Mafic Fragments Research Articles

Final Amalgamation Processes of the Southern Altaids: Insights from the Triassic Houhongquan Ophiolitic Mélange in the Beishan Orogen (NW China)

AbstractThe Permian–Triassic tectonic setting is still controversial in the southern Altaids. The Beishan orogen is an ideal region to address the final tectonic of the Altaids. These systematic mapping, geochemistry, and geochronology studies on the Houhongquan ophiolitic mélange in the south Beishan are conducted to address this issue. New mapping reveals that the Houhongquan ophiolitic mélange consists of blocks of gabbro, basalt, chert, granite, and strongly deformed and cleaved sandstone in the southern Beishan. The studies reveal that the mafic fragments are relics of normal-mid-ocean ridge (N-MOR) and suprasubduction zone (SSZ) types of oceanic lithosphere. The four sandstone matrix samples yield the maximum depositional ages of 222±5 Ma, 233.8±2.3 Ma, 263.4±2.5 Ma, and 263.5±2.8 Ma, respectively, indicating that the youngest sandstones were tectonic emplaced in the Houhongquan ophiolitic mélange after ca. 222 Ma. The sandstone matrices display two types of age spectra. Early Permian sandstones have a single Devonian to Early Permian peak age patterns, indicating the existence of an independent Permian intraoceanic arc. In contrast, Late Triassic sandstones have multiple peaks with some Precambrian zircons, suggesting that they were sourced from a continental arc. Accordingly, we consider that the Houhongquan ophiolitic mélange tectonic was emplaced in the intraoceanic island arc during the Middle Permian and docked to a continental margin arc during the Late Triassic. Thus, we argue that the terminal amalgamation timing of the southern Altaids was probably during ca. 222-217 Ma.

Evidence of a Palaeoproterozoic SLIP, northern Amazonian Craton, Brazil

The Orocaima SLIP consists of an association of acid-intermediate volcanic-plutonic rocks. The volcanic rocks were generated in explosive eruptions through low eruptive columns, probably associated with fissural volcanism in the north of the Amazonian Craton, Brazil, between 2.0 and 1.98 Ga. It generated ignimbrites, whose facies (volcanic breccia rich in lithic, lapilli-tuff and lithic lapilli-tuff) show the proximity of the source. The extensive area of ca. 200.000 km2 of ignimbrite, rhyolite and dacite deposits, as well as the age range (2.0–1.98 Ga) and geochemical signatures suggest that the Orocaima volcano-plutonism may correspond to one of the oldest silicic LIPs in the world. The silicic volcanism is essentially subaerial and characterized by high-grade ignimbrites (densely welded) and subordinate lava, the ages of which indicate the longevity of the volcanic event in the Orosirian. They are included in the Surumu Group and comprise rocks with high-K calc-alkaline affinities and were emplaced in a subduction-related setting, similar to the rocks that extend through Venezuela, Guyana and Suriname (Cuchivero-Surumu-Iwokrama-Dalbana metavolcanic belt - CSID). The occurrence of mafic fragments disseminated in volcanic and granitic rocks in the north of Roraima, Brazil and in other segments of the CSID belt suggests the coexistence of acid and basic magmas. Except for one sample (εNd(t) = -2.3), the Nd isotopic data of analyzed Surumu Group volcanic rocks yielded positive εNd(t) values (0.5–4.48; TDM = 2.0–2.47 Ga), suggesting generation from magmas derived from the mantle or from the melting of new juvenile crust. The Orocaima volcanism bears no evidence of involvement of Archean sources in the generation of the rocks. Thus, the Orocaima volcano-plutonism may represent one of the most significant ignimbrite eruption events during the Palaeoproterozoic in the world.

Mount Woolnough: the submerged volcano, SE of Sydney, NSW

Part of a larger investigation of the sea bed off Sydney was a study of the extinct submarine volcano Mount Woolnough. It is located approximately 41 km east of Kurnell, NSW, and protrudes 175 m above the sediment cover at depths of approximately −550 to −375 m. Volcanic rock, approximately 2.2 km in diameter, is exposed above the sediment sea floor and is much smaller than its magnetic expression (approximately 13 km in diameter). Samples dredged from Mount Woolnough were conglomerates with phosphatic nodules and volcanic fragments set in a fine foraminiferal sediment matrix. Zircons within the mafic fragments yielded a minimum age of 261 Ma.

The supra-ophiolitic sedimentary cover of the Asbestos ophiolite, Québec, Canada: First geochemical evidence of transition from oceanic to continental sediment flux

The 473 + 5/− 3 Myr Asbestos ophiolite complex of the Quebec Appalachians was formed in a forearc basin and obducted on a margin of Laurentia ~ 460 Myr ago. The complex together with its sedimentary cover is well exposed at Burbank Hill (~ 130 km SW of Québec City) where eight distinct lithologies have been identified: 1) pyroxenites and wehrlites with minor dunitic layers, (2) fractured gabbros, (3) breccia containing gabbro and diabase fragments, (4) polygenic conglomerates, (5) red mudstone/chert/sandstone succession, (6) tuffs with intercalated grey chert, (7) greenish grey mudstones and (8) slates and sandstones of the Saint-Daniel Mélange. The gabbroic and mafic volcanic rocks show the characteristics of arc tholeiites (TiO 2 ~ 0.8–0.85 wt.%) and boninites (TiO 2 < wt. 0.35% and Mg# > 45; (La/Sm) N ~ 1.9–3.3). These rock-types also occur as mafic fragments in the polygenic conglomerate. The mafic fragments (crystals and rocks) within the conglomerate and the ophiolitic sandstones also show the same greenschist facies metamorphism as the mafic igneous rocks of the complex. The conglomerate was probably formed after the forearc crust was fragmented by pre-obduction normal faults. These fault scarps would have promoted the erosion of the oceanic crust, leading to the accumulation of diverse detritus in grabens. The sedimentary lithologies which overlie the conglomerate exhibit a gradual increase in continent-derived material ((La/Sm) N ~ 5 and La/Yb ~ 20) and a decrease in ophiolitic material moving upward in the stratigraphic sequence. The sedimentary rocks at the bottom of the sequence (group 3) are mostly composed of ophiolite material with only 20% of crustal material. They also have a high content of Cr and MgO with values of 350–480 ppm and 5.7–8%, respectively. The rocks of the uppermost part of the pile (group 1) which contain up to 80% crustal material have low abundances of Cr and MgO (30–100 ppm and 2.7–4.5 wt.%). Continental detritus on the top of the Asbestos ophiolite suggests the complex formed near the Laurentian continental margin and that the eroded continental material had access to the basin where the ophiolite was formed. This is consistent with the obduction of the Asbestos and Thetford–Mines ophiolites on the Laurentian margin shortly after their formation. The proximity of thick Laurentian continental crust near the trench could explain why the subduction zone blocked-up soon after the ophiolites formation.

Sedimentation in a subaqueous arc/back-arc setting: the Bobby Cove Formation, Snooks Arms Group, Newfoundland

The Bobby Cove Formation of the Snooks Arm Group, north-central Newfoundland, displays cycles of lava- and sediment-dominated successions. The formation, located between pillow basalts and black graptolite-bearing mudstones, was deposited in a subaqueous setting. Volcaniclastic rocks were produced by calc-alkaline volcanism that either occurred during quiescence of, or was synchronous with, periods of tholeiitic volcanism. Variation in rock chemistry supports a change in tectonic setting from an expanding fore-arc oceanic crust to a progressively more mature magmatic arc/back-arc system. Combined chemical, sedimentological and detailed petrographic studies define several types of volcaniclastic sedimentary rocks of pyroclastic, epiclastic, and mixed origin. The volcaniclastic rocks were divided into three groups based of clast components and textures: (1) a monogenic facies with minor mudstone interbeds, composed of basaltic to basaltic andesite clasts, is interpreted to have originated from subaqueous vertically-directed, pyroclastic eruptions; (2) a polygenic facies with abundant mudstone interbeds composed of rounded fragments and a lack of vitric vesicular mafic fragments, was probably derived from chemical weathering and mechanical erosion of volcanic rocks (i.e. epiclastic volcanolithic sandstone); and (3) a second polygenic facies with minor mudstone interbeds is characterized by diverse clast compositions and textures, and is interpreted as remobilized deposits from previously deposited, fresh and/or altered, pyroclastic debris.

Geologic and geochemical relationships between the contact sublayer, inclusions, and the main mass of the Sudbury Igneous Complex; a case study of the Whistle Mine Embayment

More than half of the giant Ni-Cu-platinum-group element (PGE) sulfide ores of the Sudbury Igneous Complex are associated with a discontinuous unit at the base of the main mass known as the The sublayer is comprised of two fragment-rich members: (1) a metamorphic-textured footwall breccia, and (2) an igneous-textured contact sublayer. The contact sublayer occurs as a thick unit in depressions at the base of the Sudbury Igneous Complex termed embayments; it is associated with disseminated to massive sulfides and contains a range of inclusion types such as diabase, melanorite, olivine melanorite, metamorphosed melanorite, wehrlite, and dunite. We present data for a case study of the Whistle mine embayment and show that the igneous-textured sublayer matrix is geochemically unlike the main mass norites, quartz gabbros, and granophyres. For example, the igneous-textured sublayer matrix in the Whistle mine has La/Sm = 4.0, La/ Nb = 5.1, and Th/Zr = 0.02, and the main mass norites, quartz gabbros, and granophyres have ratios of 5.5 to 7, 2.8 to 4.2, and 0.04 to 0.05, respectively. This matrix contains partially digested hornfels diabase fragments and ghost textures of magnetite remaining from the melting of diabase. The igneous-textured sublayer matrix at the Whistle mine can be modeled with small amounts of assimilation of local country-rock granitoids ( approximately 10%), large degrees of assimilation of diabase that are not derived from the immediate country rocks ( approximately 70%), and small contributions from the main mass magma type ( approximately 20% mafic norite). The amount of diabase assimilation would be too large for traditional assimilation models controlled by the available heat of the mafic magma; even with a superheated magma, 70 percent assimilation is too large. A model is proposed where the melting of the target rocks initially produces a felsic melt sheet which is laden with mafic fragments. These fragments sink to the base of the melt sheet in the crater and are concentrated and further melted to produce the mafic sublayer. There are significant differences in the composition of the igneous-textured sublayer matrix between different embayments which may reflect differing degrees of digestion of compositionally different protolith fragments. The melanorite inclusions in the sublayer at the Whistle mine have ratios of the incompatible trace elements essentially similar to those of the inclusions in the igneous-textured sublayer matrix, but they have similar high incompatible element concentrations (e.g., olivine melanorites have 20-65 ppm Ce in rocks with 15-21 wt % MgO), 1 to 10 percent interstitial sulfide, up to 0.5 percent apatite, 1 to 15 percent biotite, and 1.85 Ga age zircon and baddeleyite. The mafic-ultramafic inclusions are interpreted to be the broken-up remnants of an earlier cumulate formed at depth from a main mass magma; this parental magma has compositional traits which suggest that crystal accumulation took place from a magma which contains a large contribution from the diabase. Olivine compositional data for the mafic inclusions (Fo (sub 71-79) ; 500-3,800 ppm Ni), in the absence of reequilibration, indicate that olivine crystallization both predated and postdated sulfur saturation of the magma. Cr-rich spinels from these rocks confirm that the parental magma was especially Cr rich, and based on the olivine compositional data, had a tholeiitic Mg/Fe ratio.