Calc-alkaline Volcanic Arc Research Articles

Turonian highly differentiated pyroclastic xenoliths and Cretaceous sedimentary record challenge mantle-plume view of Costa Rican forearc basement successions

The Mesozoic basaltic basement of southern Central America is believed to consist of proxy sections of mantle-plume events leading to the Caribbean igneous province. A significant xenolith assemblage of radiolarite, tuffite, and Turonian (∼93 Ma) highly differentiated pyroclastic rock discovered in basalt of the Costa Rican forearc basement fundamentally challenge this long-held view, revealing bimodal Cretaceous island-arc activity and forearc basin development instead. The pyroclastic materials originally accumulated with monotonous radiolarite deposits in a deep-marine forearc basin slope environment to build part of the Mesozoic sedimentary cover of the evolving arc. Xenolith formation was related to widespread Turonian to Campanian intrusive basaltic activity on the Nicoya Peninsula that affected Albian-Turonian and Berriasian-Barremian radiolarite and pyroclastic deposits as well as Jurassic siliceous strata. Strong correlation with Cretaceous biotite- and hornblende-bearing pyroclastic beds reveals explosive calc-alkaline volcanic arc episodes alongside basaltic activity linked to marine forearc basin development. These materials provide decisive evidence of explosive calc-alkaline volcanic activity of the Cretaceous Costa Rican arc as well as the probable intraoceanic-arc nature of its basaltic forearc basement and a ∼90 Ma aged komatiite suite found at Tortugal. The resulting tectonic scenario shows the Caribbean Plate was separated in southern Central America from any potential Pacific or Galápagos hotspot sources by a subduction zone. This favours in-place origin of the Caribbean Plate and its igneous province between the Americas. The findings also reveal the unsuitability of basaltic forearc basement successions to deduce with certainty mantle-plume events and hotspot evolutions in adjoining oceanic domains.

Suprasubduction ophiolite (SSZ) components in a middle to lower upper Jurassic Hallstatt Mélange in the Northern Calcareous Alps (Raucherschober/Schafkogel area)

The Northern Calcareous Alps in the Western Tethys realm were affected in Middle to Late Jurassic times by a mountain building process triggered by ophiolite obduction similar to that in the Inner Western Carpathians or Inner Dinarides. In contrast to these other mountain ranges, in the Northern Calcareous Alps the obducted ophiolites or ophiolite derived components in the Bathonian-Oxfordian mélanges are missing. Cr-spinels in Kimmeridgian basinal deposits are the oldest known relics of a Jurassic ophiolite obduction. This study reveals new data from a newly detected Hallstatt Mélange below the Late Jurassic Plassen Platform in the southeastern Northern Calcareous Alps (Raucherschober/Schafkogel area). Upper Triassic Hallstatt Limestone blocks from the former distal northwestern continental margin of the Neo-Tethys Ocean, as well as ophiolite and radiolarite blocks from the Neo-Tethys Ocean floor rest within an upper Middle to lower Upper Jurassic radiolaritic-argillaceous matrix. Ophiolitic blocks show calc-alkaline volcanic arc affinity, defining the rocks as the product of intra-oceanic subduction and the formation of an early arc during stacking of the oceanic crust. Resedimented ribbon radiolarite blocks deposited above the newly formed suprasubduction (SSZ) ophiolites in the Neo-Tethys Ocean east of the island arc have a Middle Jurassic age. Later, at a time of decreasing tectonic activity, the Hallstatt Mélange was sealed by the Kimmeridgian-Tithonian Plassen Carbonate Platform, showing a shallowing-upward trend.

First paleoseismological results in the epicentral area of the sixteenth century Ameca earthquake, Jalisco – México

The Trans-Mexican Volcanic Belt (TMVB) is a calc-alkaline volcanic arc cut by different active crustal fault systems that have originated several destructive historical earthquakes. Located in the central part of Mexico this region offers exceptional climatic, and fertility of soil conditions, which is the reason why more than 50% of the Mexican population now live here, increasing the seismic risk. Determining the seismic potential of these fault systems is important in the western section of the TMVB, in the vicinity of the city of Guadalajara, where more than 5 million inhabitants are concentrated in a densely populated urban area.We focus here on the epicentral area of the MW 7.2 sixteenth century Ameca earthquake, one of the first earthquakes described to take place in the American continent and which also may be the largest crustal earthquake to have occurred in the TMVB in the historical record. According to some historical sources, this earthquake would be associated with the Ameca-Ahuisculco Fault but no neotectonic study has been carried out so far to characterize this fault. Here, we describe the geomorphology of the fault escarpment and the characteristics of different fault segments. This first step allowed to select a suitable site for a paleoseismological study to track the historic event. The results of the interpretation of two trenches are consistent, showing evidence of net activity of the fault in the tectono-sedimentary record with two and possibly three seismic events. The older one of these is not well recorded and interpreted as a possible event that could have occurred after 27,91 ± 0,4 cal ka BP and before 5,67 ± 0,064 cal ka BP. The second one and best recorded event occurred around 5,67 ± 0,064 cal ka BP whilst the last one occurred after 0,985 ± 0,065 cal ka BP and is likely to be the geological record of the Ameca sixteenth century earthquake. Considering the potential rupture lengths and the coseismic displacement measured in the trenches, this fault system seems capable of generating earthquakes of magnitude 6.9 to 7.3 and represents a major source of earthquake hazard to the city of Guadalajara.

Sedimentology, Provenance and Radiometric Dating of the Silante Formation: Implications for the Cenozoic Evolution of the Western Andes of Ecuador

The Silante Formation is a thick series of continental deposits, exposed along a trench-parallel distance of approximately 300 km within the Western Cordillera of Ecuador. The origin, tectonic setting, age and stratigraphic relationships are poorly known, although these are key to understand the Cenozoic evolution of the Ecuadorian Andes. We present new sedimentological, stratigraphic, petrographic, radiometric and provenance data from the Silante Formation and underlying rocks. The detailed stratigraphic analysis shows that the Silante Formation unconformably overlies Paleocene submarine fan deposits of the Pilalo Formation, which was coeval with submarine tholeiitic volcanism. The lithofacies of the Silante Formation suggest that the sediments were deposited in a debris flow dominated alluvial fan. Provenance analysis including heavy mineral assemblages and detrital zircon U-Pb ages indicate that sediments of the Silante Formation were derived from the erosion of a continental, calc-alkaline volcanic arc, pointing to the Oligocene to Miocene San Juan de Lachas volcanic arc. Thermochronological data and regional correlations suggest that deposition of the Silante Formation was coeval with regional rock and surface uplift of the Andean margin that deposited alluvial fans in intermontane and back-arc domains.

The “intraorogenic” Svecofennian Herräng mafic dyke swarm in east-central Sweden: age, geochemistry and tectonic significance

ABSTRACTThe Herräng mafic dykes form an E-W-trending dyke swarm within the Bergslagen lithotectonic unit of the Svecofennian orogen in east-central Sweden. They intrude the Svecofennian supracrustal rocks and early-orogenic granitoids, but are themselves cut by late Svecofennian pegmatites, and have undergone Svecofennian amphibolite-facies metamorphism, leading to their classification as “intraorogenic” Svecofennian dykes. They can be assigned an age between 1870 and 1850 Ma, with metamorphism of the dykes dated at 1848 ± 13 Ma by U-Pb in titanite. Their current mineralogy is dominated by metamorphic plagioclase and amphibole, with variable amounts of quartz and biotite, and minor to accessory titanite, apatite, epidote, pyrite, magnetite, ilmenite and zircon. Textures range from massive to strongly foliated. Twenty samples of dyke rocks from three subareas in the Roslagen region, including the Herräng type area, range in composition from basaltic to andesitic with 47 to 60 wt% SiO2, broadly similar to the Dannemora dykes and the Avesta-Östhammar gabbros and diorites. Initial 87Sr/86Sr ratios (at 1870 Ma) varies between 0.7026 and 0.7038, corresponding to initial εSr between +5 and +21, and initial εNd between −0.4 and +1.3, suggesting a slightly enriched to mildly depleted mantle source, similar to other Svecofennian mafic rocks. The dykes dominantly show a calc-alkaline volcanic arc signature related to subduction. They formed during an extensional episode, possibly related to incipient back-arc spreading or subduction roll-back following the main early-orogenic subduction-related Svecofennian magmatism, but penecontemporaneous with amphibolite-facies metamorphism in the area.

Geochemistry, mineralogy, and radioactivity of the Abu Furad Area, Central Eastern Desert, Egypt

Abu Furad is in the Central Eastern Desert, Egypt. It comprises metasediments, metavolcanics, metagabbros, syn- to late-orogenic granites, and post-orogenic granites, in addition to numerous dykes and veins of different shapes and composition invading all the older rocks cropping out in the study area. Field, petrographic, mineralogical, and chemical investigations led to the classification of Abu Furad granites as quartz diorite, tonalite, granodiorite, and syenogranites.Major oxide and trace element data revealed that the syn- to late-orogenic granites and post-orogenic granites are peraluminous. Syn- to late-orogenic granites originated from calc-alkaline volcanic arc–related magma; while the post-orogenic granites (syenogranite) are highly fractionated, calc-alkaline granite from a within plate regime. Radiometrically, the studied quartz diorite, tonalite, and granodiorite had lower uranium and thorium contents and higher eU/eTh ratios than the syenogranites. This may indicate that the syn- to late-orogenic rocks originated from sources depleted in these elements. The average eTh/eU ratio of syenogranites was higher than that of average continental crust, suggesting that the syenogranites are relatively depleted in U. The studied altered syenogranites were strongly enriched in U and Th compared to the Earth’s crust. On the other hand, the average of eU in pegmatites is lower than the global average for uraniferous pegmatites. The most recorded accessory minerals in the altered syenogranites were thorite, fergusonite, samarskite, columbite, zircon, monazite, xenotime, apatite, fluorite, sphene, atacamite, and malachite, in addition to chromium and nickel inclusions.

Arc related dioritic–granodioritic magmatism from southeastern Peninsular Malaysia and its tectonic implication

In Peninsular Malaysia, Late Cretaceous (95-76 Ma) plutons are found within the Eastern Belt. Their compositions are diverse, ranging from biotite granite, hornblende granodiorite, diorite, and gabbro. The Aur Pluton is the easternmost body and is exposed in the Johore Archipelago, southeast of Peninsular Malaysia. The rock types vary from diorite, quartz monzodiorite and granodiorite. Ar-Ar stepheating age of the diorite and zircon U/Pb age of the granodiorite yielded ages of 77.9 ± 2 Ma and 79 ± 0.7 Ma, respectively. The overall major and trace element trends are consistent with two magmatic pulses, i.e., diorite-quartz monzodiorite and granodiorite. The diorite-quartz monzodiorites are characterized by high Fe2O3 (6.7–8.3 wt.%), CaO (5.5–7.6 wt.%), MgO (2.9–4.1 wt.%) and high but variable Sr concentrations (570–1300 ppm). The granodiorites have significantly higher total alkali content (8.49–8.91%), with high K2O concentrations (3.8–4.6%) and low Fe2O3 (4.7–6.5%), CaO (2.7–3.9%) and MgO (1.4–2.0%) concentrations. Low Nb/U and distinct Pb anomalies suggest that both rock types are likely derived from the crust. Furthermore, elevated LREE and trace element contents for the granodiorites, e.g., FTh (10–20 ppm), Pb (10–30 ppm), and U (2–5 ppm), also indicate that upper crustal components are involved in the magma generation. The dioritic rocks are chemically classified as calc-alkaline volcanic arc (VAG) rocks, but they also have field relationships and mineralogies typically associated with an active continental margin (ACM).

Characteristics and variations of sinters in the Coromandel Volcanic Zone: application to epithermal exploration

ABSTRACTThe Hauraki Goldfield of the Coromandel Volcanic Zone, North Island, New Zealand, hosts up to 19 known siliceous hot spring deposits, or sinters, some directly associated with Au–Ag ore bodies, in a Miocene–Pliocene, sub-aerial, calc-alkaline volcanic arc overlying Jurassic meta-sedimentary basement. Sinters are surface expressions of predominately adularia-sericite epithermal systems, potentially linked to Au–Ag mineralised conduits at depth. They provide a paleosurface marker to determine the level of erosion, and help determine zones of thermal fluid upflow and/or lateral outflow. Mapped sinters of the Coromandel Volcanic Zone are mainly affiliated with rhyolite domes, occur along structural corridors of the volcanic arc, and contain characteristic textures, mineralogy and pathfinder elements typical of near-neutral pH alkali chloride fluid discharge derived from convectively circulating magmatic fluids diluted by meteoric waters. Textures and trace element compositions of the Coromandel sinters are similar to those of the Taupo Volcanic Zone, New Zealand, and the Jurassic Au–Ag mining districts of the Deseado Massif, Argentine Patagonia. Thus, detailed mapping, textural analysis and trace-element studies demonstrate the prospecting potential of siliceous hot spring deposits for locating ore deposits, and in particular to help define proximal to distal positions relative to vent zones at the paleo-surface.

Volcano stratigraphy and physical characterization of the San Mateo Pomez Unit, Cerritos Member: Temascalcingo Volcano, Central Mexico

The Temascalcingo volcano is a dacitic stratovolcano located at the eastern part of the Acambay graben, a tectonic basin in the central sector of the Trans-Mexican Volcanic Belt (TMVB), which is an active, mostly calc-alkaline volcanic arc that crosses Mexico from the Pacific Ocean to the Gulf of Mexico. This study characterises the explosive activity of the San Mateo Pumice eruption that is the most recent and largest paroxysmal episode of this volcano and the last unit of the Cerritos Member. Indeed, this eruption is associated with an undated debris avalanche event consisting of a major sector collapse developed on the western flank of the volcano.Stratigraphic data suggest a highly explosive eruption that generated a relatively thick (up to 10 m) and widespread pumice fallout deposit, trending to the NE, and an aerial distribution of at least 100 km2. This subplinian type eruption of VEI 4 lasted about 9 h and produced a plume height up to 12 km above the vent (i.e., ~14 km above sea level). This type of explosive eruption is relatively common in the TMVB, so a comprehensive geological study, with the corresponding numerical simulations like the one that was carried out, contributes to the understanding of the explosive volcanism of modest size stratovolcanoes that are common in the TMVB.

Permo-Carboniferous granitoids with Jurassic high temperature metamorphism in Central Pontides, Northern Turkey

In the northern part of the Central Pontides (N Turkey) there are different metamorphic rocks exposed, notably the Devrekani metamorphic rocks. Here, upper amphibolite-lower granulite facies metamorphic rocks contain predominantly paragneiss, orthogneiss and metacarbonate, and to a lesser extent, amphibolite and quartzite, with cross-cutting aplite, pegmatite and granite veins. This is the first report of these rocks and includes new data on the petrochemistry, geochronology and metamorphic evolution of the Devrekani orthogneisses from the Central Pontides. The orthogneisses show five different mineral parageneses with the characteristic mineral assemblage quartz + K-feldspar + plagioclase + biotite ± hornblende ± opaque (± ilmenite and ± magnetite), and accessory minerals (zircon, sphene and apatite). These metamorphic rocks exhibit generally granoblastic, lepidogranoblastic and nematolepidogranoblastic with locally migmatitic and relic micrographic textures. They have well-developed centimeter-spaced gneissic banding and display gneissose structure with symmetric, asymmetric and irregular folds. The petrographic features, mineralogical assemblages and weak migmatization reflect high temperature conditions. Thermometric calculations in the orthogneisses indicate metamorphic temperatures reached 744 ± 33 °C. Field relations, petrography and petrochemistry suggest that the orthogneisses have predominantly granodioritic and some granitic protoliths, that show features of I-type, medium to high-potassic calc-alkaline volcanic arc granitoids. The orthogneisses have high contents of LILEs and low contents of HFSEs with negative Nb and Ti anomalies, which are typical of subduction-related magmas. The orthogneisses also show significant LREE enrichment relative to HREE with negative Eu anomalies (EuN/Eu* = 0.33–1.07) with LaN/LuN = 6.98–20.47 values. Based on U-Pb zircon dating data, the protoliths are related to Permo-Carboniferous (316–252 Ma) magmatism. It is likely that peak metamorphism took place during the Jurassic as reflected by the U-Pb zircon ages (199–158 Ma) and also 40Ar/39Ar from hornblende/biotite (163–152 Ma). The four biotite 40Ar/39Ar average ages from the rock samples are ca. 156 Ma, suggesting that the metamorphic rocks cooled to 350–400 °C at ca. 156 Ma. Conclusively, the Devrekani metamorphic rocks can be ascribed as products of Permo-Carboniferous continental arc magmatism overprinted by Jurassic metamorphism in the northern Central Pontides.

Age and duration of intra-oceanic arc volcanism built on a suprasubduction zone type oceanic crust in southern Neotethys, SE Anatolia

The southeastern Anatolia comprises numbers of tectono-magmatic/stratigraphic units such as the metamorphic massifs, the ophiolites, the volcanic arc units and the granitoid rocks. All of them play important role for the late Cretaceous evolution of the southern Neotethys. The spatial and temporal relations of these units suggest the progressive development of coeval magmatism and thrusting during the late Cretaceous northward subduction/accretion. Our new U-Pb zircon data from the rhyolitic rocks of the wide-spread volcanic arc unit show ages of (83.1 ± 2.2)–(74.6 ± 4.4) Ma. Comparison of the ophiolites, the volcanic arc units and the granitoids suggest following late Cretaceous geological evolution. The ophiolites formed in a suprasubduction zone (SSZ) setting as a result of northward intra-oceanic subduction. A wide-spread island-arc tholeiitic volcanic unit developed on the top of the SSZ-type crust during 83–75 Ma. Related to regional plate convergence, northward under-thrusting of SSZ-type ophiolites and volcanic arc units was initiated beneath the Tauride platform (Malatya-Keban) and followed by the intrusion of I-type calc-alkaline volcanic arc granitoids during 84–82 Ma. New U-Pb ages from the arc-related volcanic-sedimentary unit and granitoids indicate that under-thrusting of ophiolites together with the arc-related units beneath the Malatya-Keban platform took place soon after the initiation of the volcanic arc on the top of the SSZ-type crust. Then the arc-related volcanic-sedimentary unit continued its development and lasted at ∼75 Ma until the deposition of the late Campanian–Maastrichtian shallow marine limestone. The subduction trench eventually collided with the Bitlis-Pütürge massif giving rise to HP-LT metamorphism of the Bitlis massif. Although the development of the volcanic arc units and the granitoids were coeval at the initial stage of the subduction/accretion both tectono-magmatic units were genetically different from each other.

Preservation of terranes during the assembly of the Annieopsquotch Accretionary Tract: Inferences from the provenance of a Middle Ordovician ophiolite to arc transition, central Newfoundland Appalachians1Geological Survey of Canada Contribution 20100463.2This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The Newfoundland Appalachians are a classic area for studying the record of arc development and terrane accretion processes with excellent exposure to different crustal levels that are minimally deformed and metamorphosed. This area also provides a link between the once continuous Appalachian and Caledonian orogens. The Annieopsquotch Accretionary Tract lies along the Red Indian Line, within the peri-Laurentian realm of the central Newfoundland Appalachians. The Darriwilian (468–461 Ma) tectonostratigraphic units of the Annieopsquotch Accretionary Tract are commonly characterized by polymictic volcanogenic conglomerate horizons. A conglomerate horizon at the interface between a suprasubduction zone ophiolite and its calc-alkaline volcanic arc cover sequence is herein investigated for zircon and geochemical provenance. Geochronology revealed a maximum age of deposition of 467 ± 4 Ma with zircon inheritance ranging from ca. 500 to 2800 Ma, consistent with a peri-Laurentian continental basement source. Four types of volcanogenic conglomerate clasts are noted on the basis of lithogeochemistry: arc andesite; calc-alkaline basalt; tholeiitic basalt; and non-arc rhyodacite. Tholeiitic basalt clasts are likely locally derived, perhaps from the underlying Skidder Formation. Other volcanic clasts do not have any known geochemical equivalents in the Annieopsquotch Accretionary Tract and hence appear to be exotic. The dominant zircon population suggests that the exotic clasts were derived from a ca. 467 Ma peri-Laurentian andesitic volcanic arc that once formed part of the Annieopsquotch Accretionary Tract.

Geochemistry of volcanic rocks in a traverse through Nicaragua

Major element composition and preliminar trace element data for 138 samples of mainly basic and intermediate volcanic rocks from a cross section between the Pacific and Atlantic coasts of Nicaragua suggests that the present tectonic setting-subdution of an oceanic plate below the western margin of Central American can be extrapolated back to the middle Tertiary. The samples can be divided into three groups with regard to their chemistry: (a) the Recent to Tertiary volcanics from the entire traverse, (b) Cretaceous lavas from the Siuna mining district in NE Nicaragua, and (c) young basalts from Corn Island off Bluefields on the Atlantic coast. Teh first (and by far the largest) group are calc-alkaline volcanic arc lavas beloging to the high-alumina series. Many of them (especially those of the Tamarindo Formation and Recen lavas in NW Nicaragua) have tholeiitic affinities, consistent with the thin crust in this part of Central America. However, a common feature is an anomalous enrichement in Ba-rich active ridge sediments of the subducting oceanic plate. Crustal contamination is another possible explanation for the high values of Ba and other large-ion lithophile elements, especially for the volcanics in NE Nicaragua, where the influence of a towards NE, away from the trench. Crustal contamination is clarly evident in the second group, the Siuna lavas, which have a shoshonitic affinity. The third group -the Corn Island basalts- have chemical features inconsisten with a subdution.related origin, being similar to some within-plate basalt of oceanic islands.

Did the Kohistan-Ladakh island arc collide first with India?

Research Article| March 01, 2009 Did the Kohistan-Ladakh island arc collide first with India? Shuhab D. Khan; Shuhab D. Khan † 1Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA †E-mail: sdkhan@uh.edu Search for other works by this author on: GSW Google Scholar Douglas J. Walker; Douglas J. Walker 2Department of Geology, University of Kansas, Lawrence, Kansas 66045-7613, USA Search for other works by this author on: GSW Google Scholar Stuart A. Hall; Stuart A. Hall 3Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA Search for other works by this author on: GSW Google Scholar Kevin C. Burke; Kevin C. Burke 3Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA Search for other works by this author on: GSW Google Scholar Mohammad T. Shah; Mohammad T. Shah 4National Center of Excellence in Geology, University of Peshawar, Peshawar, 25120, Pakistan Search for other works by this author on: GSW Google Scholar Lisa Stockli Lisa Stockli 5Department of Geology, University of Kansas, Lawrence, Kansas 66045-7613, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2009) 121 (3-4): 366–384. https://doi.org/10.1130/B26348.1 Article history received: 19 Oct 2007 rev-recd: 17 Jul 2008 accepted: 18 Jul 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Shuhab D. Khan, Douglas J. Walker, Stuart A. Hall, Kevin C. Burke, Mohammad T. Shah, Lisa Stockli; Did the Kohistan-Ladakh island arc collide first with India?. GSA Bulletin 2009;; 121 (3-4): 366–384. doi: https://doi.org/10.1130/B26348.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Kohistan-Ladakh block occupies the northwestern corner of the Himalayan mountains and has long been recognized to represent an island arc constructed on ocean floor during Jurassic and Cretaceous times. Because the Kohistan-Ladakh block now lies within the Asian continent, it is important to know how and when it became sandwiched between India and the rest of Asia. We have found from analysis of paleomagnetic data that in Late Cretaceous–early Paleocene times, the Kohistan-Ladakh island arc could not have been far from the equator. India was close to the equator, but the southern margin of Asia was more than 3000 km to the north. Our new U-Pb zircon age results from rocks of the Kohistan-Ladakh block show that calc-alkaline volcanic arc igneous activity ended in the Kohistan-Ladakh arc by 61 Ma. We interpret that cessation to date the collision of Kohistan with India. This new timing is confirmed by evidence that a Southern Hemisphere enriched DUPAL mantle source was involved in the generation of the latest Cretaceous Teru Volcanic Formation rocks of the Kohistan-Ladakh arc. Further confirmation of the collision of the Kohistan arc with India in early Paleocene times comes from evidence of the timing of obduction of ophiolites and from the unconformity of postcollisional sedimentary rocks onto the Indian continental margin in northwestern and western Pakistan. Final incorporation of India, now carrying the Kohistan-Ladakh block in its NW corner, into Asia took place at the Shyok suture. The best evidence for the timing of that suturing ca. 50 Ma comes from two postcollisional granites (ages 47 Ma and 41 Ma) in northern Kohistan, which show in their zircon isotopic compositions evidence of the involvement of ancient Asian continental crust that did not exist under Kohistan before the suture formed. The 50 Ma age for Shyok suturing against the then-active Karakoram Andean arc fits well with the extension of the suture beyond the eastern end of the Kohistan-Ladakh block to join the precisely dated ca. 51 Ma Yarlung–Tsang Po suture between India and the southern (Lhasa block) margin of Tibet, which at that time was also occupied by an Andean arc, the Gangdese arc. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Detrital garnet and chromian spinel chemistry of Permian clastics in the Renge area, central Japan: Implications for the paleogeography of the East Asian continental margin

The Japanese Islands comprise a complex assembly of allochthonous continental shelf deposits and subduction-related accretionary complexes. The paleogeography of Permian clastic strata was restored based on chemical analysis of detrital garnets and chromian spinels, which are useful for provenance discrimination. Detrital garnet assemblages from the Permian clastic rocks in Japan are characterized by a grandite-rich assemblage and a grandite-poor assemblage. Most trench-fill deposits in Permian accretionary complexes contain the grandite-poor assemblage, which is composed of spessartine-rich almandine, grossular-rich almandine, and rarely pyrope-rich almandine, and is commonly associated with chromian spinels and volcanic rock fragments. It is suggested that the sediments were derived from a volcanic arc and associated non-calcareous contact-metamorphic rocks along the North China Block, where pelitic granulites including pyrope-rich almandine garnets are extensively exposed in the northern margin. Ophiolite and high-P/T schist also crop out along the continental margin. Most Permian shelf deposits and the trench-fill deposits of the central part of the accretionary complexes in Japan contain the grandite-rich assemblage. Analyses suggest provenance from a calc-alkaline volcanic arc and association with calcareous contact-metamorphic rocks including skarn deposits. Sediments lack ophiolite detritus. A part of the grandite-poor trench-fill deposits of the accretionary complexes is overlain by grandite-rich clastic rocks. This indicates that the grandite-rich detritus increased and the supply extended through the fore-arc basin to the trench. The trench-fill deposits in the northernmost part of the Permian accretionary complex in the Renge area yield a grandite-poor assemblage, particularly uvarovite-rich andradite garnets and chromian spinels derived from a back-arc basin basalt, which is very similar to the clastic composition of the Permian back-arc basin strata of Japan. Hence, we conclude that a back-arc basin existed in the northernmost region of the Permian subduction zone along the North China Block.

Spatial, temporal and geochemical characteristics of Silurian collision-zone magmatism, Newfoundland Appalachians: An example of a rapidly evolving magmatic system related to slab break-off

Silurian plutonic suites in the Newfoundland Appalachians include abundant gabbro, monzogabbro and granite to granodiorite and lesser quartz diorite and tonalite. Most are medium- to high-K, but included are some low-K and shoshonitic mafic compositions. Felsic rocks are of both alkaline (A-type or within-plate granite (WPG)) and calc-alkaline volcanic arc granite (VAG) affinity. Mafic rocks include both arc-like (Nb/Th < 3) calc-alkaline and non-arc-like (Nb/Th > 3) transitional calc-alkaline basalt to continental tholeiitic affinity compositions. ε Nd( T) values range from − 9.6 to + 5.4 and δ 18O (VSMOW) values range from + 3.1 to + 13.2‰. A rapid progression from exclusively arc-type to non-arc-like mafic and then contemporaneous WPG plus VAG magmatism has been documented using precise U–Pb zircon dating. Earlier arc-like plutonism indicates subduction, while asthenosphere-derived mafic magmas support slab break-off, due to subduction of a young, warm back-arc basin. Contemporaneous mafic magmas with arc and non-arc geochemical signatures may reflect tapping of asthenospheric and subcontinental lithospheric mantle (SCLM) sources and/or contamination of asthenosphere-derived magmas by SCLM or crust. The brevity (< 5 Ma) of the mafic magmatic pulse agrees with the transient nature of magmatism associated with slab break-off. The subsequent ca. 1 to 2 m.y. period of voluminous WPG and VAG plutonism likely reflects mafic magma-driven partial melting of both SCLM and crustal sources, respectively. Continuation of VAG-like magmatism for an additional 2 to 5 m.y. may reflect lower solidus temperatures of crustal materials, enabling anatexis to continue after mantle melting ceased. East to west spatial variation of ε Nd and (La/Yb) CN in Silurian plutons suggests a transition from shallow melting of juvenile sources proximal to the collision zone to deeper melting of old source materials in the garnet-stability field further inboard. Previous work has demonstrated that geochemical discriminaton of post-collisional granitoid magmatism (PCGM) is difficult in the absence of other constraints. Our example should contribute to the understanding and identification of PCGM if it can be employed as a ‘fingerprint’ for slab break-off-related PCGM within the Paleozoic geological record.

Geochronology and nature of the Palaeoproterozoic basement in the Central African Copperbelt (Zambia and the Democratic Republic of Congo), with regional implications

U–Pb SHRIMP zircon age data, together with geochemical analyses, from the basement to the Katanga Supergroup in the Central African Copperbelt reveal the existence of a widespread Palaeoproterozoic magmatic arc terrane. The Lufubu schists represent a long-lived calc-alkaline volcanic arc sequence and, where dated in both Zambia and the Democratic Republic of Congo (DRC), yield ages of 1980 ± 7, 1968 ± 9, 1964 ± 12 and 1874 ± 8 Ma. The oldest dated unit from the region, the Mkushi granitic gneiss from south-east of the Zambian Copperbelt, has an age of 2049 ± 6 Ma. The copper-mineralized Mtuga aplites, which crosscut the foliation in the Mkushi gneisses, have mainly xenocrystic, zoned zircons with cores dated at ca. 2.07–2.00 Ga. Overgrowths on these cores are dated at 1059 ± 26 Ma, which is interpreted as the intrusive age of the aplites. An augen gneiss from the Mulungushi Bridge locality yielded an emplacement age of 1976 ± 5 Ma. The Mufulira Pink Granite has an age of 1994 ± 7 Ma, while the Chambishi granite has been dated at 1983 ± 5 Ma, an age within error of Lufubu schist metavolcanics from elsewhere in the Chambishi basin. The gneisses, granitoids and acid-intermediate calc-alkaline metavolcanics are considered to represent stages in the evolution of one or more magmatic arcs that formed episodically over a 200 million year period between 2050 and 1850 Ma. We suggest naming this assemblage of rocks the “Lufubu Metamorphic Complex”. The rocks of the Lufubu Metamorphic Complex are interpreted to be part of a regionally extensive Palaeoproterozoic magmatic arc terrane stretching from northern Namibia to northern Zambia and the DRC. This terrane is termed the Kamanjab–Bangweulu arc and is inferred to have collided with the Archaean Tanzanian craton during the ca. 2.0–1.9 Ga Ubendian orogeny, to produce a new composite minicontinental entity that we term the “Kambantan” terrane. The Kambantan terrane was accreted onto the southern margin of the Congo craton during the ca. 1.4–1.0 Kibaran orogeny.

A reappraaisal of the geology, geochemistry, structures and tectonics of the Mozambique belt in Kenya, east of the rift system

The largest segment of the Neoproterozoic Mozambique belt in Kenya occurs east of the north-south oriented Rift system. Geological works carried out in the country during the last few decades have progressively revealed the complexity of the geology, structures and tectonics of the Mozambique belt in the region. Important high grade tectono-thermal events in the belt took place between about 845 and 715 Ma. The tectonothermal events attained P/T conditions of 5.5 - 7.1 kbars and 500 - 750o C. The subsequent cooling and uplift of the basement has been traced by K-Ar dates on biotites, which range between 528 and 438 Ma. New lithological units established in the last two decades include widespread granitoid, anorthositic, gabbroic to ultramafic intrusions and limited andesitic volcanics in the central region of this eastern segment of the Mozambique belt (EMBS). Previously the geology of this central region had been considered to consist predominantly of pelitic and semi-pelitic schists and gneisses, migmatites and amphibolites. Further north in the Chanler's Falls and Archer's Post areas, this segment is dominated by paragneisses and schists, marbles and calc-silicate rocks, granulites, as well as metamorphosed mafic and ultramafic rocks. A similar lithological metamorphic assemblage as the one in the north is repeatedly noted in the southern part of the belt. Here in the Taita Hills region, close to the Tanzanian border, paragneisses, marbles, amphibolites and metamorphosed ultramafic rocks, with ophiolitic signatures are widespread. West of the Taita Hills in Kajiado district, quartzites are an additional rock unit while meta-ultramafic rocks are notably absent. Complex folding, exhibiting at least three fold generations, and which occasionally have formed elliptical dome and basin structures, are widespread. These are particularly common in the central region of the EMBS. The entire segment however, has several Neoproterozoic faults, thrust and shear zones, some of which are of regional dimensions. For example, the Yatta shear zone, extending for over 300 km in a NW-SE direction, obliquely subdivides this segment into two. Cenozoic reactivation of some of the fault and shear zones is quite evident. Petrography coupled with limited geochemical analyses indicate the probable existence of Neoproterozoic island arcs in the central region of this segment. In particular, while the sequences in the northern section of the EMBS indicate an ophiolitic- midocean ridge- to island arc setting, the central section show an affinity ranging from volcanic arc- calc-alkaline volcanic arc- to within-plate volcanics setting. The southern section of the EMBS indicate a setting sequence ranging from continental shelfophiolitic sature - midocean ridge – within plate to subduction related volcanics and island arc setting. The sequences in the nothern and southern sections of the belt are interpreted to be parts of the previously deep marine “proto Mozambique ocean” prior to the protracted Mozambique orogenic event. Some of the shallow marginal zones of this ocean are likewise inferred to have been in the west, where extensive quartzite horizons are presently found. The authors strongly recommend a more enhanced multi-disciplinary study of the EMBS in Kenya. Key Words: Overview, geology, geochemistry, structures, Mozambique belt in Kenya African Journal of Science and Technology Vol.4(2) 2003:51-71

Oligocene reefal deposits in the Pisang Ridge and the origin of the Lucipara Block (Banda Sea, eastern Indonesia)

In 1998 we dredged early Oligocene/late Oligocene boundary, shallow-water reefal deposits and pelagic, early Pliocene muds in the submerged Pisang Ridge. The shallow-water deposits were identified as moderate- to low-energy coral-reef depositional environments, which confirms that the Pisang Ridge belonged to the continental to continental-arc Lucipara Block which also includes the Tukang Besi, Lucipara and Rama ridges. On the basis of these findings, it was possible to reconstruct parts of the general geological evolution of the Lucipara Block, which drifted away from northern Irian Jaya during mid-Miocene times and collided with the Kolonodale Block during the late Miocene. A late-early Oligocene calc-alkaline volcanic arc developed in the Weber Trough area, then uplifted to a shallow-water position at the early–late Oligocene boundary in the Pisang Ridge. Late Oligocene–early Miocene metamorphism subsequently developed, prior to the deposition of early Miocene coral reefs in the Rama Ridge. Locally, late Miocene metamorphism was identified in the Lucipara Ridge, prior to the latest Miocene–Pliocene general drowning and the splitting of the Lucipara Block into several small blocks throughout the Banda Sea region.

The interplay of lithospheric flexure and thrust accomodation in forming stratigraphic sequences in the southern Apennines foreland basin system, Italy

Most of the regional shortening in the type area of the southern Italy orogenic belt, Calabria and southern Apennines, was accomodated by displacement on the Calabrian Terrane (Oligocene to middle Miocene), the Liguride and Sicilide Complex (lower to middle Miocene), the Alburni-Cervati-Pollino-Monti della Maddalena units (inner carbonate platform and slope; middle to upper Miocene), Lagonegro units (upper Miocene), the Monte Alpi unit (lower Pliocene) thrust systems. Inception of each thrust system generated synorogenic sediment associated with frontal thrust-tip anticlines or triangle zones and older thrust sheets that were elevated above major ramps farther toward the hinterland. The Neogene kinematic and sedimentary evolution of the southern Italy is related by shortening of the Apennines and Calabria and stretching of the Tyrrhenian Basin. The southern Italy orogenic belt consists of tectonic assemblages of Paleozoic crystalline rocks (various metamorphic rocks and plutonics), Triassic through upper Miocene platform to deep-marine carbonatic and siliceous rocks, Jurassic to Cretaceous ophiolites and related oceanic sedimentary strata, Paleogene to lower Miocene Subduction Complex assemblage, and lower Miocene through Pleistocene foreland clastic strata. Triassic through Quaternary clastic sediments are accumulated in tectonically diverse sedimentary basins, recording the key geodynamic events affected the Mediterranean region after the Late Paleozoic-Early Mesozoic breakup of Pangea. These sediments are generated following the onset of the sequential rift development and continental separation (Triassic to Early Jurassic), the Tethyan proto-oceanic rifting and open-ocean phases (Early-Middle Jurassic to Early Cretaceous), the Tethyan subduction and onset of Alpine orogenesis (Middle-Late Cretaceous to Paleogene), the Adria (or Apennines) orogenesis (early Miocene to present), and the Tyrrhenian backarc rifting (or Tyrrhenian phase; 15 Ma to present). The oldest (early-middle Triassic) siliciclastic sediments of southern Italy are on the Apulia plate (the buried base of the Apulia platform), the Monte Facito Formation of the Lagonegro Basin, and redbeds of Sardinia (Iberian plate). During Upper Triassic to Middle Jurassic, continental redbeds (rift-valley phase), and shelf-slope-basin sediments (proto-oceanic rift phase) having quartzose composition were accumulated directly on the crustal basement of the Iberia plate margin, in Calabria-Peloritani domain, but siliciclastics are absent on the southern Apulia plate. These sediments record the initial phases of the Tethyan rifting processes. Siliciclastic accumulations in southern Italy assemblages during the main alpine phases (Cretaceous to Paleogene) are scarse or absent. The Cretaceous to early Miocene alpine tectonic phases in southern Italy include remnants of the Alpine orogen (Calabrian Block) and quartzofeldspathic siliciclastic deposits of the Oligocene to early Miocene Paludi, Stilo-Capo d’Orlando and Frazzano formations, and volcanolithic, quartzolithic and quartzofeldspathic sands of the Calabro-Lucanian Flysch Unit. The siliciclastic deposits are directly related to new subduction plane of the Adriatic-Ionian oceanic lithosphere beneath the Iberia plate, generating an oceanic subduction complex (Liguride Complex of southern Italy), and a calcalkaline volcanic arc (Sardinia). From late Paleogene to the present, the siliciclastic sedimentary sequences of southern Italy filled basins that are directly related to this convergent setting, causing consumption of the oceanic lithosphere, and subsequent accretion of the Calabrian allochthonous terranes over the Adria-Africa plate, generating post-Oligocene foreland basin systems. The composition as well as stratigraphic and structural relations of clastic sediments derived from erosion of the Calabrian microplate provide constraints for paleogeographic and tectonic models of southern Italy. Clastic detritus in the following sedimentary assemblages were derived mainly from provenance terranes within the strongly deformed Calabrian Arc allochthon: (A) the Paleogene «Calabro-Lucanian Flysch Unit» accretionary wedge, deposited in a remnant ocean basin that lay east of the Calabrian Arc and west of the Adria margin; (B) Burdigalian to lower Messinian foreland basin successions, widely developed in the southern Apennines, representing progressively shifted foredeep basins and related wedgetop basins; (C) Upper Tortonian to Messinian nonmarine to shallow-marine and deep-marine successions, cropping out on the western and eastern Calabrian Arc, representing synrift clastic wedges related to backarc rifting in the peri-Tyrrhenian area (western sequences), or foreland clastic wedges in the peri-Ionian area (eastern sequences); (D) Pliocene and Quaternary of the northern Calabrian Arc, represented by foredeep and related wedge-top basins on the eastern side (Gulf of Taranto and Corigliano Basin), and a slope basin on the western side, the eastern Tyrrhenian margin (Paola Basin). These receive detritus primarily from deep erosion of northern Calabria. The modern deep-marine basins of offshore northern Calabria have many similarities to the middle to upper Miocene clastic sequences in both foreland and backarc regions of the southern Italy. The type of sedimentary provenance of the southern Italy foreland basin system, providing an example of the changing nature of the orogenic belt through time, may contribute to general understanding and application of other major orogens.