cannot be excluded that Oligocene volcanic rocks protected the ore deposits from erosion, even if they remained relatively thin (<500 m-thick). We propose that eastern Sonora was characterized by Basin and Range extension distributed on numerous normal faults with small offsets, which allowed to preserve the upper crustal part of the Cretaceous-Paleogene magmatic arc and its numerous associated porphyry copper systems.

A new perspective on Cenozoic calc-alkaline and shoshonitic volcanic rocks, eastern Saveh (central Iran)

ABSTRACT Late Eocene – Oligocene volcanic rocks in the eastern Saveh region of the Urumieh-Dokhtar magmatic arc (UDMA) are representative of Paleogene magmatic activity in Iran. They show a wide range of silica-undersaturated to silica-oversaturated compositions, from basalt-trachy basalt and tephri-phonolite to trachyte-latite. Whole rock chemical compositions define a continuous assemblage of mafic and felsic rocks in terms of SiO2 (46.3 to 71.1 wt.%), Al2O3 (12.5–19.2 wt.%), K2O (2.06–12.23 wt.%) and TiO2 (0.41–1.75 wt.%) contents, Mg number (6.8–51) and K2O/Na2O ratios (0.53–30.2). Abundances of rare earth elements in the tephri-phonolite group are much higher than in the trachyte-latite and basalt-trachy basalt groups. Elevated contents of large ion lithophile elements such as Ba (191–7311 ppm) and Pb (6.49–118 ppm) as well as depletions in high field strength elements such as Nb (2.43–40.6 ppm), Ta (0.15–2.24 ppm), Zr (40.8–258 ppm) and TiO2 (0.40–1.77 wt.%) characterize these rocks. Initial ratios of 87Sr/86Sr from 0.7046 to 0.7074 and ɛNd(t) from +0.9 to +3.3 show that these were derived from parental magma extracted from partial melting of metasomatized mantle, namely amphibole- and/or phlogopite-bearing peridotite. Fractionation of feldspar, clinopyroxene, amphibole and phlogopite, and minor contamination with upper crust was responsible for the large variation of the rock types observed. We explain contemporaneous late Eocene – Oligocene calc-alkaline and high-K magmatic activity in the UDMA as due to the partial melting of both hot metasomatized asthenospheric and subcontinental lithospheric mantle in an extensional tectonic regime. This extensional regime probably developed due to the rollback of the subducted Neo-Tethys oceanic plate.

Epithermal systems of the Torud–Chah Shirin district, northern Iran: Ore-fluid evolution and geodynamic setting

The Torud–Chah Shirin (TCS) ore district in northern Iran is defined by an NE alignment of Tertiary Ag-Au and base metal-rich epithermal systems, and it is part of the eastern Alborz orogenic belt of Iran. Intermediate- to high-sulfidation mineralization occurs as veins hosted by the Eocene−Oligocene volcanic, subvolcanic and volcaniclastic rocks. The TCS district is characterized by three fault system populations including ∼70°, ∼270°, and ∼340°−trending faults, and detailed structural mapping show that overall strike of the TCS vein system is 320°–340° but varies from ∼290° to ∼350°. The ∼N70°−trending faults are parallel to the Anjilow and Torud regional faults in the TCS ore district. Green- to grey-schist and metamorphosed dolomite and limestone are the oldest units (Ordovician–Silurian) in the TCS. Sedimentary rocks were initiated by limestone, dolomite and green shale in the Cretaceous and continued with conglomerate into the Paleocene (Fajan Formation). Magmatism mainly occurred sporadically from the Eocene to Oligocene (ca. 55–24 Ma), with two major episodes between early to middle Eocene (ca. 55 and 37 Ma, EME) and early to late Oligocene (ca. 34 and 24 Ma, ELO). Whole rock geochemical data of EME and ELO rocks of the TCS district shows a range from basalts to rhyolites with low-K calc-alkaline and shoshonitic affinity. Their rare earth elements (REEs) and high field strength elements (HFSE) signatures indicate the occurrence of a supra-subduction zone magmatism and all rocks have been sourced from the same parent melt. Samples from ELO display higher alkali contents compare with EME but have a similar trace element characteristics.Hydrothermal alteration is pervasive in volcanic and subvolcanic rocks but is mainly localized near the veins and ore zones. Alteration assemblages include quartz-sericite (±pyrite ± adularia), quartz-illite (kaolinite ± pyrite), carbonate-quartz and chlorite + calcite + epidote (propylitic). Homogenization temperatures of fluid inclusions in the epithermal prospects is in range of 125–375 °C, and ore-forming fluids were mainly of magmatic-hydrothermal origin (e.g., Gandy and Abolhassani prospects), with some contributions from meteoric water. The epithermal prospects have more or less similar salinities, ranging from 2 to 18 wt% NaCl equiv., with a distinct cluster between 6 and 15 wt% NaCl equiv.Our shreds of evidence suggest that the EME and ELO calc-alkaline volcanic activity in TCS occurs occurred in tension fractures related to orthogonal plate convergence that postdates the NE-trending strike-slip regional faults, and plays an important role in the development of the epithermal prospects. Simultaneous with, or soon after crustal heating related to the magmatism, strike-slip faults movement may also have been critical in the ore-forming process, leading to trans-tension of local compressive forces, enhancement of crust-scale permeability, and promotion of mixing of ore-forming fluids.

Eocene-Oligocene volcanic units of momen abad, east of Iran: petrogenesis and magmatic evolution

This study investigates petrology and major, minor, and rare earth elements ‎geochemistry of ‎East Iranian Eocene–Oligocene volcanic rocks in Sistan suture ‎zone, to examine their ‎petrogenesis and magma evolution. The volcanic rocks include andesite, trachy-andesite, dacite ‎and rhyolite. ‎These calc-alkaline rocks of high-K series are enriched in Large ‎Ion Lithophile ‎Elements of Rb and Ba and depleted in High Field Strength Elements ‎(e.g. Ti and Nb). The REE ‎pattern shows Eu negative anomaly. These ‎features are comparable with rocks in subduction ‎zone of continental ‎margin setting. ‎The petrographic features and the geochemical variation ‎of major oxides and trace ‎elements against SiO2 can be related to fractional ‎crystallization in parent magma. The trace ‎element ratios, e.g. Nb/La, Nb/U, Ba/Rb, Nb/Y and Rb/Y, verify crustal ‎contamination with a remarkable upper ‎crustal contamination as the main ‎process in the ‎formation of volcanic series. ‎The ratios of Nb/Ba and Nb/Zr indicate that magmatic activity in Momen Abad had ‎resulted ‎from sub-continental lithospheric mantle. In addition, Nb/Y and Zr/Yb versus ‎Ta/Yb reveals an ‎E-MORB like mantle source. A spinel lherzolite composition ‎with significant role of the ‎AFC processes are deduced from the concentrations of Yb and ‎Y, and the ratios of Th/Yb and ‎La/Yb. ‎The Ba/Nb, Ba/Th and Th/Nb ratios presume a metasomatized mantle source ‎resulted from ‎the melting of upper crust sediments inherited from the subduction of ‎Neotethys ocean beneath ‎the Lut Block. ‎

Pantasma: Evidence for a Pleistocene circa 14 km diameter impact crater in Nicaragua

AbstractThe circa 14 km diameter Pantasma circular structure in Oligocene volcanic rocks in Nicaragua is here studied for the first time to understand its origin. Geomorphology, field mapping, and petrographic and geochemical investigations all are consistent with an impact origin for the Pantasma structure. Observations supporting an impact origin include outward‐dipping volcanic flows, the presence of former melt‐bearing polymict breccia, impact glass (with lechatelierite and low H2O, &lt;300 ppm), and also a possible ejecta layer containing Paleozoic rocks which originated from hundreds of meters below the surface. Diagnostic evidence for impact is provided by detection in impact glass of the former presence of reidite in granular zircon as well as coesite, and extraterrestrial ε54Cr value in polymict breccia. Two 40Ar/39Ar plateau ages with a combined weighted mean age of 815 ± 11 ka (2 σ; P = 0.17) were obtained on impact glass. This age is consistent with geomorphological data and erosion modeling, which all suggest a rather young crater. Pantasma is only the fourth exposed crater &gt;10 km found in the Americas south of N30 latitude, and provides further evidence that a significant number of impact craters may remain to be discovered in Central and South America.

The Kighal Porphyry Cu–Mo Deposit, NW Iran: Insights into Origin and Evolution of the Mineralizing Fluids

Abstract —The Kighal porphyry Cu–Mo deposit, located at 46°42′36″ N and 38°37′06″ E, occurs 120 km north of Tabriz and 12 km north of Varzeghan in the Arasbaran magmatic belt, NW Iran. The geologic units in the Kighal area are Eocene and Oligocene volcanic rocks associated with Miocene intrusive and subvolcanic rocks. Quartz-monzonite porphyry in the Kighal area hosts porphyry style Cu–Mo mineralization consisting of four alteration zones (potassic, phyllic, argillic, and propylitic) and abundant quartz veinlets. Based on mineralogy and geochemistry data, the intrusive and subvolcanic rocks belong to quartz-diorite, diorite, and granite units with high-K calc-alkaline to calc-alkaline affinities. All samples are enriched in LILE and depleted in HFSE, pointing to a mantle magma source contamination with subducted oceanic-crust material. Three types of fluid inclusions are identified in quartz from different quartz-sulfide veinlets, including multiphase (LVS) and liquid-rich (LV) inclusions in the potassic zone, liquid-rich (LV) and vapor-rich (VL) inclusions in the phyllic zone, and liquid-rich (LV) inclusions in the silicic zone. Microthermometry studies showed that Th in multiphase fluid inclusions in the potassic zone varied from 265 to 450 °C and salinity, from 38 to 59 wt.% NaCl-equiv. These values are higher than those of two-phase fluid inclusions in the phyllic zone (Th of 163 to 466 °C and salinity of 0.3 to 11 wt.% NaCl-equiv.). The calculated δ18OH2O and δDH2O values in biotite, sericite, and quartz from potassic, phyllic, and silicic zones showed that it is predominantly magmatic water with a lesser amount of meteoric water that is responsible for mineralization in the potassic zone. Light δDH2O values of biotite in the potassic zone suggest a magma degassing process and/or a change in the fluid composition with magmatic water influx to a hydrothermal system.

New high-precision 40Ar/39Ar ages on Oligocene volcanic rocks of northwestern Kenya

New, high-precision 40Ar/39Ar ages from volcanic rocks in northwestern Kenya are provided for some areas of exposure in this remote area. We report seven 40Ar/39Ar ages generated from single crystal total fusion experiments on alkali feldspar separated from volcanic rocks in the Mogila, Songot, and Lokwanamur Ranges and the Gatome valley. A rhyolite from the lower part of the sequence in the Mogila Range yielded ages of 32.31 ± 0.06 Ma and 32.33 ± 0.07 Ma, and a rhyolite near the top of that sequence yielded 31.67 ± 0.04 Ma. A single sample from the Songot Range yielded an age of 32.49 ± 0.07 Ma, slightly older than the rocks collected from Mogila. In both ranges the early Oligocene rhyolites are underlain by basalts, as is also the case in the Labur Range. Ages of 25.95 ± 0.03 Ma, 25.91 ± 0.04 Ma, and 27.15 ± 0.03 Ma were measured on alkali feldspar from rhyolites from the Lokwanamur Range, and the nearby Gatome valley. All of these rocks are part of an episode of widespread volcanism in northwestern Kenya in the mid-to late Oligocene that is not currently known from the Ethiopian Rift Valley.

Multiple geophysical methods used to examine neotectonic structures in the western foothills of the Sierra de El Maitén (Argentina), North Patagonian Andes

ABSTRACTA high‐resolution geophysical study was carried out in a region of the retroarc of the Patagonian Andes located on the western slope of the Sierra de El Maitén. This structure is characterized by an imbricated west‐vergent fault system developed in the orogenic front area of the North Patagonian Andes that has uplifted. Oligocene volcanic rocks (Ventana Formation) affect Miocene to Quaternary sediments. Even though neotectonic fault scarps are affecting Quaternary deposits in the foothills of this range, no direct observation of slip in Quaternary strata was determined. The main objective of this study is to determine geometry of recognized neotectonic structures, characterizing them by variations in magnetic susceptibility, density, and p‐wave velocities. The combined application of different geophysical methods has allowed the characterization of the bedrock geometry and the determination of neotectonic displacements along faults. The potential field model and its integration with a seismic profile show the accurate geometry of this tectonic zone, which is crucial for seismogenic hazard analysis, in the area of northern Patagonia, a highly significant economic zone due to tourism with several towns (El Maitén, Esquel, and San Carlos de Bariloche) dispersed throughout the area of young tectonic activity.

Geochronology of the Turkana Depression of Northern Kenya and Southern Ethiopia

Mesozoic and Cenozoic sedimentary rocks in the Turkana Depression of northern Kenya and southern Ethiopia rest on basement rocks that yield K/Ar cooling ages between 433 and 522 Ma. Proven Cretaceous strata are exposed in Lokitaung Gorge in northwest Kenya. Eocene basalts and rhyolites in Lokitaung Gorge, the Nabwal Hills, and at Kangamajoj, date between 34 and 36 Ma, recording the earliest volcanism in the region. Oligocene volcanic rocks, with associated fossiliferous sedimentary strata at Eragaleit, Nakwai, and Lokone, all west of Lake Turkana, are 23 to 28 Ma old, as is the Langaria Formation east of Lake Turkana. Lower and Middle Miocene volcanic and sedimentary sequences are present both east and west of Lake Turkana, where ages from 17.9 to 9.1 Ma have been measured at many levels. Upper Miocene strata are presently known only at Lothagam, with ages ranging from 7.4 to 6.5 Ma. Deposition of Pliocene strata of the Omo Group begins in the Omo-Turkana, Kerio, and South Turkana basins -4.3 Ma ago and continues in parts of those basins until nearly the present time, but with some gaps. These strata are linked through volcanic ash correlations at many levels, as are Pleistocene strata of the Omo Group (principally the Shungura, Koobi Fora, and Nachukui formations). (40) Ar/(39) Ar dates on many volcanic ash layers within the Omo Group, supplemented by K/Ar ages on intercalated basalts and paleomagnetic polarity stratigraphy, provide excellent age control from 4.2 to 0.75 Ma, although there is a gap in the record between -1 Ma and 0.8 Ma. Members I to III of the Kibish Formation in the lower Omo Valley record deposition between 0.2 and 0.1 Ma ago; Member IV, correlative with the Galana Boi Formation, was deposited principally between 12 and 7 ka BP.

中央北海道南部の後期漸新世火山岩の岩石学的検討―背弧海盆拡大の初期火山活動―

The Oligocene volcanism in central Hokkaido may have taken place as the volcanic arc of the Eurasian continental margin prior to the formation of the Japan Sea back-arc basin. The Oligocene volcanic rocks in the Minami-Naganuma Formation recovered from the Nanporo drill cores consist of alkali olivine basalt, basaltic andesite, and andesite, and is classified into Lower (3742.2-3744.6 m depth), Middle (3488.0-3489.4 m), and Upper (3243.2-3245.1 m) members. The Oligocene volcanic rocks are characterized by greater large ion lithophile element and rare earth element enrichments, and also show a depletion in high-field strength elements. The Oligocene volcanic rocks have enriched isotopic compositions (higher 87Sr/86Sr and lower 143Nd/144Nd), similar to the Early Miocene lavas in the precursory stage of the Japan Sea. The geochemical characteristics of the Oligocene volcanic rocks are consistent with magma generation beneath central Hokkaido area from subduction-modified lithospheric mantle. The felsic andesites from the Middle member have adakite-like characteristics; high Sr/Y ratios, low Y contents, which can be produced in garnet stability field. In contrast to the felsic andesites, the basalts and basaltic andesites in the Lower and Upper members have similar isotopic compositions with systematically different REE abundances. Alkali basalts from the Upper member was inferred to have generated through small degrees of partial melting at pressure >2 GPa (>70 km depth), which is similar to that for the Quaternary NE Honshu back-arc basalts. In comparison with the NE Honshu arc magma, petrological evidence of the Oligocene basalts is consistent with the existence of a thick lithospheric lid beneath central Hokkaido.

Oligocene Laramide deformation in southern New Mexico and its implications for Farallon plate geodynamics

The Silver City Range in southwest New Mexico contains Proterozoic basement rocks that are overlain by a sequence of Paleozoic, Mesozoic, and Paleogene strata. These rocks are folded in a broad, NW-SE–trending, east-facing monocline that lies structurally above an east-directed thrust fault. The youngest rocks folded in the Silver City monocline are similar to other late Eocene and early Oligocene volcanic rocks of the Mogollon-Datil volcanic field; an ash-flow tuff near the bottom of the volcanic sequence gives an 40 Ar/ 39 Ar age on sanidine of 34.9 ± 0.4 Ma (2σ), and another tuff near the top of the section contains zircons that yield a weighted 206 Pb/ 238 U age of 34.6 ± 0.6 Ma (2σ). We interpret similar structures in the Little Burro Mountains, Lone Mountain, and Bayard area, immediately east and west of the Silver City monocline, to all be genetically related to a system of basement-involved thrust faults. Modeling of these structures from the Mangas Valley in the southwest to the Mimbres Valley in the northeast, suggests ∼17% total shortening. We conclude that Laramide shortening was active in southwest New Mexico generally, and the Silver City region in particular, from the Cretaceous until the earliest phase of Mogollon-Datil volcanism beginning at ∼37 Ma, during which time the earliest extension in the southern Rio Grande rift was initiated. The final stage of Laramide shortening, recorded in the Silver City monocline, took place during a lull of volcanism (and extension) from ∼31.5 to ∼29.3 Ma. We explain the contemporaneity of shortening, significant ignimbrite eruptions, and crustal extension as the consequence of intermittent slab breakoff and renewed underthrusting of the downgoing Farallon plate.

The effect of early Alpine thrusting in late-stage extensional tectonics: Evidence from the Kulidzhik nappe and the Pelevun extensional allochthon in the Rhodope Massif, Bulgaria

In the northeastern Rhodope Massif, the Kulidzhik nappe exposes a unique juxtaposition of a high-grade basement allochthon onto a low-grade Mesozoic unit, and the counterpart Pelevun extensional allochthon belonging to the same unit. The Kulidzhik nappe tectonostratigraphy comprises structurally upward: (i) a lower unit consisting of high-grade basement orthogneisses; (ii) a low-grade greenschist-phyllite unit consisting of Jurassic extrusive rocks and metasedimentary rocks; (iii) the nappe allochthon built by the lower high-grade basement unit orthogneisses; and (iv) Eocene sedimentary rocks and Oligocene volcanic cover rocks. The Pelevun extensional allochthon is heterogeneous, and consists of Mesozoic low-grade unit marbles and greenschists and the upper high-grade basement unit. We have combined structure and kinematics, with lithological information and 40Аr/ 39Аr geochronology to constrain the tectonic evolution and regional significance of the Kulidzhik nappe and the Pelevun extensional allochthon. Mineral chemistry reveals igneous phases of the granitic protolith of the allochthonous orthogneisses with textures related to ductile deformation and metamorphism higher than 500 °C. Their trace element patterns are indistinguishable from the high-grade basement orthogneisses in the eastern Rhodope. Mineral chemistry of the metamorphic assemblage in the underlying greenschists is consistent with medium-grade greenschist-facies metamorphism at temperatures well below 450 °C, whose geochemistry defines transitional MORB to IAT affinities with a strong arc imprint. The greenschist's composition is extremely similar to the composition of a supra-subduction zone Jurassic arc extrusive suite that occurs in the low-grade unit of the eastern Rhodope. The structural elements in all of the tectono-metamorphic units and the nappe surface indicate top-to-the NNE tectonic transport. Deformation evolved from ductile to brittle conditions coevally with a progressive decrease from lower amphibolite to weak greenschist-facies metamorphism towards the structural top. Two klippen gave plateau 40Аr/ 39Аr mica ages of 154.23 ± 0.66 Ma and 156.70 ± 0.81 Ma, respectively, which reflect cooling following the greenschist-facies event below ∼ 350 °C, during shallow crustal level thrust emplacement and exhumation of the Kulidzhik allochthon. The hanging-wall Pelevun extensional allochthon preserves an internal NNE-directed ductile tectonic transport trend, but is underlain by a SSW-directed ductile–brittle extensional detachment and has experienced NE-SW brittle extension on high-angle normal faults. An amphibole 40Аr/ 39Аr inverse isochron age of 156.58 ± 0.60 Ma constrains a Late Jurassic upper greenschist-facies tectono-metamorphic event, whereas a white mica plateau age of 39.66 ± 0.47 Ma provides evidence for Middle Eocene cooling and exhumation of the Pelevun extensional allochthon in the hanging wall of the detachment. We relate the Kulidzhik nappe to Late Jurassic crustal deformation during arc-continental margin collision that involved NNE-directed nappe staking and metamorphism of continental margin basement and island arc units. The nappe shares a tectono-metamorphic history with the nappes of the adjacent Strandzha Massif, implying a region-wide early Alpine orogenic system. Our results reveal a record of early Alpine thrust tectonics and show the significance of crustal accretion-related assembly for the tectonic evolution of the Rhodope Massif. Both the Late Jurassic thrusting event and the subsequent Cretaceous thrusting event thickened the Rhodope crust creating crustal instability, which influenced Tertiary crustal extension.

Structural and 40Ar/39Ar age constraints on the Kulidjik nappe: A record of an early Alpine thrust tectonics in the northeastern Rhodope Massif, Bulgaria

In the northeastern Rhodope Massif, a unique juxtaposition of assumed high-grade basement allochthon onto a low-grade Mesozoic unit is exposed in the Kulidjik nappe. The thrusting is interpreted as post-Jurassic to pre-Late Eocene. The tectonostratigraphy comprises the following units: (i) an upper unit of high-grade basement of the eastern Rhodope consisting of various amphibolite facies rocks with metaophiolite lenses; (ii) an overlaying Mesozoic (including Jurassic radiolarians) low-grade unit; (iii) a structurally overlaying allochthon of the Kulidjik nappe; and (iv) unconformable Late Eocene sedimentary and Oligocene volcanic cover rocks. The basement unit contains a NNE-SSW oriented lineation rarely associated with top-to-the NNE ductile shearing. Greenschist facies retrogression of this unit is diagnostic of local thrusting of the overlying low-grade unit. The greenschists of the low-grade unit represent arc metavolcanic rocks overlain by phyllites. The structural elements in this unit have similar attitudes as the basement unit. Kinematic indicators in the low-grade unit demonstrate internal top-to-the NNE ductile shearing during greenschist facies conditions. The allochthon consists of klippen composed of muscovite-albite gneisses. The flat-lying thrust contact with ductile-brittle deformation contains mylonites and cataclasites. Previously reported local top-to-the SSW ductile shearing in the allochthon is probably a consequence of shear instabilities. Deformation-crystallization relationships revealed fabrics in the allochthon compatible with greenschist up to lower amphibolite facies conditions without retrogression and similar ENE-verging fold patterns in the low-grade unit and the allochthon at the thrust contact. Two muscovite samples from distinct klippen yielded inverse isochron 40Ar/39Ar ages of 156.07±1.30 Ma and 162.62±2.69 Ma, which reflect cooling below 350°C of the allochthon associated with its exhumation during emplacement after greenschist-lower amphibolite facies metamorphism. Structural data indicate NNE-directed tectonic transport of the low-grade footwall unit, which is consistent with emplacement in comparable metamorphic conditions, and the thrust direction of the allochthon in the hanging wall. The subduction-accretionary origin of the low-grade unit and the new age results allow us to link this tectono-metamorphic event to Late Jurassic thrust imbrication of arc units on the Rhodope basement, which is the first time that early Alpine thrust tectonics in the northeastern Rhodope Massif have been recognized.

Transitional field clusters from uppermost Oligocene volcanic rocks in the central Walker Lane, western Nevada

Three sections of the Candelaria Hills volcanic sequence, west-central Nevada, appear to have recorded parts of two transitional field records or reversal excursions. Paleomagnetic data and 40 Ar / 39 Ar laser fusion sanidine age estimates for pyroclastic rocks and associated flows show that these rocks recorded the unusual field behavior at about 25.7 Ma and about 23.8 Ma. Fifteen sites yield northeast declination, moderate to shallow negative inclination mean directions and 16 sites yield west to southwest declination, moderate negative inclination directions. Both populations of site mean directions, representing a total of 12 independent eruptive units, are highly discordant to a time-averaged late Tertiary field direction, and neither can be explained by a geologically reasonable magnitude of vertical axis rotation. Virtual paleomagnetic poles (VGPs), estimated from the directional data, lie at low to intermediate latitudes; 29 of the 31 flows at intermediate latitudes (<60°), and 11 at very low latitudes (<30°). Two well-grouped VGP clusters are defined by these data with each cluster roughly corresponding to one of the age groups. Stratigraphically corrected VGPs from most of the 23.8 Ma group roughly cluster at intermediate to low latitudes at about 150°E longitude. The cluster at about 150°E corresponds to VGP clusters that have been interpreted to reflect a long lasting near-dipole configuration during several field reversals. The second stratigraphically corrected cluster lies at intermediate to low latitudes at about 80°E longitude and, notably, is defined by pyroclastic flows of the 25.7 and 23.8 Ma age groups. The VGP data at about 80°E do not fall into any previously identified preferred longitudinal band, however, they are consistent with data from some sedimentary records of reversal excursions in western North America. We recognize that the VGPs returned to a preferred location in both age populations, which we interpret as a preferred directional position, thus reflecting a potentially stable non-dipole component during a complete reversal or a reversal excursion. The observation that the VGPs maintained a preferred location during separate high amplitude events supports the hypothesis that preferred VPG clusters and thus persistent non-dipole field components can factor into the behavior of the geomagnetic field during full reversals or reversal excursions.

Porphyry-Style Alteration and Mineralization of the Middle Eocene to Early Oligocene Andahuaylas-Yauri Belt, Cuzco Region, Peru

Originally known for its Fe-Cu skarn mineralization, the Andahuaylas-Yauri belt of southeastern Peru is rapidly emerging as an important porphyry copper province. Field work by the authors confirms that mineralization in the belt is spatially and temporally associated with the middle Eocene to early Oligocene (~48–32 Ma), calc-alkaline Andahuaylas-Yauri batholith, a composite body with an areal extent of ~300 × 130 km emplaced into clastic and carbonate strata (e.g., Yura Group and Ferrobamba Formation) of Jurassic to Cretaceous age. Batholith emplacement included early-stage, mafic, cumulate gabbro and diorite between ~48 and 43 Ma, followed by pulses of granodiorite and quartz monzodiorite at ~40 to 32 Ma. Coeval volcanic rocks make up the middle Eocene to early Oligocene Anta Formation, a sequence of >1,000 m of andesite lava flows and dacite pyroclastic flows with interbedded volcaniclastic conglomerate. Sedimentary rocks include the red beds of the Eocene to early Oligocene San Jeronimo Group and the postmineralization late Oligocene to Miocene Punacancha and Paruro formations. Eocene and Oligocene volcanic and sedimentary rocks are interpreted to have accumulated largely in both transtensional and contractional synorogenic basins. New and previously published K-Ar and Re-Os ages show that much of the porphyry-style alteration and mineralization along the belt took place during the middle Eocene to early Oligocene (~42–30 Ma). Thus, batholithic magma emplacement, volcanism, and sedimentation are inferred to have accompanied a period of intense deformation, crustal shortening, and regional surface uplift broadly synchronous with the Incaic orogeny. Supergene mineralization is inferred to have been active since the Pliocene on the basis of geomorphologic evidence and a single K-Ar determination (3.3 ± 0.2 Ma) on supergene alunite. The belt is defined by 31 systems with porphyry-style alteration and mineralization, including 19 systems grouped in 5 main clusters plus 12 separate centers, and by hundreds of occurrences of magnetite-rich, skarn-type Fe-Cu mineralization. Porphyry copper stocks are dominated by calc-alkaline, biotite- and amphibole-bearing intrusions of granodioritic composition, but monzogranitic, monzonitic, quartz-monzonitic, and monzodioritic stocks occur locally. Hydrothermal alteration includes sericite-clay-chlorite, and potassic, quartz-sericitic, and propylitic assemblages. Calcic-potassic and advanced argillic alteration associations are locally represented, and calc-silicate assemblages with skarn-type mineralization occur where carbonate country rocks predominate. Porphyry copper deposits and prospects of the belt range from gold-rich, molybdenum-poor examples (Cotabambas), through deposits carrying both gold and molybdenum (Tintaya, Los Chancas), to relatively molybdenum-rich, gold-poor end members (Lahuani). Gold-only porphyry systems are also represented (Morosayhuas). Gold-rich porphyry copper systems are rich in hydrothermal magnetite and display a positive correlation between Cu and Au in potassic alteration. The bulk of the hypogene Cu (-Au, -Mo) mineralization occurs in the form of chalcopyrite and bornite, in intimate association with early-stage potassic alteration which, in many deposits and prospects, is variably overprinted by copper-depleting sericite-clay-chlorite alteration. Most porphyry copper systems of the belt lack economically significant zones of supergene chalcocite enrichment. This is due primarily to their relatively low pyrite contents, the restricted development of quartz-sericitic alteration, and the high neutralization capacities of both potassic alteration zones and carbonate country rocks as well as geomorphologic factors. Leached cappings are irregular, typically goethitic, and contain copper oxide minerals developed by in situ oxidation of low-pyrite, chalcopyrite (-bornite) mineralization. Porphyry copper-bearing stocks emplaced in the clastic strata of the Yura Group and certain phases of the Andahuaylas-Yauri batholith may develop appreciable supergene chalcocite enrichment in structurally and lithologically favorable zones. A model for the region suggests that the calc-alkaline magmas of the Andahuaylas-Yauri batholith and subsequent porphyry-style mineralization were generated during an event of subduction flattening which triggered the crustal shortening, tectonism, and uplift assigned to the Incaic orogeny. Shortening of the upper crust would have impeded rapid magma ascent favoring storage of fluid in large chambers which, at the appropriate depth in the uppermost crust, would have promoted large-scale porphyry copper emplacement. Geodynamic reconstructions of the late Eocene to early Oligocene period of flat subduction in the central Andes suggest that emplacement of the Andahuaylas-Yauri batholith took place at an inflection corridor in the subduction zone broadly coincident with the position of the present-day Abancay deflection. Similarly, evidence from southeastern Peru suggests that the Andahuaylas-Yauri belt may be continuous with the late Eocene to early Oligocene porphyry copper belt of northern Chile and that the process of subduction flattening in southern Peru also may have taken place in northern Chile between ~45 and 35 Ma.

Paleomagnetic and paleointensity study of Oligocene volcanic rocks from Chihuahua (northern Mexico)

A detailed rock-magnetic, paleomagnetic and paleointensity study was carried out on Oligocene volcanic formations from Chihuahua (northern Mexico) in order to obtain some decisive constraints for the tectonic evolution of the southern Cordilleran Orogenic Belt of North America and for the geomagnetic field strength during Oligocene time. Age of the volcanic units lies between 33 and 25 Ma according to available radiometric data. Rock-magnetic experiments reveal that remanence is carried in most cases by Ti-poor titanomagnetite, resulting of oxy-exsolution of original titanomagnetite during the initial flow cooling. Unblocking temperature spectra and relatively high coercivities point to ‘small’ pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. Nine sites yield reverse polarity magnetization, three are normally magnetized and one unit shows intermediate polarity magnetization. An evidence of strong lightning-produced magnetization overprint was detected for one site. Combining all paleomagnetic data currently available for northern Mexico, we obtained a well-defined Eocene–Oligocene mean paleomagnetic direction with I=48.5°, D=337.1°, k=20, α 95=6.8°, N=24, which deviates counterclockwise from the expected direction estimated from the North American apparent polar wander path. This suggests a vertical-axis tectonic counterclockwise rotation of about 16° relative to stable North America. Transition from Laramide compression to Basin and Range extension occurred during Oligocene, around 32–30 Ma. Tectonic rotation may then reflect east–northeast extension in the mid- to late-Cenozoic. Twenty-three samples were pre-selected for Thellier palaeointensity experiments because of their low viscosity index, stable remanent magnetization and reasonably reversible continuous thermomagnetic curves. Only 12 samples, coming from three individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 3.96 to 4.65×10 22 Am 2. Combination of Mexican data with the available comparable quality Oligocene paleointensity results yield a mean VDM of 4×10 22 Am 2, which is comparable or slightly higher than the mean VDMs calculated for the Mesozoic low field period, but significantly lower than present day value. This low intensity may correlate with the relatively high paleosecular variation rate invoked around 30 Ma by several authors.