Andesite Research Articles

Petrological characterization of the Cenozoic igneous rocks of the Tafresh district, central Urumieh-Dokhtar Magmatic Arc (Iran)

We report a petrographic and whole-rock geochemical characterization of the Cenozoic volcanic rocks cropping out in the Tafresh area of the central Urumieh-Dokhtar Magmatic Arc of Iran. The investigated rocks range mainly from basaltic andesite to dacite, and are considered to be genetically linked by (mostly) closed-system evolutionary processes involving fractionation of ferromagnesian minerals and plagioclase first, then of plagioclase and lesser amphibole (plus minor clinopyroxene) and finally of plagioclase with lesser alkali feldspar and minor amphibole. These represent a typical calcalkaline series emplaced in a subduction-related setting, producing the observed LILE-enriched and HFSE-depleted geochemical signature. The basaltic andesite compositions likely derived from an unsampled hydrous primitive melt equilibrated in a spinel-bearing metasomatized peridotite source, evolving at shallow to moderate crustal depths.

Thermographic analysis of failure for different rock types under uniaxial loading

Mining activities focus on the production of mineral resources for energy generation and raw material requirements worldwide and it is a known fact that shallow reserves become scarce. For this reason, exploration of new resources proceeds consistently to meet the increasing energy and raw material demand of industrial activities. Rock mechanics has a vital role in underground mining and surface mining. Devices and instruments used in laboratory testing to determine rock mechanics related parameters might have limited sensing capability of the failure behavior. However, methodologies such as, thermal cameras, digital speckle correlation method and acoustic emission might enable to investigate the initial crack formation in detail. Regarding this, in this study, thermographic analysis was performed to analyze the failure behaviors of different types of rock specimens during uniaxial compressive strength experiments. The energy dissipation profiles of different types of rocks were characterized by the temperature difference recorded with an infrared thermal camera during experiments. The temperature increase at the failure moment was detected as 4.45oC and 9.58oC for andesite and gneiss-schist specimens, respectively. Higher temperature increase was observed with respect to higher UCS value. Besides, a temperature decreases of about 0.5-0.6oC was recorded during the experiments of the marble specimens. The temperature change on the specimen is related to release of radiation energy. As a result of the porosity tests, it was observed that increase in the porosity rate from 5.65% to 20.97% can be associated to higher radiation energy released, from 12.68 kJ to 297.18 kJ.

Effect of rock mineralogy on mortar expansion

Alkali-silica reaction (ASR) is among one of the most important damaging mechanisms in concrete, depending primarily on aggregates which contain reactive minerals. However, expansion in concrete may not directly relate to the reactive minerals. This study aims to investigate the influence of ASR and the expansion of mortar bars depending on aggregate type containing various components such as quartz, clay minerals (montmorillonite and kaolinite) and micas (muscovite and biotite). In this study, the accelerated mortar bar tests (AMBT) were performed in two conditions (mortar bars in the same and sole NaOH solutions). Petrographic thin section studies, X-ray diffraction (XRD) analysis (Rietveld method), scanning electron microscopy (SEM) and chemical analyses were carried out. This study showed that quartzite bars led to increase in expansion values of mortar bars in diabase-1 and andesite when these were in the same NaOH solution. However, three samples (basalt, quartzite and claystone) were found having ASR expansion based on the AMBT when the special molds were used for each sample. SEM study revealed that samples which exhibit highest expansions according to AMBT had a generally rough surface and acicular microstructures in or around the micro-cracks. Basalt and quartzite showed more variable in major oxides than those of other samples based on the chemical analyses, SEM studies and AMBT. This study revealed that the highest expansions were observed to source not only from reactive aggregates but also from alteration products (silicification, chloritization, sericitization and argillisation), phyllosilicates (muscovite, biotite and vermiculite) and clays (montmorillonite and kaolinite).

Extensional magmatism in a continental collision zone,Tafresh area, western central Iran: structural, geochemical and mineralogical considerations

The Tafresh area, located on the middle part of Urumieh-Dokhtar magmatic arc, is a suitable area for considering the effects of tectonic structures on development of the volcanic facies in the magmatic provinces. The main structure in the area is a dextral fault (so called Rahjerd Fault) that has produced the local extensional basins and also the parallel dyke swarms (or feeder dykes) on its both sides. So, the pyroclastic deposits, basaltic-andesitic lava flows, parallel dyke swarms were appeared by explosive subaqueous and then effusive subaerial eruptions and eventually a dioritic stock intruded in the volcanic pile. Petrologically, the magmatic rocks are belonged to the calcalkaline suite that had been changed and evolved in the magma chambers of local extensional basins in a collisional tectonic setting. The mineral chemistry and geochemical modeling as well as the coexistence of different mineral paragenesis of plagioclases (An47-72 and An17-37), pyroxenes (diopside and pigeonite) and amphiboles (magnesiohastingsite and magnesiohornblende) reveal that the AFC process is a dominant process in the magma chamber. Also, according to geothermobarometric calculations, the investigated volcanic rocks of the area could be grouped into two types: one type with higher P-T (>6Kbar and about 900 °C) including Eocene pigeonite-magnesiohastingsite bearing andesites (PHA) and Miocene andesites and other with lower P-T (1-4 Kbar and 600- 800 °C) including the Eocene diopside-magnesiohornblende bearing andesites (DHA) and the dioritic stock.

Detection of near surface rock fractures using ultrasonic diffraction techniques

Ultrasonic Time-of-Flight Diffraction (TOFD) techniques are useful methods for non-destructive evaluation of fracture characteristics. This study focuses on the reliability and accuracy of ultrasonic diffraction methods to estimate the depth of rock fractures. The study material includes three different rock types; andesite, basalt and ignimbrite. Four different ultrasonic techniques were performed on these intact rocks. Artificial near-surface fracture depths were created in the laboratory by sawing. The reliability and accuracy of each technique was assessed by comparison of the repeated measurements at different path lengths along the rock surface. The standard error associated with the predictive equations is very small and their reliability and accuracy seem to be high enough to be utilized in estimating the depth of rock fractures. The performances of these techniques were re-evaluated after filling the artificial fractures with another material to simulate natural infills.

Petrology and geochemistry of sheeted dykes and pillow lavas from the Sabzevar Ophiolitic Mélange (Northeast Iran): new Constraints for the Late Cretaceous evolution of the Neo-Tethys Oceanic Basin between the Central Iranian Microcontinent and Eurasia

TheSabzevarophiolite-bearingmelangemainlyincludestectonic slicesofophiolitesregardedas remnants of the Late Cretaceous oceanic basin located between the northern margin of the Central Iran Microcontinent and the southern margin of the Turan (Eurasia) plate, as well as remnants of its northern continental margin. We present here new mineral chemistry and whole rock chemical data on the sheeted dyke complex and pillow lavas series from this ophiolite. The sheeted dykes consist of basalts and basaltic andesites showing island arc tholeiitic (IAT) affinity (Group 1). They display low TiO 2 (0.37-0.81 wt%) contents and N-MORB normalized incompatible element patterns featuring Th enrichment and Ta and Nb depletion. The pillow lavas show different chemical features compared to the sheeted dykes and can be subdivided in two distinct geochemical groups. The first Group of pillow lavas (Group 2) mainly consists of andesites showing a clear calc-alkaline (CA) affinity with low TiO 2 (0.71-0.81 wt%) contents. Their N-MORB normalized incompatible element patterns display marked enrichments in Th, U, La, and Ce, and depletion in Ta, Nb, and Ti. The second Group of pillow lavas (Group 3) is mainly represented by alkaline basalts with high TiO 2 (1.54-3.45 wt%) contents. N-MORB normalized incompatible element patterns show significant Th, Ta, and Nb enrichments. Trace element modeling shows that CA primary melts were generated by 20% partial melting of a depleted lherzolite mantle source that had been enriched by continental crust chemical components. IAT basalts were generated from various degrees of partial melting of either depleted lherzolitic or harzburgitic mantle sources bearing no or negligible influence from continental crust components. Alkaline basalts were generated by polybaric partial melting of an enriched within-plate oceanic mantle source. A new tectono-magmatic model that can explain the different rock association cropping out in the Sabzevar ophiolite is proposed. This model implies the existence, on the southern margin of the Turan plate during the Late Cretaceous, of a continental arc-forearc system where CA and IAT rocks were erupted, respectively. Alkaline basalts were erupted in a subduction-unrelated oceanic basin and then tectonically accreted to the forearc as a consequence of the collision between the seamount and the arc-forearc system.

Geochemical features of magmatic rocks and mineral deposit genesis of xinjiang aktas gold deposit

There is a close spatial relationship between the ore body of Aktas gold deposit and the contact zone of the rock body of different periods. This paper by conducting a detailed study of geological and geochemical characteristics with 15 samples of andesitic porphyrite, diorite, slanted granite and medium fine-grained granite, analyzes the relationship between the geochemical features and formation mechanism of magmatic rocks and its mineralization process. It is found that the average content of SiO2 in volcanic rock of Aktas is 49.69wt%, which suggests that those volcanic rocks are alkaline or calc-alkaline rocks and has no direct influence on mineralization. Besides, the average content of SiO2 in intrusive rock is 63.17wt%, which means that this kind of rock belongs to medium acid rock, the main source of metallogenic material. The paper also found that the light rare earth is relatively rich while heavy rare earth few, and the LREE/HREE ration of volcanic rock is lower than that of intrusive rock, showing that some crustal substances may be involved in the ascending process of deep magmatic rocks and mixed with mantle material. The source substance of intrusive rock mainly comes from the earth's mantle, therefore the intrusive rock, especially the plagiogranite in this area, may be closely related to the formation of Aktas gold deposit. Combining with the evolution mechanism theory of regional tectonism, this paper concludes that the metallogenic material of Aktas gold deposit originates from the combined action of the ore-bearing fluid from deep mantle and the fluid from shallow crust.

Post-collisional Plio-Pleistocene Anar - Dehaj adakitic subvolcanic domes in the central volcanic belt of Iran: geochemical characteristics and tectonic implications

In the area between Anar and Dehaj, located in the Urumieh–Dokhtar magmatic assemblage (UDMA), there are some Plio-Pleistocenic subvolcanic porphyritic andesitic-dacitic domes. The subvolcanic domes, as a part of Dehaj-Sarduieh belt in Kerman province, show tafoni erosion and magma mingling evidences. Petrographically, the domes are andesite and trachyandesite in composition and are characterized by microlitic-porphyritic, glomeroporphyritic, trachytic and aphanitic textures. Mineralogically, the domes consist of plagioclase, amphibole, clinopyroxene and opaque minerals. Geochemical studies reveal that Anar-Dehaj subvolcanic domes can be divided two groups, named group 1 and group 2, on the basis of the silica content. Group 1 shows high-K calc- alkaline andesite- trachyandesite composition; whereas group 2 is medium-K calc-alkaline dacite. The absence of a distinct Eu anomaly in all samples suggests contemporaneous crystallization of plagioclase and amphibole, and/or oxidation state of magma. Trace element discrimination diagrams along with chondrite-normalized rare earth element patterns show that the Anar- Dehaj subvolcanic rocks formed in a subduction related environment. Moreover, high Sr/Y and La/Yb ratios along with low Y and HREE contents are consistent with adakitic natures. It is suggested that the Anar- Dehaj subvolcanic domes formed in a post-collisional setting due to slab melting or underplating of basaltic magmas under thick Plio-Pleistocine continental crust.

Mineralization and trace element distribution in pyrite using EMPA in exploration drill holes from Cheshmeh Zard gold district, Khorasan Razavi Province, Iran

ناحیه مطالعاتی در شمال استان خراسان رضوی و 45 کیلومتری جنوب نیشابور قرار دارد. رگه های طلادار درون سنگهای گرانیت، گرانودیوریت، گرانودیوریت پورفیری، آندزیت، برش و توف نفوذ کرده اند. رگه ها از جنس کوارتز و کلسیت هستند. مناطق اصلی دگرسانی شامل منطقه سرسیتیک، سیلیسی، پروپلیتیک و منطقه کربنات است. کانی زایی به‌طور نزدیکی با دگرسانیهای سیلیسی و سرسیتیک به شکل رگه‌چه و با منطقه پروپلیتیک به شکل افشان همراه است. کانی شناسی کانیهای باطله کوارتز، کلسدونی، سرسیت، آدولاریا، کلسیت، دولومیت، ایلیت، دیکیت، آلبیت، کائولینیت و کلریت است. کانی زایی رگه ای عمدتا با رگه‌چه ها، برش، افشان و استوک ورک همراه هستند. رگه‌چه ها از پیریت، مارکازیت، آرسنوپیریت و کمتر کالکوپیریت، اسفالریت، گالن، مگنتیت و هماتیت تشکیل شده اند. بیشترین عیار طلا در رگه های سیلیسی دیده می شود. پیریت کانی سولفیدی اصلی در کانی سازی اولیه است. سه نوع پیریت بر اساس ترکیب شیمیایی شناسایی شده است: پیریت خالص، پیریت غنی از آرسنیک و پیریت تیتانیوم- وانادیوم دار. تجزیه های ریزکاوشگر الکترونی بیشترین غلظت آرسنیک، تیتانیوم و وانادیوم را به ترتیب 62/3، 91/3 و 53/0 درصد وزنی در پیریتهای رگه های طلادار نشان می دهند. طلا معمولا همراه با پیریت آرسنیک دار و پیریت تیتانیوم- وانادیوم دار است. براساس مطالعه بافت و ترکیب پیریتها، رگه‌چه‌های پیریت آرسنیک دار همراه با کانیهای سولفیدی آرسنوپیریت، کالکوپیریت، گالن، اسفالریت در دمای بالا و با کاهش دما و فشار، پیریتهای فرامبوئیدال با حاشیه حاوی آرسنیک و طلا شکل گرفته اند و در ادامه پیریتهای درشت تیتان دار به‌وجود آمده اند. پیریتهای خالص و تأخیری درشت بلور آخرین فاز کانی ساز در منطقه هستند.

Corrigendum to 'Magma Evolution in the Primitive, Intra-oceanic Tonga Arc: Petrogenesis of Basaltic Andesites at Tofua Volcano' and 'Magma Evolution in the Primitive, Intra-oceanic Tonga Arc: Rapid Petrogenesis of Dacites at Fonualei Volcano'

Primitive, Intra-oceanic Tonga Arc: Petrogenesis of Basaltic Andesites at Tofua Volcano’ and ‘Magma Evolution in the Primitive, Intra-oceanic Tonga Arc: Rapid Petrogenesis of Dacites at Fonualei Volcano’ J. T. Caulfield*, J. Blichert-Toft, F. Albarede and S. P. Turner Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia and Laboratorie de Geologie de Lyon, Ecole Normale Superieure de Lyon, Universite Claude Bernard Lyon 1, CNRS UMR 5276, 46 Allee d’Italie, 69007 Lyon, France

IDENTIFATION OF LANSLIDE DIMENSION BASED ON SEISMIC REFRACTION SURVEY IN TENGKILK VILLAGE, KARANGANYAR, CENTRAL JAVA

A seismic refraction survey was conducted at a landslide site, which had destroyed a road and several houses in Tengklik Village, Karanganyar, Central Java, Indonesia. The survey provided insight into the slope stratigraphy, as well as the dimensions of sliding area and depths of the slope failure planes. Field observations and drill-hole data were used to verify the accuracy of seismic refraction data. The seismic velocity measured in the top - sliding layer of sandy silt was less than 900 m/sec, while that in the underlying layer of silty sandstone ranged from 2400 to 900 m/sec. The stable bedrock of andesite breccia underneath the sliding soil layers was about 2500 m/sec. It was apparent that the failure surface was irregular, varies in depth. This failure plane exists at the contact of the andesitic breccia (bedrock) and the sliding layer of silty sandstone. The thickness of the sandy silt ranged from 3 to 5 m at lower part of sliding mass, while at the upper part ranged from 4 to 8 m. The estimated depth of the failure surface ranged from 12 to 24 m. The volume of mass movement is estimated about 111.386 m3 . It was also found that the presence of mountain block at the bedrock of andesitic breccia may resist the movement.

پترولوژی، ژئوشیمی و کانی سازی سنگهای آتشفشانی ترشیری و توده های نفوذی و نیمه نفوذی همراه آنها در منطقه ارغش- قاسمآباد (شمالشرق ایران) با گرایشی ویژه به سن و منشأ گرانیت ها

منطقه ارغش- قاسم آباد در شمال‌شرق ایران و در حاشیه زون ساختاری سبزوار قرار دارد. رخنمونهای ترشیری شامل گرانیت و کوارتز مونزودیوریت پورفیری همراه با سنگهای آتشفشانی از نوع داسیت و آندزیت هستند. در منطقه مورد مطالعه، دایک‌هایی از نوع کوارتز گابرو و کوارتز مونزودیوریت، سنگهای قدیمی تر را مورد نفوذ قرار داده اند. گرانیت‌ها و سنگهای آتشفشانی، میزبان رگه های کوارتز-‌کلسیت طلا و آنتیموان دار هستند. رگه های کانی سازی عمدتا در اطراف دایک‌ها و در نیمه جنوبی منطقه دیده می شوند. مطالعات ژئوشیمیایی حاصل از این پژوهش نشان می دهد که گرانیت‌ها ماهیت کالک‌آلکالن پتاسیم بالا داشته، و از نوع گرانیتوئیدهای انتقالی بین دو نوع A و I هستند. سنگهای آتشفشانی نیز با ترکیب آداکیتی، سرشار از استرانسیم و باریم می‌باشند. دایک‌های منطقه مورد مطالعه، ویژگیهای سنگهای لامپروفیری را نشان می دهند. سن سنجی اورانیم- سرب بر روی زیرکن در گرانیت، سن 2/2 ± 4/55 میلیون سال (ائوسن زیرین) را نشان می دهد. نسبتهای 87Sr/86Sr و eNd اولیه در گرانیت‌ها به‌ترتیب 704142/0 و 84/5+ می-باشند. داده های نسبتهای ایزوتوپی 87Sr/86Sr اولیه، وجود ماگمایی با منشأ گوشته تهی‌شده را تأیید می کند. سن به‌دست آمده نشان می دهد کانی سازی طلا- آنتیموان از ائوسن زیرین جوانتر است. براساس شواهد صحرایی، ارتباط کانی سازی با دایک‌های لامپروفیری، بیش از ارتباط آن با سنگهای آداکیتی است.

MINERALOGY AND ORIGIN OF COPPERGOLD BEARING SKARN WITHIN THE BATU HIJAU PORPHYRY DEPOSIT, SUMBAWA ISLAND, INDONESIA

The aim of this study ist o emphasize on the origin of copper-gold bearing skarn mineralization at the Batu Hijau deposit which is located at the southwestern corner of Sumbawa Island, Indonesia. Although most skarn are derived from limestones, nolimestone is known in the Batu Hijau deposit. Ca-rich andesitic volcaniclastic host rocks favor skarn alteration within the Batu Hijau deposit. The t ype of skarn can be classified as calcic-exoskarn, and locally controlled by faults and fractures. Two major stages consisting of four sub-stages of skarn forming processes can be divided by the mineral assemblages of skarn as prograde and retrograde stages. The prograde skarn consists of clinopyroxene and garnet ± magnetite formed at the trapping temperature of 440°-480 °C with 34-38 wt% NaCl eq. while retrograde skarn alteration is dominated by Fe-rich minerals such as amphibole and epidote formed at the trapping temperature down to 340°-360°C with 4-8 wt% NaCl eq. Opaque minerals include chalcopyrite, pyrite, sphalertite, and minor galena and bismuth- telluride. Gold was precipitated in the retrograde stage associated with bismuthtelluride minerals. The sulfur isotope data of skarn ranges from +0.1 to +1.7‰ (sulfide), and porphyry systems range from 0.04 to1.4‰ and 10‰ to 15‰ (sulfide and sulfate respectively). According to the fluid inclusion and sulfur isotope data, the origin of skarn and porphyry system can be suggested to be that the magmatic origin. Furthermore, the sulfur isotope data of the deposit evidently shows that a porphyry- related skarn mineralization exhibiting transition from one style to the next can be relatively rapid. The result of this research has indicated that the range of porphyry-related deposits, skarn and porphyry systems can form during a single prolonged hydrothermal event.

Alteration, zoning model, and mineralogical structure considering lithogeochemical investigation in Northern Dalli Cu–Au porphyry

Dalli Cu–Au porphyry deposit was occurred in the igneous diorite, quartz diorite porphyry (QDP), and volcanic rocks such as porphyritic amphibole andesite, andesite (AND), dacite, and pyroclastics during the late Miocene to Pliocene. Regolith investigations and Advanced Spaceborne Thermal Emission and Reflection Radiometer images were used to identify the anomalous areas. According to lithogeochemical survey (from boreholes and trenches) in Northern Dalli Cu–Au porphyry, the potassic, chlorite, sericite, propylitic, and argillic alterations have been found and mineralization was basically associated with potassic and quartz–sericite alterations. The alteration is dominantly moderate quartz chlorite ± sericite magnetite with 1–10 mm wide quartz ± magnetite veinlets. The elevated copper–gold values are correlated with density of stockworking and mineralization. The intensity of the mineralization is high in the contact of QDP and AND with increases in pyrite and chalcopyrite values. Malachite, native Cu, and bornite were used to identify supergene, transition, and hypogene zone. In addition, molybdenum increased near to the center of granodiorite intrusion. And besides, from depth to surface in DDH03 and wall rock to mineralization zones, a sequence of Mo→Cu (Au)→Au (Cu) was recorded and the mineralization temperature cooled down (from high to low). The alteration is characterized by specific pattern and structure in Dalli Cu–Au porphyry deposit. The alteration model was followed from the modified Lowell and Gilbert model. The porphyry is stockworked by quartz veins and by quartz magnetite veins. Vein distribution and ore mineralogy vary between the different alteration zones. Due to the formation of an iron cap in the supergene, especially in the southern hills, supergene grade was higher than hypogene zone. Also, hematite, as a dominant Fe oxide in DDH03 borehole with minor limonite, jarosite, and goethite created thickness about 150–270 m in supergene zone; finally, this finding show a possibility of an extensive mineralization.

Biological degradation of pyrogenic organic matter in temperate forest soils

Pyrogenic organic matter (PyOM), derived from the incomplete combustion of plant biomass and fossil fuels, has been considered one of the most stable pools of soil organic matter (SOM) and a potentially important terrestrial sink for atmospheric CO2. Recent evidence suggests that PyOM may degrade faster in soil than previously thought, and can affect native SOM turnover rates. We conducted a six-month laboratory incubation study to better understand the processes controlling the degradation of PyOM in soils using dual-enriched (13C/15N) PyOM and its precursor wood (Pinus ponderosa). We examined the effects of soil type and inorganic N addition on PyOM and wood C and N mineralization rates, microbial C utilization patterns, and native SOM turnover rates. PyOM charred at 450 °C or its precursor pine wood was incubated in two temperate forest subsoils with contrasting short range order (SRO) clay mineralogy (granite versus andesite parent material). Duplicates of experimental treatments with and without PyOM added were sterilized and abiotic C mineralization was quantified. In a second incubation, PyOM or wood was incubated in granitic soil with and without added NH4NO3 (20 kg N ha−1). The fate of 13C/15N-enriched PyOM and wood was followed as soil-respired 13CO2 and total extractable inorganic 15N. The uptake of 13C from PyOM and wood by soil microbial community groups was quantified using 13C-phospholipids fatty acids (PLFA). We found that (1) The mean residence time (MRT) of PyOM-C was on a centennial time scale (390–600 yr) in both soil types; (2) PyOM-C mineralization was mainly biologically mediated; (3) Fungi more actively utilized wood-C than PyOM-C, which was utilized by all bacteria groups, especially gram (+) bacteria in the andesite (AN) soil; (4) PyOM-N mineralization was 2 times greater in granite (GR) than in AN soils; (5) PyOM additions did not affect native soil C or N mineralization rates, microbial biomass, or PLFA-defined microbial community composition in either soil; (6) The addition of N to GR soil had no effect on the MRT of C from PyOM, wood, or native SOM. The centennial scale MRT for PyOM-C was 32 times slower than that for the precursor pine wood-C or native soil C, which is faster than the MRT used in ecosystem models. Our results show that PyOM-C is readily utilized by all heterotrophic microbial groups, and PyOM-C and -N may be more dynamic in soils than previously thought.

Evolution of Rajabasa Volcano in Kalianda Area and Its Vicinity, South Lampung Regency

DOI: 10.17014/ijog.v7i1.132Rajabasa Volcano (± 1281 m) and Lampung Tuff, located in the South Lampung Regency, is the main point in order to understand the evolution of Quaternary volcanism in the area. A remote sensing analysis and field geologic work are the methods of the study. The volcanism began with the construction period of the Pre-Rajabasa composite cone which was followed by the destruction period of the cone to form the Pre-Rajabasa Caldera having ca. 25 km in diameter. The present Rajabasa Volcano, along with cones of flank eruptions and monogenesis, has appeared in the Pre-Rajabasa Caldera depression. Those volcanic activities are considered as the second construction period. During the first and the second construction periods, basaltic to andesitic lava flows, pyroclastic breccias, and tuffs were erupted. The Rajabasa eruption points moved in WNW - ESE direction, which were possibly controlled by a subsurface weak zone. The Pre-Rajabasa Caldera erupted voluminous Lampung Tuffs having rhyolite in composition, and they are considered as a combination of pyroclastic falls, flows, and surges, or pyroclastic density currents.

Mineralogical, IR-spectral and geochemical monitoring of hydrothermal alteration in a deformed and metamorphosed Jurassic VMS deposit at Arroyo Rojo, Tierra del Fuego, Argentina

Fil: Biel, C.. Universidad de Zaragoza; Espana. Productos Minerales para la Industria S.A.; Espana

Contact relationships of Tertiary volcanic units from the Andes of central Chile (33°S): a reinterpretation based on 40Ar/39Ar dating.

Se dan a conocer ocho edades 40Ar/39Ar de rocas de dos sectores de la Cordillera de los Andes cerca de los 33°S donde se presentan unidades volcanicas separadas por discordancias angulares. Estos sectores corresponden a la Cuesta de Chacabuco en la continuacion norte del Valle Central y al Cerro Las Ollas en la parte oeste de la Cordillera de los Andes. En Cuesta de Chacabuco se reconocio la unidad Chacabuco, formada principalmente por basaltos, andesitas basalticas y rocas sedimentarias clasticas continentales. Esta unidad esta cubierta, con discordancia angular, por flujos piroclasticos de la unidad Algarrobo, un complejo volcanico erosionado. Una edad plateau 40Ar/39Ar (roca total, basalto) de 28,8 ± 0,3 Ma fue interpretada como la mejor estimacion de la edad del volcanismo para la unidad Chacabuco, mientras que las edades plateau obtenidas sobre anfibolas de una lava de andesita basaltica y de un dique andesitico para la unidad Algarrobo, fueron 19,6 ± 0,3 y 18,6±0,4 Ma, respectivamente. Se detecto asi un hiatus del orden de 8,6 Ma entre estas dos unidades. El Cerro Las Ollas ha sido un lugar clasico en la discusion acerca de la naturaleza del contacto entre unidades que han sido atribuidas a las formaciones Abanico y Farellones el que ha sido interpretado como discordancia de plegamiento y erosion o como contacto tectonico. Las nuevas edades obtenidas sobre biotita en tobas muestran una continuidad temporal para rocas situadas bajo y sobre el plano de contacto, evidenciando la pertenencia de estas tobas a un mismo evento volcanico. La mejor estimacion de edad para este evento es la edad media ponderada de 20,1 ± 0,1 Ma. En consecuencia, la relacion discordante observada en el cerro Las Ollas puede ser interpretada como intraformacional.

Upper cretaceous magmatic suites of the Timok magmatic complex

The Upper Cretaceous Timok Magmatic Complex (TMC) developed on a continental crust composed of different types of Proterozoic to Lower Cretaceous rocks. The TMC consists of the magmatic suites: Timok andesite (AT) - Turonian-Santonian, Metovnica epiclastite (EM) - Coniacian-Campanian, Osnic basaltic andesite (AO) and Jezevica andesite (AJ) - Santonian-Campanian, Valja Strz plutonite (PVS) - Campanian and Boljevac latite (LB). The sedimentary processes and volcanic activity of the TMC lasted nearly continuously throughout nearly the whole Late Cretaceous. The sedimentation lasted from the Albian to the Maastrichtian and the magmatism lasted for 10 million years, from the Upper Turonian to the Upper Campanian. The volcanic front migrated from East to West. The volcanic processes were characterized by the domination of extrusive volcanic facies, a great amount of volcanic material, a change in the depositional environment during the volcanic cycle, sharp facial transitions and a huge deposition of syn- and post-eruptive resedimented volcaniclastics.

Paleogene adakitic rhyolite to high-Mg andesites and Early Cretaceous adakitic zoned plutons in the Kitakami Mountains, Japan

花崗岩質大陸地殻の形成機構を明らかにすることは，地球の進化過程の解明に通じる重要な研究課題である．本案内書では，北上山地の前期白亜紀アダカイト質累帯深成岩体と古第三紀浄土ヶ浜流紋岩類の主要な岩体について，岩石学的特徴とその成因について述べた．これらのアダカイト類の岩石化学的多様性の成因には，スラブメルトとマントルおよび下部地殻との反応の程度の差が主要な役割を果たしていたと考えられ，スラブメルトの発生から上昇・定置に至る現象の解明に重要な位置を占めると考えられる．