The Sierra de Calalaste in Northwestern Argentina, a counterpart of the peri-Gondwanan Ellsworth Terrane in the northern Appalachians?
The Ellsworth terrane, coastal Maine, is one of a number of fault-bounded blocks that occur along the eastern margin of Ganderia, a peri-Gondwanan terrane that underlies the bulk of the central part of the northern Appalachians. The Ellsworth block is considered part of the Ganderia microcontinent based on its geological history during the Early Paleozoic lithological characteristics and abundance of Early Cambrian detrital zircon. U-Pb zircon geochronology of rhyolitic schists indicates that both the Ellsworth schists (508 ± 0.8 Ma) and overlying Castine volcanics (503 ± 2.5 Ma) are middle Cambrian in age. The rhyolitic tuffs are interlayered with lava flows of enriched MORB affinity. The ε Nd (500) is depleted + 5.8 to +7.5, and Pb isotopic composition is unradiogenic to relatively enriched. Serpentinezed ultramafic sheets and massive sulfide deposits are present. The geologic, geochemical and isotopic characteristics of the bimodal volcanic sequences led recent workers to interpret the Ellsworth terrane as the remnants of a proto-oceanic rift system. The occidental sector of Puna in NW Argentina contains Cambrian to Ordovician magmatic associations distributed in two belts, the Occidental Eruptive Belt and the Oriental Eruptive Belt. Bimodal, basalts and rhyolites of the Occidental eruptive belt correspond to subalkaline, tholeiitic, low-K sequences developed in a suprasubduction zone environment. The Sierra de Calaste Ranges of the Occidental Eruptive Belt comprise a sequence of metaquarzites, rhyolitic flows and tuffs, mafic gabbroic rocks and some serpentinezed ultramafic sheets. Small massive sulfide deposits are locally also present. U-Pb zircon ages on rhyolites are Upper Cambrian (495 ± 3Ma). Their ε Nd values are +4 to+7. Pb isotopic compositions are unradiogenic. The rocks are interpreted to belong to a rift setting similar to that proposed for the Ellsworth terrane, which is nearly coeval in age. Recent reconnaissance fieldwork highlight strong similarities between the Sierra de Calalaste rocks and the Ellsworth terrane. A prominent NNE-SSW shear zone, named the Tipa Shear Zone is a major terrane boundary between the Western Pampia and the Famatina terranes and could represent an older terrane boundary that accommodated the separation of Ganderia or an along strike-equivalent (Carolinia?) from Gondwana. We believe that the Ellsworth terrane and the Calaste Ranges, are pieces of a related proto-oceanic setting developed along the western margin of Gondwana during the Middle to Late Cambrian immediately before Ganderia detached from the Gondwanan margin.
Paleomagnetic Progress in Peri-Gondwanan Terranes of Cape Breton Island, Nova Scotia
Paleopoles from primary Ediacaran magnetization directions established the Gondwanan origin of northern Appalachian Avalonian terranes, but magnetic overprints in the same rocks also provide useful tectonic information. Thus, in the Southeastern New England Avalon Zone, virtual geomagnetic poles (VGPs) calculated from magnetic B and C components in both 595 Ma Lynn-Mattapan volcanic rocks and 490-488 Ma Nahant Gabbro track mid- and late Paleozoic segments of the North American apparent polar wander path (APWP), suggesting the influence of Acadian and Neo-Acadian accretionary events. We report here on multi- vectorial magnetizations in pilot samples from Cape Breton Island, Nova Scotia where the Bras d'Or and Mira terranes represent both Ganderian and Avalonian elements transferred from Gondwana. Overprint relationships in these terranes may constrain their amalgamation with each other as well their docking with Laurentia. As in southeastern New England, secondary remanences can be identified in Cape Breton Island as consistent magnetization directions in rocks of differing ages. The S- to SSE-trending and gently downward pointing direction reported in 1985 by Johnson and Van der Voo in Middle Cambrian sedimentary rocks of the Bourinot Group (Bras d'Or terrane), for example, is also present in the 563 Ma Main à Dieu Formation and in 620 Ma Chisholm Brook Granite and East Bay Hill rhyolite (Mira terrane). This magnetization represents the C component already found around Boston, MA. The resulting VGPs in both areas occupy positions on the North American APWP consistent with a Neo-Acadian overprint, possibly related to the docking of the Meguma terrane against previously accreted Avalonia. Other overprint directions encountered in this investigation give rise to VGPs that do not coincide with the North American APWP, hence appear to reflect tectonic events independent of Laurentia. One such cluster comprising both Mira and Bras d'Or VGPs includes the paleopole also reported by Johnson and Van der Voo for volcanic rocks in the Bourinot Group. This relationship suggests that the two terranes were neighbors at moderate southerly paleolatitudes by ca. 505 Ma and allows the possibility that Bourinot volcanism played a role in overprinting older rocks including 620 Ma East Bay Hills rhyolite (Mira) and 553 Ma Creignish Hills granite (Bras d'Or). Another intriguing observation from our preliminary data is a low-latitude cluster of VGPs that lies near the Late Ordovician overprint VGPs from Swedish limestones (after rotation into a North American reference frame). Other workers have previously interpreted these Baltic VGPs to reflect the collision between East Avalonia and Baltica during closure of the Tornquist Sea. Peeling away the overprints reveals a possible primary direction in 620 Ma rocks of the Mira terrane. Three pilot samples from two sites show normal and reverse polarity and a positive tilt test. VGPs from these samples resemble the VGP obtained from the 609 Ma Dedham Granite in Boston, Massachusetts.
Acritarchs in Cambrian and Lower Ordovician Rocks of Nova Scotia and New Brunswick, Canada: New Constraints on Correlations and Paleogeography
Most of 200 samples collected from Avalonian Cambrian and Lower Ordovician sequences in Nova Scotia and New Brunswick, as well as age-equivalent units in the Meguma terrane of southern Nova Scotia, have yielded acritarchs. Preliminary results show that they include taxa not reported previously from western Avalonia (Newfoundland), and indicate revised stratigraphical ranges for several taxa, thus increasing potential for stratigraphic comparisons and regional and global correlations, in particular to Baltica and northern Gondwana. Well-preserved material was obtained from the upper part of the MacLean Brook Formation (early Furongian) in the Mira River area of southeastern Cape Breton Island, including Petaloferidium lacrimiferum, Stelliferidium albanii and S. magnum and the first record in western Avalonia of Pirea orbicularis, a species which permits precise correlation to Baltica and northern Gondwana. Acritarchs from the MacNeil Formation in the Mira River area include Cristallinium randomense and Stelliferidium cortinulum. These findings suggest that any stratigraphic gap between the MacLean Brook Formation and the overlying MacNeil Formation is smaller than previously interpreted. In New Brunswick, the classic Lower to Middle Cambrian Hanford Brook section yielded acritarchs throughout the sequence. The upper half of the Ratcliffe Brook Formation (Early Cambrian) contains Archaeodiscina umbonulata and abundant Skiagia ornata, S. orbiculare and S. scottica. This assemblage of acritarchs has a global first appearance generally thought to be close in time to the rise of trilobites, potentially in conflict with the established view that all of the Ratcliffe Brook Formation is pre-trilobitic. The upper part of the Hanford Brook Formation yielded well- preserved material of Comasphaeridium silesiense, Heliosphaeridium notatum, Eliasum llaniscum and Liepania plana, taxa with a first appearance close to the base of the Middle Cambrian. Further studies of the Hanford Brook section promise to help constrain the age and regional correlation of the Protolenus elegans Trilobite Zone, which at present are problematic. Acritarchs from the middle part of the King Square Formation include Eliasum llaniscum, Timofeevia lancarae, Cristallinium cambriense, C. dubium and Polygonium sp. This assemblage suggests that the medusoid-bearing portion of the King Square Formation is late middle Cambrian (Cambrian series 3) and probably older than Agnostus pisiformis. Acritarchs close to the transition between the King Square and Silver Falls formations include Petaloferidium lacrimiferum, Stelliferidium albanii and S. magnum, consistent with correlation to the MacLean Brook Formation in the Mira River area, but not to its upper part. In the Meguma terrane of southern Nova Scotia, acritarchs have been recovered from various levels of the Halifax Group in sections at Bear River and Black River. Preliminary identifications include several species of Acanthodiacrodium, Veryhachium and Stelliferidium trifidumv, consistent with the Tremadocian age for this unit inferred from a sparse graptolite record. Acritarchs offer possibility for better constraints on the age of the upper part of the Halifax Group.
Geology of the Governor Lake Area ('Liscomb Complex'), Meguma Terrane, Nova Scotia, Canada
In 2008, a detailed (1:10 000 scale) bedrock mapping and sampling project was initiated in the Governor Lake area ('Liscomb Complex') in the central Meguma terrane to better constrain the metamorphic and structural history of the area. The oldest units exposed are the Lower Paleozoic Goldenville and Halifax groups. The Goldenville Group is composed of the lower metasandstone-dominated Governor Lake and Taylors Head formations and overlying Beaverbank Formation, a coticule-bearing metasiltstone. Units in the overlying Halifax Group include the slate-rich Cunard Formation and Glen Brook Formation. These metasedimentary units are similar to those established elsewhere in the Meguma terrane. They are deformed into regional, east- to northeast-trending folds with well-developed axial planar cleavages and northeast- and southwest-plunging intersection lineations, produced during the Middle Devonian to Early Carboniferous Neoacadian Orogeny. Deformation was accompanied by greenschist-facies (chlorite-grade) regional metamorphism. A suite of ca. 385 to 370 Ma igneous units, the Trafalgar Plutonic Suite, intruded the Goldenville and Halifax groups in the Governor Lake area. Based on field evidence combined with geochronology the order of intrusion is: (1) tonalite to quartz diorite with magma-mingling textures, and minor gabbroic enclaves; large garnet crystals are locally abundant; (2) granodiorite with magma-mingling textures and tonalitic enclaves; (3) coarse- grained to megacrystic biotite-muscovite monzogranite; (4) medium- to coarse-grained muscovite-biotite monzogranite; and (5) fine- to medium-grained muscovite monzogranite to syenogranite. The granodioritic units are locally protomylonitic and cut by undeformed monzogranite, suggesting some deformation was synchronous with intrusion. These plutons produced a narrow contact metamorphic zone (less than 500 m in width), consisting of spotted hornfels to granofels containing sillimanite, andalusite, cordierite, +/- garnet, and +/- staurolite superimposed on chlorite-zone regional metamorphic assemblages. The results of this project do not support previous interpretations of the presence in the Govenors Lake area of basement gneissic units with upper amphibolite- to granulite-facies metamorphic assemblages. Units previously identified as mafic and quartzofeldspathic gneiss appear to be igneous units with magma-mingling textures or superimposed protomylonitic fabrics. The often-cited but unpublished granulite-facies P-T conditions inferred for the complex seem inconsistent with the field evidence. No extensional structures were observed to support the previously proposed core-complex scenario. Therefore we suggest that the 'Liscomb Complex' is not as complex as previously interpreted, and that the term should be abandoned.
Altering Avalonia - Tracking fluid-rock interaction in the Mira Terrane, Cape Breton Island, Nova Scotia.
Pervasive oxygen-isotope depletion of the Neoproterozoic rocks that make up West Avalonia characterizes an area from Newfoundland to Boston. This depletion provides an additional geochemical way of distinguishing Avalonian terranes from other tectonostratigraphic units comprising the Appalachian Orogen. The 18O- depletion may have been produced by fluid infiltration and hydrothermal alteration of these rocks during initial rifting and extensional-related magmatism in the late Neoproterozoic. A study of the vein systems in such Neoproterozoic rocks of the Mira terrane, Cape Breton Island, is underway to determine the origin and composition of the fluids that penetrated the Avalonian crust at this time. The Neoproterozoic volcanic units of the Mira terrane contain abundant vein systems that are also present in the associated intrusive rocks but noticeably absent in the younger Paleozoic intrusions. Six vein types have been identified with a paragenetic sequence, from oldest to youngest, as follows: (1) quartz-albite, (2) quartz- epidote, (3) quartz, (4) quartz-chlorite ± calcite, (5) quartz-calcite, and (6) massive calcite. Primary mono- and bi-phase aqueous-dominant inclusions occur in all vein types. Last ice-melting temperatures of inclusions in most veins indicate low salinity fluids (0 to 3.0 wt% equiv. NaCl), suggesting a meteoric-dominated fluid source. The exceptions are two quartz-albite and the quartz-chlorite ± calcite veins, which have calculated salinities of ~3.5 to 7.0 wt% equiv. NaCl. Homogenisation temperatures (to liquid), coupled with mineral-pair oxygen-isotope results, indicate that most vein types formed between 200 and 300°C. The early quartz-albite veins formed between 250 and 300°C, the later quartz-epidote and quartz-calcite veins formed between 200 and 250°C, and the massive calcite veins formed at ∼200°C. Mineral δ18O values, coupled with formation temperatures, indicate that the oxygen-isotope fluid composition in equilibrium with these veins was -1 to +2 ‰ for the early quartz-albite veins, -3 to 0 ‰ for the quartz-epidote/quartz veins and -1 ‰ for the quartz-calcite veins. Two quartz veins with very low δ18O values (+2.0 - +3.5 ‰) may indicate equilibration with a fluid with still lower δ18O values of -9 to -6 ‰. The late massive calcite veins are an exception, with δ18Ocal values of +22 ‰, indicating either precipitation/re-equilibration at much lower temperatures (<70°C) or precipitation from a high-18O fluid (∼ +12.5 ‰). These results show that predominantly low-salinity meteoric waters were involved in vein formation and hydrothermal alteration of the host rocks. The calculated fluid values overlap but trend towards greater 18O-depletion in the younger veins, suggesting progressively greater infiltration of meteoric water into the Avalonian crust.