Ophiolitic Remnants Research Articles

Palaeoceanography and tectonic setting of the Jurassic Coast Range ophiolite, central California: evidence from the extrusive rocks and the volcaniclastic sediment cover

The primary thesis of this paper is that the character of sedimentary rocks within or directly on top of an ophiolite complex can be used to constrain tectonic models of oceanic crustal genesis. Late Jurassic ( c. 169-161 Ma) Coast Range ophiolite remnants are exposed E and W of the San Andreas Fault in central California. Ophiolite volcanic rocks are mainly sub-alkaline tholeiitic basalts, but include subordinate intermediate and silicic flows and breccias. Available ‘immobile’ major and trace element data indicate that the Coast Range ophiolite formed above a subduction zone, possibly in a fore-arc setting, as suggested by the presence of high magnesian andesites and boninites. The ophiolite extrusives mainly comprise pillow lava and lava breccia formed on a rugged fault-controlled ocean floor. Interlava pelagic sediment is restricted to very small amounts of interstitial red chert and nannoplankton limestone. Widespread occurrence of vesicular lavas probably reflects high primary volatile contents rather than eruption in shallow water. Local interbeds of arc-derived silicic tephra in the uppermost extrusives suggest that the ophiolite and cover sediments formed together in a single tectonic setting. Depositionally overlying deep-water successions up to 600 m thick comprise radiolarian-rich tuffaceous and volcaniclastic sediments of ?Callovian-mid-Tithonian age (165-148 Ma). Depositional processes included airfall, turbidity currents, mass flow and current reworking. Small SiFeMn lenses along the lava-sediment contact formed by hydrothermal replacement of basal siliceous tuffaceous sediments or by direct chemical precipitation. At one locality (Llanada), the ophiolite is overlain by up to 1.8 km of younger (?Tithonian) basic-intermed, and acid lava flows, submarine lahars and mainly silicic pyroclastic sediments, of inferred volcanic arc origin. The volcaniclastic cover passes conformably upwards into late Tithonian and younger terrigenous deep-water clastic sediments of the Great Valley Group, deposited in a fore-arc setting. The Coast Range ophiolite and its cover are interpreted as an incipient intraoceanic volcanic arc, possibly developed above an eastward (continent-ward)-dipping subduction zone within the equatorial Pacific Ocean. In this scenario, short-lived (5–8 Ma) supra-subduction zone spreading to form the ophiolite was followed by ?Callovian-Tithonian arc volcanism, shedding tuffaceous debris onto the adjacent fore-arc. Subduction was active separately along the western American continental margin with the result that the Coast Range arc-ophiolite complex eventually collided with North America, triggering the Late Jurassic Nevadan orogeny and initiating Franciscan subduction-accretion. The Coast Range ophiolite remnants, at least those west of the San Andreas Fault, were apparently translated northwards to their present positions after Late Cretaceous.

Geologic structure of the northern New Caledonia Ridge, as inferred from magnetic and gravity anomalies

Bathymetric, gravity, and magnetic data collected in the southwest Pacific Ocean over the northern New Caledonia ridge show that the main geological units known from the island of New Caledonia extend northward from this island, beneath the Grand Lagon Nord, the Grand Passage, and the d'Entrecasteaux reefs. These data support the model of tectonic evolution of the New Caledonia region proposed by Kroenke [1984]. We interpret a linear axial gravity low that extends from southern New Caledonia to the Grand Passage as evidence for the continuity of the thick pre‐Permian to Jurassic core of the island. The Belep magnetic pattern, which covers the southwestern half of the Grand Lagon Nord, shows linear, high relief (1000–2000 nT) anomalies that are locally associated with a 120‐mGal gravity high. These magnetic and gravity anomalies resemble anomalies measured over the west coast of New Caledonia, suggesting that the Cretaceous to Eocene basaltic complex of the coastal area is overlain by ophiolite remnants as far north as the western d'Entrecasteaux reefs. The similarity between the Belep magnetic pattern and a highly magnetic province evident 200 km southwest across the New Caledonia basin, along the Fairway ridge, indicates that volcanic rocks lie symmetrically on both sides of the New Caledonia basin. We suggest that part of these volcanic rocks were emplaced during the middle Cretaceous when the New Caledonia ridge rifted from the Australian margin. Simple gravity models of an elongated gravity high, having peak values in the range of +130 to +150 mGal, suggest that the ophiolite, which was thrust over New Caledonia during the Upper Eocene, extends along the east side of the Grand Lagon Nord and as far north as the d'Entrecasteaux reefs. Gravity and magnetic models suggest that in the area of the d'Entrecasteaux reefs, the ophiolite belt includes two subunits. The first subunit underlies the Huon‐La Surprise platform and may include highly serpentinized ultramafic rocks without basaltic oceanic crust. The second subunit, which lies beneath the Guilbert ridge, may include ultramafic rocks as well as thin imbricate slices of oceanic crust. At the northern termination of the New Caledonia ridge, seismic and geopotential data evidence a major east‐west trending tectonic zone that separates the basement of the New Caledonia ridge from the d'Entrecasteaux zone, an arcuate oceanic feature extending northward from the ridge. Differences in structure, geophysical signatures and morphology evident between areas north and those south of the Grand Passage, together with the nearness of the Le Noroit massif west of the Grand Passage, suggest that contemporaneously with Eocene to early Oligocene subduction along the western New Caledonia margin, an arc‐ridge collision may have occurred near the northern termination of this subduction zone.

The Devils Elbow ophiolite remnant and overlying Galice Formation: New constraints on the Middle to Late Jurassic evolution of the Klamath Mountains, California

The Josephine ophiolite and its sedimentary cover (Galice Formation) of the western Jurassic belt, Klamath Mountains, northern California, are the expression of a Late Jurassic rift basin which developed between an active Late Jurassic arc to the west and a remnant Middle Jurassic are to the east. The Devils Elbow ophiolite remnant (DEO) and its sedimentary and volcanic cover represent the southern continuation of the Josephine ophiolite and overlying Galice Formation. The DEO consists of a dike complex, volcanic sequence, and ophiolitic breccia. Overlying strata consist of terrigenously derived turbidites and andesitic to dacitic volcaniclastic rocks. The following relations suggest formation of the DEO in a marginal basin setting, in accordance with the setting inferred for the Josephine ophiolite basin: (1) dikes and lavas are characterized by an abundance of clinopyroxene phenocrysts, by crystallization of clinopyroxene before plagioclase, and by trace-element geochemistry which is transitional between IAT and MORB; (2) overlying sedimentary rocks appear to have been derived from erosion of older Klamath Mountain terranes and were deposited into the DEO basin shortly after and/or during its formation; (3) the texture and composition of the volcanic deposits interlayered with sedimentary strata indicate the presence of a nearby active volcanic arc source. The period of rifting which culminated in the formation of the Josephine ophiolite/DEO basin began by at least 164 Ma, the age of the DEO, but is unlikely to have begun much earlier, as the region was experiencing a major period of compression just prior to this time. Rifting began at least 5 m.y. prior to cessation of magmatism in the rifted Middle Jurassic arc. The overlap in rifting and arc magmatism can be explained if spreading in the Josephine ophiolite/DEO basin was asymmetric relative to the axis of Middle Jurassic are magmatism and/or if transform motion approximately parallel to the Middle Jurassic are was important in the spreading geometry of the basin. The western boundary fault of the western Jurassic belt, and Klamath Mountain province, is the South Fork fault (previously referred to as part of the Coast Range thrust). In the southern western Jurassic belt, a serpentinite matrix meelange (SMM) occurs along this fault. Blocks in the SMM consist largely of greenstones with the trace-element geochemistry of MORB. Other block types include gabbroic to dioritic intrusive rocks, diabase, harzburgite, amphibolite, chert, sandstone, and shale. Generally similar serpentinite matrix melanges have been described from other locations along the South Fork fault. This spatial association suggests that the South Fork fault provided a pathway for the bouyant rise of serpentinite or serpentinite matrix melange from a source at depth. Obvious sources for the SMM include parts of the Franciscan complex, the Josephine ophiolite/DEO, or possibly the Coast Range ophiolite. The geochemistry and relative abundances of different lithologies in the SMM, however, indicate that none of these could represent the source. In contrast, the SMM is nearly identical in block content and block geochemistry to serpentinite matrix melange of the Rattlesnake Creek terrane which structurally overlies the western Jurassic belt to the east. This suggests that a previously unrecognized part of the Rattlesnake Creek terrane may underlie the Josephine ophiolite/DEO at depth. As the Rattlesnake Creek terrane formed the western part of the basement for the rifted Middle Jurassic arc, the part of the Rattlesnake Creek terrane which underlies the Josephine ophiolite/DEO may represent a fragment of crust which was rifted oceanward with the opening of the Josephine ophiolite/DEO basin and then subsequently thrust beneath the basin during the Late Jurassic Nevadan Orogeny.

Significance of xenocrystic Precambrian zircon contained within the southern continuation of the Josephine ophiolite: Devils Elbow ophiolite remnant, Klamath Mountains, northern California

The Josephine ophiolite of the western Jurassic belt, Klamath Mountain province of California and Oregon, is the expression of a well-documented Late Jurassic suprasubduction zone rift basin that formed between an active Late Jurassic arc to the west and a remnant Middle Jurassic arc to the east. The Devils Elbow ophiolite remnant (DEO) exposed along the South Fork of the Trinity River represents the southernmost continuation of the Josephine ophiolite and provides important new constraints on the early history of this rift basin. The DEO consists of pillow lavas and breccias and a well-developed sheeted dike complex that are depositionally overlain by a clast-supported breccia derived almost exclusively from ophiolitic detritus. Dikes and irregular pods of plagiogranite are conspicuous elements of the DEO and exhibit mutually crosscutting relations with mafic dikes; thus, they are interpreted as genetically related elements of the dike complex. Zircon separates from two widely separated plagiogranite dike localities yielded two distinct zircon populations: a clear, euhedral, magmatic population and a reddish, rounded xenocrystic population. The isotopic systematics of four zircon fractions from these two dike localities indicate a crystallization age for the DEO of 164 ±1 Ma and an age of ∼1.7 Ga for the xenocrystic zircon component. The occurrence of a xenocrystic Precambrian zircon component within the plagiogranites of the DEO provides unequivocal evidence that rifting occurred within preexisting zircon-bearing crustal rocks of the Klamath Mountains. As there is no Precambrian crust within the Klamath Mountains, the xenocrystic zircon population must have been derived from supracrustal sedimentary sequences. The most likely source for the older zircon component is a terrigenous metasedimentary sequence contained within the Rattlesnake Creek terrane, which formed part of the structural basement of the rifted Middle Jurassic arc. The incorporation of xenocrystic Precambrian zircon from the rifted basement of the Middle Jurassic arc could only have occurred during the earliest stages of rifting. Thus, the DEO must represent a fragment of an initial rift that ultimately formed the Late Jurassic basin now floored by the Josephine ophiolite. In support of the zircon data, dikes and lavas of the DEO have geochemical characteristics transitional between island arc and mid-ocean ridge magma series, as would be expected within an initial rift setting of a suprasubduction zone ophiolite.

Geochemical evidence for the tectonic setting of the Coast Range ophiolite: A composite island arc–oceanic crust terrane in western California

The Middle to Late Jurassic age Coast Range ophiolite (CRO) of California contains two geochemically distinct volcanic rock associations that formed in different tectonic settings. Volcanic rocks from the southern CRO (Point Sal, Cuesta Ridge, Stanley Mountain, Llanada, Quinto Creek, and Del Puerto) and parts of the northern CRO (Healdsburg, Elder Creek) are similar to low-K tholeiites and calc-alkaline rocks of the island-arc suite. The thin volcanic sections of these ophiolite remnants suggest formation by intra-arc rifting. In contrast, volcanic rocks from Stonyford seamount and Paskenta in the northern CRO are transitional subalkaline metabasalts with geochemical characteristics similar to enriched mid-ocean ridge basalts or ocean-island tholeiites. These rocks are associated with Tithonian radiolarian cherts and may be part of the Franciscan Complex. Alternatively, they may represent a change in tectonic setting within the CRO during the Late Jurassic. Regardless, the CRO as currently conceived cannot be considered a single terrane with one mode of origin.

Tectonic history of the ophiolitic rocks of the highland border fracture zone, Scotland: Stable isotope evidence from rock-fluid interactions during obduction

The mafic and ultramafic rocks of the Highland Border Fracture Zone are ophiolitic remnants of a pre-Grampian marginal basin that opened either within, or to the north of, the Dalradian sedimentary pile. Closure of the basin was achieved through a combination of northerly-directed subduction, and obduction of ophiolitic thrust-slices onto the basin's southern margin. During the early stages of obduction, young hot peridotite slabs were thrust over the cold upper surfaces of lower thrust sheets, producing a dynamothermal metamorphic sole. Serpentinisation of these peridotites, whilst they were still cooling, occurred in a near-surface position through the interaction of meteoric waters. Subsequently, the ophiolitic thrust-sheets, which comprise lizardite serpentinites, spilitic pillow lavas, and aureole rocks, were thrust over the uppermost Dalradian nappes which were themselves being expelled southwards, thereby accommodating basement shortening. Grampian regional metamorphism of the nappe pile and overlying Highland Border Suite ophiolitic thrust sheets, produced greenschist metaspilites from the spilitic pillow lavas, induced minor retrogression in the aureole rocks, and caused the lizardite in the serpentinites to be recrystallised and replaced by antigorite. The Highland Border Suite greenschist facies metamorphic fluids were D-enriched compared with low-grade Dalradian metamorphic waters, and may have been mixtures of the latter and D-rich dehydration fluids released from the mafic rocks during dynamothermal metamorphism. Brittle fracturing and shearing in the serpentinites were responses to late deformation at different crustal levels during the final stages of emplacement, which involved gravity-sliding as well as downbending of the Dalradian nappes and ophiolitic thrust-sheets against the elevated Midland Valley block.

The geology and structural history of the Cape Ray Fault Zone in southwestern Newfoundland

Structural studies within the Cape Ray Fault Zone, southwestern Newfoundland indicate that three tectonic events (D1 to D3) have deformed all regional lithologic units. These lithologies are distributed as a tonalitic terrain (intrusive into ophiolitic remnants) to the northwest, a central volcanic–sedimentary terrain, and a staurolite–kyanite gneiss terrain to the southeast. D1 produced the dominant northeasterly striking planar fabrics in the host rocks that are axial planar to isoclinal folds. The fold axes parallel a synkinematic mineral elongation lineation. D2 formed open asymmetrical folds with curvilinear fold axes and local crenulation cleavage. The D3 event produced local conjugate kink folds. Radiometric dates on deformed plutons indicate these deformational events were post-369 ± 12 Ma (post-Middle Devonian) and pre-352 ± 6 Ma (pre-Late Carboniferous).The fault was previously interpreted as a "cryptic suture" along which opposed margins of the Iapetus Ocean were juxtaposed. Supposedly this suture was subsequently partially covered by volcanic and sedimentary rocks. The present work indicates that the deformational events that produced the fault zone overprinted all regional lithologies in a similar manner and that the delineating mylonites were the products of ductile simple shear due to intense localized deformation. The fault itself is a large-scale shear zone and is not the end result of intercontinental collision.

Crystallization and age of zircon from Corsican ophiolitic albitites: consequences for oceanic expansion in Jurassic times

In Corsica, albitites and albite-granites are considered to be the latest products of an ophiolitic magma. Two albitites have been selected for a study of the morphology, typology and age of their zircons, applying for the first time the U-Pb method to Western Tethyan ophiolites. Both samples come from the Rospigliani series, possibly formed in a transform fault zone. The first one shows a cumulate texture, whereas the second has features indicative of more rapid cooling. Zircons have precipitated at the end of the fractional crystallization process after a strong enrichment in alkali and incompatible elements. In a typological diagram they plot at high temperature and high agpaicity indexes. These abundant large, euhedral to subhedral zircons, sometimes with fine zoning, may also contain inclusions of granitic composition which testifies to K fractionation in ophiolitic magmas. The U-Pb age of 161±3Myr is in the range of the ages obtained for Alpine and Apennines ophiolites. All these ophiolitic remnants belonging to the Liguro-Piedmont Basin may represent a portion of the Central Atlantic Ocean created during Middle and Upper Jurassic times.