Petrolgy of the volcanic/subvolcanic members of the volcano-sedimentary Maden Complex in Eastern Turkey

<p>Maden Complex exposed in Eastern Turkey, is a succession of volcano-sedimentary rocks and tectonically overlain by Bitlis Metamorphics and Cretaceous ophiolitic rocks. The succession includes shallow-water deposits and deep marine pelagic sediments intercalated with pillow lavas ranging from a few centimeters to ten meters in diameter. The planktonic foraminiferal assemblages from micritic limestones and zircon U-Pb ages from selected sedimentary rocks indicate the age of Late Ypresian - Early Lutetian. Plagioclase and  clinopyroxenes are the main mineral phases, olivine rarely found as altered phenocrysts. Clinopyroxenes are augite and diopside, and their compositions are ranging between Wo<sub>44-51</sub>, En<sub>27-43</sub>, Fe<sub>10-21</sub>. The anorthite contents of plagioclases are between 32- 67 % in unaltered grains. The crystallization temperatures and pressures obtained from clinopyroxene chemistry are ranging from 1126 to 1250<sup>o</sup>C and 3 to 8 Kbar, respectively. The majority of the volcanic/subvolcanic rocks are subalkaline-tholeiitic basalts however; a few andesitic and rhyolitic derivatives are also present. The whole – rock and  Sr-Nd-Pb isotope compositions reveal that the  basaltic rocks are originated from E-MORB like asthenospheric mantle source without a subduction component.</p>

Stress-cycle driven evolution of faulting in a plate boundary transition zone

<p>Upper plate faults along the Cascadia subduction margin of North America go through a 3 stage evolution over millions of years as a consequence of the migrating Mendocino Triple Junction (MTJ). Initially, NE-directed cyclic shortening produced by the Cascadia subduction earthquake cycle drives reverse dip-slip motion on trench-parallel faults. As the triple junction moves north, NNW-shortening associated with the Mendocino Crustal Conveyor (MCC, Furlong & Govers, 1999) is superimposed on the cyclic subduction-earthquake-cycle regional stress field. As the triple junction migrates further north, and these faults transfer from the subduction to transform plate boundary, they become part of the San Andreas system and are loaded by right-lateral shear. In this work we investigate how the faulting behavior in northern California evolves through time from first being driven by cyclic subduction zone stresses (superimposed on a NNW-oriented shortening field) to eventually forming the primary structures within a dominantly strike-slip stress regime.</p><p>We decompose the observed horizontal GPS velocity field in southern Cascadia to determine a subduction coupling component and a NNW-directed displacement component to separate the subduction cycle effects from other tectonic effects on the behavior of upper plate faulting and its evolution through time. Since the MCC processes acts over millions of years, we assume that the effects associated with the NNW-directed signal can be represented by a constant stress field over subduction earthquake cycle timescales. Early in the subduction earthquake cycle, the principal stresses north of the MTJ are oriented in this NNW-SSE direction and rotate clockwise as the subduction component increases. This stress cycle then resets following each large megathrust event. Coulomb stress analyses indicate that the cyclic nature of the regional stress field, changes the likelihood of faulting and slip behavior on faults in southern Cascadia over time intervals of 100s of years. Trench-parallel faults are most likely to exhibit right-lateral or oblique motion early in the seismic cycle, however by ~100-200 years following a megathrust event, they are more likely to exhibit reverse dip-slip motion as the stress effects from the subduction component increase.</p><p>Though the NNW-oriented displacement field is assumed to be temporally constant over subduction earthquake cycle timescales, the spatial extent of this deformation field constrains strain localization within the upper plate. For example, a steep decrease in GPS velocities from SW to NE in southernmost Cascadia indicates right-lateral strain is accumulating adjacent to the relatively rigid Klamath Mountain Province. This region of localized right-lateral shear coincides with the location of the development of several regional-scale right-lateral strike slip faults. We hypothesize these faults, formed within the subduction regime, evolve to become regional-scale 'San Andreas-type' plate boundary faults. Understanding the implications of such time- and space-variable stress regimes provides insight into interpreting geologic estimates of the slip history of faults along the Cascadia and northern San Andreas margins of North America, and also a framework for understanding how a new plate boundary develops following a major change in plate interaction.</p>

Identification, classification, and interpretation of boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics

Boninites are rare, high-Si, high-Mg, low-Ti lavas that have considerable tectonic significance, especially for recognizing and interpreting episodes of subduction initiation in the geologic record. Formal identification and classification of boninites may be carried out using MgO-SiO2 and MgO-TiO2 diagrams to find compositions that satisfy modified International Union of Geological Sciences (IUGS) criteria of Si8 > 52 and Ti8 < 0.5, where Si8 and Ti8 refer to concentrations of the oxides at 8 wt% MgO. However, screening of highly metasomatized rocks and accurate classification require precautions, including normalization to a 100% volatile-free basis. The MgO-SiO2 diagram can also be used for subdivision into low-Si boninites (Si8 < 57) and high-Si boninites (Si8 > 57). Satisfying one but not both of the boninite criteria are rocks with Si8 > 52 but Ti8 ≥ 0.5 (siliceous high-magnesium basalts) and rocks with Si8 ≤ 52 but Ti8 < 0.5 (low-Ti basalts). We tested the classification methodologies using ∼100 low-Ti lava suites dating from the present-day back to the Eoarchean. We conclude that, of those classifying as “boninite series,” Izu-Bonin-Mariana arc–type subduction initiation terranes provide the dominant setting only back as far as ca. 2 Ga, which marks the maximum age of extensive clinopyroxene-undersaturated melting and eruption of high-Si boninites. From 2 to 3 Ga, most boninites formed in intraplate settings by melting of refertilized, depleted cratonic roots. Prior to 3 Ga, hot, depleted mantle plumes provided the main boninite sources. Nonetheless, arc-basin boninites, though rare, do extend back to 3.8 Ga, and, together with the inherited subduction component in intracratonic boninites, they provide evidence for some form of subduction during the Archean.

GEOCHEMICAL CHARACTERISTICS OF THE MAFIC ENCLAVES AND THEIR HOSTS FROM NEOGENE ERENLERDAGI VOLCANITES, AROUND YATAGAN VILLAGE AND SAĞLIK TOWN (KONYA), CENTRAL TURKEY

Late Miocene to Pliocene volcanism is represented by development of lava domes, nuée ardentes and pyroclastic fall and flow (ignimbrites) deposits in the WSW and NW of Konya city. The lava dome contains various mafic microgranular enclaves (MMEs), which have various size (a few cm to a few meters), shape (ellipse/sphere to rounded), with a well-developed chilly zone. The MMEs samples are situated on mostly basaltic andesite and andesite, and a few MME samples on basaltic trachyandesite area while the host rocks are concentrated on dacite and andesite areas. The felsic samples have more fractionated chondrite-normalised REE pattern (La/YbN: 9.5-18.1) than MMEs (6.7-16.0) ones, but both have slightly developed negative Eu anomaly (Eu/Eu*: 0.67-0.89 in felsic rocks, 0.68-0.87 in MMEs). In primitive mantlenormalized spider diagram, the MMEs and felsic rocks have negative Nb, Ta, P and Ti anomalies, indicating some subduction component in their genesis. Based on geochemical data, the MMEs are suggested to have been formed by hybridization of basic magma mingled with partially crystallized felsic magma.

GEOCHEMICAL VARIATIONS ON HOSTED VOLCANIC ROCKS OF CIBALIUNG EPITHERMAL GOLD MINERALISATION, BANTEN – INDONESIA: IMPLICATIONS FOR DISTRIBUTION OF SUBDUCTION COMPONENTS

Subduction of the Indo-Australian Plate beneath the Eurasian Plate formed at least seven magmatic arcs in Indonesia. One of the magmatic arcs is the Neogene Sunda-Banda arc hosts various style of gold mineralisation such as Cibaliung epithermal gold mineralisation. Major and trace element data for host volcanic rocks to the Cibaliung epithermal gold mineralisation is provided by this study to identify the magmatic arc system and the distribution of subduction components. Enriched LILE (Large Ion Lithopile Element) and LREE (Light Rare Earth Element) compositions for basaltic andesite – rhyodacitic samples from the Cibaliung district are characteristic of calc-alkaline arcs. In this typical volcanic arc, the subduction component can be shown to make a dominant contribution to its content of LILE such as Rb, K, Th, and Ba enriched (more than 88%) relative to the mantle and within plate inputs. The incompatible elements (Hf, Zr, and Nb) cannot be observed in the subduction component and thus assumed to be derived from trace element enriched sub-continental lithosphere. These incompatible elements are defined as conservative elements therefore it suggests that the magma occurrence is related to a hydrous slab component. Keywords: Subduction, Indo-Australian plate, magmatic arcs, volcanic rocks, Cibaliung, epithermal gold.