Andesitic Dykes Research Articles

MARIALITIC SCAPOLITE OCCURENCES FROM THE KIMMERIA-LEFKOPETRA METAMORPHIC CONTACT, XANTHI (N. GREECE)

Emplacement of the Xanthi Plutonic Complex within the Rhodope Massif of N. Greece created an extensive metamorphic aureole around the plutonite. The aureole contains two areas of intense scapolitization in the contacts between granodiorite and biotitegneiss and between monzonite and sandstone, the latter cross-cut by andesite dykes. This paper reports the results of a mineralogical and geochemical study into the formation of the scapolites and particularly the nature of the plutonite-derived hydrothermal fluids from which scapolites were formed.

Volcanic sequence in Late Triassic – Jurassic siliciclastic and evaporitic rocks from Galeana, NE Mexico

In northeastern Mexico, volcanic rocks interbedded with Late Triassic–Jurassic siliciclastic and evaporitic strata have been linked to magmatic arcs developed in the Pangea western margin during its initial phase of fragmentation. This work provides new petrographic and geochemical data for volcanism included in the El Alamar and Minas Viejas formations outcropping in the Galeana region. Andesitic dykes and sills (n= 10) in the El Alamar redbeds show SiO2= 47.5–59.1% and MgO= 1.2–4.2%, as well as a geochemical affinity to island arc magmas. This work represents the first report of this tectonic setting in the region. Geological and petrographic evidence suggest that this arc system likely developed after ~220 and before ~193Ma. Trachy-andesitic and rhyodacitic domes (n= 20) associated with the Minas Viejas gypsum-carbonates sequence show SiO2= 61.8–82.7% and MgO= 0.1–4.0% with a tectonic affinity to continental arc. A rhyodacite sample from this region has been dated by U-Pb in zircon, yielding an age of 149.4 ± 1.2Ma (n= 21), being the youngest age related to this arc. Finally, we propose a threestep model to explain the tectonic evolution from Late Triassic island arc to Jurassic continental arc system in the northeastern Mexico.

Analysis of fabric evolution and metamorphic reaction progress at Lago della Vecchia-Valle d’Irogna, Sesia-Lanzo Zone, Western Alps

ABSTRACTThe Lago della Vecchia-Valle d’Irogna rocks are part of the Eclogitic Micaschists Complex (EMC) of the Sesia-Lanzo Zone, western Austroalpine domain. The 1:10,000 scale map includes metaintrusive, minor micaschist, banded gneiss, and metabasic boudins. The multiscale structural analysis reveals successive magmatic and tectono-metamorphic stages: during M0 the metaintrusive protoliths emplaced; D1 took place under eclogite-facies conditions; during D2 stage, a pervasive foliation developed under retrograde blueschist-facies conditions; D3–D4 and D5 structures developed under greenschist-facies conditions; during M6 andesitic dykes intruded. The mapped degree of fabric evolution (FE) and metamorphic transformation (MT) related to D2-foliation shows that the MT was not only controlled by bulk rock and mineral compositions, but also by FE. The development of a pervasive blueschist-facies D2-foliation is in contrast with the eclogitic dominant fabric generally recorded in the EMC. This difference suggests that FE and MT are potentially responsible for km-scale heterogeneities in the tectono-metamorphic record.

Ediacaran (∼620 Ma) high-grade regional metamorphism in the northern Arabian Nubian Shield: U-Th-Pb monazite ages of the Elat schist

Ediacaran times witnessed a hemisphere-scale orogenesis, forming the extensive Pan-African mountain ranges and resulting in the final assembly of the Gondwana supercontinent. The Elat metamorphic association (S Israel), located at the northernmost tip of the Arabian Nubian Shield (ANS), comprises amphibolite-facies pelitic and psammitic schists and orthogneisses and was presumably shaped by Ediacaran East African orogenesis. However, the timing, number and duration of metamorphic events in Elat and elsewhere in the ANS are inconclusive and a major emphasis was previously given to pre-Ediacaran island-arc related tectonics. Here U-Th-Pb dating of monazite, a precise chronometer of metamorphic mineral growth, is systematically applied to the Elat schist and unveils the tectono-metamorphic evolution of the Elat association.In-situ analysis of metamorphic monazites by LASS-ICPMS (Laser-Ablation Split-Stream Inductively Coupled Plasma Mass Spectrometry) yields two concordant age clusters at ∼710 and 620Ma, recording two regional metamorphic events. The corresponding REE patterns of the dated monazite grains indicate that garnet porphyroblast growth took place only during the younger event (M2). Likewise, the regional south-dipping penetrative foliation (S2), common to the Elat schist and the surrounding orthogneisses, formed during the Ediacaran event (M2). The peak M2 staurolite-garnet-sillimanite assemblage was overprinted by retrograde andalusite and cordierite during decompression, followed by the intrusion of andesitic dykes that were immediately metamorphosed at greenschist-facies conditions. This metamorphic P-T-d-t path corresponds to the collision of the already accreted arc terranes of the ANS with the Saharan Metacraton, as also suggested by the large geochronological database of 610–580Ma post collisional granitoid batholiths throughout NE Africa and Arabia. The Cryogenian M1 metamorphism therefore records arc-arc collision during major convergence forming the proto-ANS to the south.

Comparison of NATM and umbrella arch method in terms of cost, completion time, and deformation

In all kinds of tunnel excavations, especially those excavated in cities, it is important to control surface settlements and prevent damage to the surface and subsurface structures. For this purpose, in weak rocks and soils, the umbrella arch method (UAM) has been used in addition to the new Austrian tunneling method (NATM). NATM and UAM are the best-known classical methods used in tunnel excavation. In classical tunneling, NATM is usually preferred in normal rocks. However, in some cases, NATM may be insufficient. UAM is a very effective alternative especially in soils and weak rocks. In soil and weak rocks, UAM is especially necessary to prevent excessive deformations. Selection of UAM or NATM is based on the following factors: cost comparison of NATM and UAM, allowable deformations, quality of rock or soil, application time of NATM and UAM, availability of skilled workers, and qualification level of the workers. Therefore, selecting the excavation method in these kinds of grounds is vital in terms of achieving the project goals in time, managing the project costs effectively, and controlling the probable deformations on nearby structures. A critical issue in successful tunneling application is the ability to evaluate and predict the deformations, costs, and project time. In this paper, application times, costs, and deformation effects are compared between NATM and UAM in sensitive regions at the Uskudar-Umraniye-Cekmekoy metro project (UUCMP). Also, efficiency of the deformation control of UAM is demonstrated by using the 2D numerical analysis method. UUCMP is part of the Istanbul metro network. The tunnels have a cross section of 75.60 m2 for NATM and 83.42 m2 for UAM. Geology in this section is composed of weak sandstone. Diabase and andesite dykes are also rarely observed. This study shows that the construction cost of UAM are 1.7 times more expensive than NATM. Although application time of UAM is 2.5 times longer than NATM, it is 2.5 times more efficient in controlling the deformations. This efficiency in controlling the deformations is confirmed via two-dimensional numerical analyses.

Probe Drilling Ahead of Two TBMs in Difficult Ground Conditions in Turkey

This paper summarizes the results of probe drilling carried out ahead of TBMs in two difficult tunneling projects in Turkey. The tunnels have completely different geological characteristics which necessitated two different methods of data analysis. Melen Water Tunnel was excavated under Istanbul Bosphorus within sedimentary rocks which are cut frequently by andesitic dykes, fracturing the surrounding rocks and creating a potential risk for water ingress into the tunnel. At the beginning of Melen Project, pioneering probe drillings with petrographic analysis and strength tests were performed on samples collected from TBM muck. This analysis allowed identifying some critical normalized probe drilling rate values for predicting potential weak zones created by andesitic dykes. These studies gave a sound basis for further interpretation of TBM and geologic data for the same tunnel. The second set of probe drilling analysis was from Kargi Tunnel. The North Anatolian Fault highly affected the tunnel excavation by fractured rock formations. Although the change in normalized probe drilling data was a good indicator of fractured zones, the diversity of rock formations made it difficult to interpret the data.

Primary Magmas in Continental Arcs and their Differentiated Products: Petrology of a Post-plutonic Dyke Suite in the Tertiary Adamello Batholith (Alps)

Determining the primary compositions of arc magmas is fundamental in retracing the chemical differentiation processes responsible for the formation of juvenile arc crust and the thermal structure of the mantle wedge. We have investigated a series of post-plutonic dykes that intruded the gab- broic to tonalitic southern part of the Tertiary Adamello Batholith in the Alps. The dyke rocks range in composition from primary, hydrous high-Mg basalts to basalts, basaltic andesites, andesites and dacites. Field relationships and high-precision U–Pb dating of titanite and zircon show that the dyke suite ranges in age from 41.67 ± 0.06 Ma for the high-Mg basalt to 38.62 ± 0.12 Ma for the youngest dacitic dykes, closely associated with plutonic activity from 42.5 to 39.0 Ma. Andesites and dacites have primitive 87Sr/86Sri (0.7032–0.7038) and 143Nd/144Ndi (eNdCHUR +3.5–3.2) isotopic signatures strongly limiting the extent of crustal assimilation, whereas some of the high-Mg basalts have se- lectively assimilated pelitic metasedimentary rocks as shown by high Cs/Rb, Rb/Sr and Rb/Zr ratios, and isotopically more enriched compositions (87Sr/86Sri 0.7039–0.7046; eNdCHUR +1.6–0.0). Primitive high-Mg basaltic dykes that escaped assimilation processes are primary mantle partial melts that were extracted from their source at pressures of 2.7 ± 0.2 GPa and temperatures of 1390 ± 30°C, conditions corresponding to the spinel–garnet transition in mantle peridotite. Major elem- ent modelling constrains the degree of melting to 20 6 2% leaving a harzburgite residue, consistent with the trace element chemistry of the high-Mg basalts, which have moderate [Gd/Yb]N ratios of 1–1.2. Differentiated basaltic andesites and dacites follow experimentally constrained liquid lines of descent for fractional crystallization at mid- to deep crustal levels. The trace element chemistry of amphiboles from basaltic andesite and andesite dykes reveals the coexistence of amphibole with primitive melts, indicating elevated pressures and H2O contents in their parental magmas. Thermobarometric constraints for amphibole phenocrysts result in pressures from 0.65 to 0.78 GPa and temperatures ranging from 930 to > 1000°C. The absence of any significant Eu-anomaly in the rare earth element patterns in these amphiboles indicates the late appearance of plagioclase in the crystallization sequence. The crystallization of amphibole drives the differentiated magmas to slightly peraluminous, corundum-normative compositions that are common for tonalites building the major part of the Adamello Batholith. Fractionation models at mid- to lower crustal conditions result in the cumulative crystallization of 17% olivine, 2% Cr-rich spinel, 18% clinopyroxene, 41% amphibole, 4% plagioclase and 0.1% magnetite to obtain an andesitic composition from a primary, hydrous high-Mg continental arc basalt. Cumulates formed during fractional crystallization atmid- to deep crustal levels are dunites and wehrlites followed by hornblendites and hornblende- gabbros. The trace element signatures of basaltic andesites and dacites display low Rb/Zr and Rb/ Sr, and are consistent with fractionation-dominated processes within the crust in an active contin- ental margin. Significant crustal assimilation is not required to obtain the trace element signatures of the evolved andesitic magmas.

Multi-scale Acheulean landscape survey in the Arabian Desert

The interpretation of Acheulean landscape use through the analysis of artefact distributions over a range of environmental settings is vital for understanding early hominin behaviour. Such approaches have been successfully applied in areas such as East Africa and South Africa, where long-term and sustained archaeological research has led to the development of robust environmental frameworks within which to analyse hominin land-use patterns. Much less is known about Acheulean hominin behaviour in the Arabian Peninsula, which is increasingly being recognised as an important area for studying early hominin dispersals and adaptations to new environments. To address this lacuna, we have undertaken the first multi-scale systematic survey of Acheulean occupation evidence at Dawadmi, in the centre of the Arabian Peninsula. Specifically, we carried out systematic transect surveys over a large andesite dyke at Saffaqah, on which the majority of 26 known Acheulean sites are associated, as well as across narrow drainage channels, desert pavements and hills located within 5 km of the dyke. Survey transects also crossed neighbouring dykes and adjacent landscape units with a 25 × 20 km area. Our surveys at Saffaqah have led to the discovery of 14 new Acheulean sites. Initial lithic analyses reveal differences between sites in terms of typology, but further work on the assemblages is required to determine if these differences are behavioural or a product of post depositional processes. A broad regional survey was undertaken to identify the full extent of Acheulean activity around Dawadmi. This led to the discovery of a further 22 sites. There is a strong correspondence between Acheulean sites and fine-grained andesite dykes, which were major lithic raw material sources. No Acheulean sites in the study area were found away from dykes or their adjacent landscape units. Based on dyke distributions, the geographic range of Acheulean activity is estimated to be 100 × 55 km, making Dawadmi one of the largest Acheulean landscapes in the world.

Constraints on the development of orogenic style gold mineralisation at Mineral de Talca, Coastal Range, central Chile: evidence from a combined structural, mineralogical, S and Pb isotope and geochronological study

Mineral de Talca is a rare occurrence of Mesozoic, gold-bearing quartz vein mineralisation situated within the Coastal Range of northern Chile. Quartz veins controlled by NNW–SSE-trending faults are hosted by Devonian-Carboniferous metasediments of greenschist facies and younger, undeformed granitoid and gabbro intrusions. The principal structural control in the area is the easterly dipping, NNW–SSE-trending El Teniente Fault, which most likely developed as an extensional normal fault in the Triassic but was later reactivated as a strike-slip fault during subsequent compression. A dilational zone in the El Teniente Fault appears to have focussed fluid flow, and an array of NW–SE-trending veins is present as splays off the El Teniente Fault. Mineralised quartz veins typically up to a metre thick occur in three main orientations: (1) parallel to and within NNW–SSE-trending, E-dipping faults throughout the area; (2) along NW–SE-trending, NE-dipping structures which may also host andesite dykes; and (3) rarer E–W-trending, subvertical veins. All mineralised quartz veins show evidence of multiple fluid events with anastomosing and crosscutting veins and veinlets, some of which contain up to 3.5 vol.% base metal sulphides. Mineralogically, Au is present in three textural occurrences, identified by 3D CT scanning: (1) with arsenopyrite and pyrite in altered wall rock and along the margins of some of the veins; (2) with Cu-Pb-Zn sulphides within quartz veins; and (3) as nuggets and clusters of native Au within quartz. Fluid inclusion work indicates the presence of CO2–CH4-bearing fluids with homogenisation temperatures of ∼350 °C and aqueous fluids with low-moderate salinities (0.4–15.5 wt% NaCl eq.) with homogenisation temperatures in the range of 161–321 °C. The presence of Au with arsenopyrite and pyrite in structurally controlled quartz veins and in greenschist facies rocks with evidence of CO2-bearing fluids is consistent with an orogenic style classification for the mineralisation. However, the significant amounts of base metals and the moderate salinity of some of the fluids and the proximity to felsic granitoid intrusions have raised the possibility of an intrusion-related origin for the mineralisation. Vein sulphides display S isotope signatures (δ34S +2.1 to +4.3 ‰) that are intermediate between the host rock metasediments (δ34S +5.3 to +7.5 ‰) and the local granitoids (δ34S +1.3 to +1.4 ‰), indicating a distinct crustal source of some of the S in the veins and possibly a mixed magmatic-crustal S source. The local granite and granodiorite give U-Pb zircon ages of 219.6 ± 1 and 221.3 ± 2.8 Ma, respectively. Lead isotopic compositions of galena in the veins are consistent, suggesting derivation from a homogeneous source. Differences, however, between the isotopic signatures of the veins and igneous feldspars from nearby intrusions imply that these bodies were not the source of the metals though an igneous source from depth cannot be discounted. The Triassic age of the granitoids is consistent with emplacement during regional crustal extension, with the El Teniente Fault formed as an easterly dipping normal fault. The change to a compressional regime in the mid-Jurassic caused reactivation of the El Teniente Fault as a strike-slip fault and provided a structural setting suitable for orogenic style mineralisation. The intrusions may, however, have provided a structural competency contrast that focused the mineralising fluids in a dilational jog along the El Teniente Fault to form WNW-trending veins. As such, the mineralisation is classified as orogenic style, and the identification of the key mineralogical, isotopic and structural features has implications for exploration and the development of similar deposits along the Coastal Range.

Geochronology and isotope geochemistry of Eocene dykes intruding the Ladakh Batholith

In order to determine the extent and timing of dyke formation related to possible E–W extension along the southern margin of Eurasia during Early Cenozoic time, we examined ca. 30 mostly andesitic dykes intruding the Ladakh batholith from 10 to 50 km west of Leh (NW India). The dykes in the east of the area trend E–NE and those in the west trend N–NW. The difference in orientation is also evident in the petrography and isotopic signatures. The eastern dykes contain corroded quartz xenocrysts and show negative ε0(Nd) and positive ε0(Sr) values, whereas the western dykes do not contain quartz xenocrysts and exhibit positive ε0(Nd) and near-zero ε0(Sr) values. The variability in Sr–Nd isotopes (ε0(Nd) = 3.6 to − 9.6, ε0(Sr) = 0.4 to 143) and the quartz xenocrysts can best be explained by (differing degrees of) crustal assimilation of the parent magma of the dykes. Separated minerals from five dykes were dated by 40Ar–39Ar incremental heating: amphibole ages range between 50 and 54 Ma, and one biotite separate dated both by Rb–Sr and 40Ar–39Ar gave an age of 45 Ma. One dated pseudotachylyte sample attests to brittle faulting at ca. 54 Ma. The combination of structural field evidence with petrographic, isotopic and geochronological analyses demonstrates that the dykes did not form from a single, progressively differentiating magma chamber, despite having formed in the same tectonic setting around the same time, and that processes such as crustal assimilation and magma mixing/mingling, also played a significant role in magma petrogenesis.

Volcanic Rocks from Q-Prospect, Chatree Gold Deposit, Phichit Province, North Central Thailand: Indicators of Ancient Subduction

Volcanic rocks exposed at Q-prospect in the northern Chatree gold deposit, Phichit province, Thailand, appear to have erupted during Permo-Triassic period; they are geochemically composed of basalt porphyry, basaltic tuff and rhyolitic/rhyodacitic tuff. These host rocks are often cross cut by basaltic andesite dyke. Zr/TiO2 versus SiO2 diagram indicates that these volcanic hosts are significantly equivalent to sub-alkali basalt and rhyodacite–rhyolite. Based on trace element compositions, they appear to have derived from tholeiitic magma originated within mantle and partially involved by crustal material; on the other hand, basaltic andesite dyke may have originated from calc-alkaline magma at a shallower depth within crustal environment. A tectonic model of subduction-related island arc is therefore suggested for magma evolution that also led to gold deposit. Consequently, it fits very well with the regional tectonic of the country.

Late-Hercynian post-collisional dyke magmatism in central Calabria (Serre Massif, southern Italy)

Widespread late- to post-collisional magmatism occurred in the Calabria-Peloritani Orogen (southern Italy) during the final stages of the Hercynian Orogeny. In the Serre Massif (central Calabria), medium- to high-K calcalkaline andesitic to dacitic-rhyodacitic dykes show typical geochemical features of subduction-related magmas (LILE and LREE enrichment, HFSE depletion, peaks at Rb, Pb and Th). The origin of late- to post-Hercynian calcalkaline rocks is usually interpreted in an extensional post-collisional framework, involving thinning of the continental lithosphere and progressive passive upwelling of the asthenospheric mantle. In such a context, pure mantle, crustal and hybrid melt production likely can occur. The andesitic dykes were produced by partial melting of an enriched mantle source metasomatized by crustal fluids/melts during former subduction and then suffered minor, if any, assimilation of lower crustal metapelites. Most dacite-rhyodacites were likely derived by hybridization in various proportions of crustal and mantle melts, whereas pure crustal metasedimentary sources, and more or less efficient restite unmixing processes, were involved in the generation of the most silica-rich rhyodacites.

Exploration of copper deposits in Wadi El Regeita area, Southern Sinai, Egypt, with contribution of remote sensing and geophysical data

Three types of hydrothermal alterations are recorded in Wadi El Regeita area, argillic, phyllic, and propylitic. Whole-rock analysis of representative samples of the alteration halo (1) shows a Cu, Au, and Ag content up to 1.7 wt.%, 1.6 g/t, and 4 g/t, respectively; in the alteration halo (2), these metal contents are up to 1.3 wt.%, 1.4 g/t, and 3 g/t, following the same order. The integrated remote sensing and geophysical data, as well as geological field verification, show that Wadi El Regeita area includes promising Cu ores within two hydrothermal alteration haloes. Spatial data analyses of lineaments from Landsat Enhanced Thematic Mapper (ETM) band ratio image (7/5, 5/4, 3/1) reveal the presence of alteration haloes that potentially may host Cu mineralization at south and north of El Regita Cu mine. Gravity interpretation indicates that the surveyed area is dissected by NE–SW fault zones in the central part, near Wadi El Regeita Cu mine. Ground magnetic survey data revealed that the surveyed area includes six magnetic bodies at depths ranging from 32 to 90 m, possibly recording the presence of mineralized and hydrothermally altered andesitic dykes. The half length of these dykes ranges from 600 to 1,070 m; their half thickness from 30 to 123 m and their half width from 48 to 531 m. Dyke locations coincide with surface alteration haloes (1) and (2) as indicated by the ETM band ratio image processing. The final assessment of the area, however, needs more detailed geological and geophysical studies with contributions of remote sensing techniques.

이종지질 분포사면에서의 3차원 사면안정해석

Three-dimensional slope stability analysis was applied to a failed dual-lithology slope containing both granite and an andesitic dyke, taking account of the differences in shear strength of the different lithologies. A direct shear test of the soil-rock boundary was performed to examine the shear strength of two different types of failure surfaces within different lithologies, and a laboratory test was performed on an upper, weathered soil layer. The test results indicate that shear strength was lower at the soil-rock boundary than within the weathered soil layer. A representative geological section was subjected to two-dimensional slope stability analysis using a limit equilibrium method to assess whether the distribution of lithologies upon the slope influences the results of stability analysis. The results were then compared with those of three-dimensional slope stability analysis, for which input parameters can be varied according to the distribution of lithologies upon the slope. The three-dimensional analysis yielded safety factors of 1.26 under dry conditions and 0.55 under wet conditions, whereas the two-dimensional analysis yielded unstable safety factors of 0.92 and 0.32, respectively. These findings show that the results of stability analysis are affected by the distribution of different lithologies upon the slope. Given that the studied slope collapsed immediately after rainfall, it is likely that the results of the three-dimensional analysis are more reliable.

The Chogat-Chamardi subvolcanic complex, Saurashtra, northwestern Deccan Traps: Geology, petrochemistry, and petrogenetic evolution

The Saurashtra region in the northwestern part of the Deccan continental flood basalt province (India) is notable for its many volcano-plutonic complexes, compositional diversity, and the abundance of rhyolite and granophyre. In this first detailed study of the Chogat-Chamardi subvolcanic complex in eastern Saurashtra, we report microgranite and granophyre plutons, gabbro plutons cut by basaltic andesite dykes, as well as rhyolite plugs. Bulk-rock geochemical (including Nd–Sr isotopic) data suggest a strong contribution from ancient, Rb-rich basement crust to the silicic magmas. The mafic rocks are also crustally contaminated but less than the silicic rocks, in conformity with observations elsewhere (e.g., the Skye and Skaergaard granophyres). The Chogat-Chamardi silicic rocks have initial 87Sr/ 86Sr (for t = 65 Ma) as high as 0.72914, and initial ε Nd values as low as −13.9. Trends defined by the Chogat-Chamardi and other Deccan rhyolitic suites on plots of Sr concentration vs. Sr isotopic ratio are modeled with concurrent assimilation and fractional crystallization (AFC) processes involving a basaltic magma and granitic basement rocks. Considering both Nd and Sr isotopic variations, the Chogat-Chamardi silicic rocks notably require crustal end members very different from those inferred in most previous isotopic studies of Deccan rocks.

The Sarcheshmeh porphyry copper deposit, Kerman, Iran: A mineralogical analysis of the igneous rocks and alteration zones including halogen element systematics related to Cu mineralization processes

The giant Sarcheshmeh porphyry Copper deposit is located 65 km southwest of Kerman City, southeastern Iran. Numerous Miocene porphyry stocks and dykes intruded thick sequences of Upper Cretaceous sedimentary and Eocene volcanic rocks. Hypogene and supergene porphyry Cu mineralization occurs within the granodioritic porphyry and host rock sequence, which was extensively altered to a dominantly potassic, phyllic, and argillic assemblage with interstitial to distal propylitic types. Biotite-bearing assemblages occur as both primary phenocrysts and secondary replacements showing variable size, colour, and shape. Fluorine contents (0.22 to 1.33 wt.%) and X Mg (0.54 to 0.71) in biotites from the potassic and phyllic zones are higher than those of non-mineralized granitoids (F = 0.09 to 0.56 wt.%, X Mg = 0.43 to 0.54), whereas their Cl contents (Cl = 0.05 to 0.24 wt.%) are lower than those of the non-mineralized granitoids (Cl = .0.11 to 0.45). The biotites from the phyllic zone have higher log ( fH 2O/ fHF) and log ( fH 2O/ fHCl) values than those of the potassic zone, as well as the granitoid and andesitic dykes. The log ( fHF/ fHCl) values determined for the granitoid, potassic and phyllic zones are similar, though more negative than those of the andesitic dykes. The log ( fHF/ fHCl) values have a similar range for biotite from the granitoid, and potassic and phyllic zones. The halogen fugacity ratios established for fluids associated with the Sarcheshmeh deposit from their F and Cl contents in biotite show that the granitoid, potassic zone and phyllic zone are increasingly affected by meteoric waters. The fluids that circulated in the phyllic zone are predominantly of meteoric origin, possibly overprinting original phyllic zone halogen contents. The Cl intercept values of biotite in the granitoid, and phyllic and potassic are similar to other ore-forming systems and tend to be more Cl-rich than Cl-intercept values of biotites in common igneous rocks. Calculated F/Cl intercept values for biotite in the granitoid and potassic zone are also consistent with many other porphyry copper forming systems.

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.

Morfometría en la cuenca hidrológica de San José del Cabo, Baja California Sur, México

La cuenca hidrológica-forestal de San José del Cabo, localizada al sur de la península de Baja California, México, es la de mayor extensión en el estado de Baja California Sur y es considerada como una de las principales fuentes de recursos hídricos. Los acuíferos que suministran agua a las áreas turísticas y urbanas están en las partes bajas (planicies aluviales) de la cuenca. Un complejo cristalino constituido por rocas ígneas y metamórficas caracteriza la región montañosa de la cuenca. Análisis de imagen Landsat ETM, fotografías aéreas y modelo digital de elevación fueron usados para cartografiar la geomorfología, geología y desarrollar un análisis morfométrico en la margen occidental de la cuenca. Rasgos estructurales tales como alineamientos rectos o curvilíneos y diques fueron obtenidos del modelo digital de elevación. Parámetros morfométricos y análisis estructural de siete subcuencas se derivaron para determinar áreas potenciales de captura hacia la Cuenca Hidrológica de San José del Cabo (CHSJC). Del análisis se identificó que la captura toma lugar en la parte serrana de la cuenca, dentro del basamento cristalino a través de un sistema de fracturamiento interconectado. Se proponen dos subcuencas como las principales áreas de recarga hacia la CHSJC. Los resultados muestran que la investigación con imagen Landsat y el modelo digital de elevación proporcionan, una fuente acertada de datos e información para la identificación de áreas de recarga y descarga a una escala regional.

Palaeomagnetic Study of Dated Pre-Cambrian Rocks of the Front Range, Colorado-Wyoming

Summary The mean direction of remanent magnetization, after partial a. c. demagnetization, of Pre-Cambrian rocks from the Front Range of Colorado and Wyoming gives a palaeomagnetic pole position at 8°N, 29°E. Rock types sampled include andesite dykes, diorite inclusions in granite, and a highly oxidized red granite, all with a radiogenic age of 1.41 G. Y. This preliminary study suggests there are at least two and probably as many as four field reversals recorded in these rocks, and that remanent directions from andesite dykes may be useful in determining tectonic rotation of the Front Range.

The Cabo de la Vela Mafic-Ultramafic Complex, Northeastern Colombian Caribbean region: a record of multistage evolution of a Late Cretaceous intra-oceanic arc

Abstract Ophiolite-related rocks accreted to Caribbean Plate margins provide insights into the understanding of the intra-oceanic evolution of the Caribbean Plate and its interaction with the continental margins of the Americas. Petrological, geochemical and isotope (K–Ar, Sr and Nd) data were obtained in serpentinites, gabbros and andesite dykes from the Cabo de la Vela Mafic–Ultramafic Complex from the Guajira Peninsula, in the northernmost Colombian Caribbean region. Field relations, metasomatic alteration patterns and whole rock–mineral geochemistry combined with juvenile isotope signatures of the different units suggest that gabbros and serpentinites formed in a slow-spreading supra-subduction zone that was brought to shallower depths and subsequently evolved to an arc setting where andesitic rocks formed with little sediment input. The tectonomagmatic evolution of the Cabo de la Vela Mafic–Ultramafic Complex involved an intra-oceanic arc that evolved from pre-Campanian time to 74 Ma. Relationships with other units from the Guajira Peninsula show either the existence of a mature arc basement or a series of coalesced allocthonous arcs, juxtaposed before accretion onto the passive continental margin of South American in pre-Eocene times.