Mid-ocean Ridge Basalt Affinities Research Articles

Multiphase ophiolite formation in the Northern Altyn Tagh Orogen, southeastern Tarim

Early Paleozoic ophiolitic mélanges in the Altyn Tagh Orogen, southeastern Tarim, yield a large range of formation ages and geochemical affinities. This study focused on the Hongliugou ophiolitic mélange in the North Altyn Tagh subduction-accretion belt and involved mineral chemical, zircon geochronological, and whole-rock elemental and isotopic investigations of the ultramafic and mafic rocks. In the studied lherzolite samples, Cr-spinel, olivine, and pyroxene show mineral chemistry akin to that of abyssal peridotite. Subhedral-anhedral Cr-spinel grains with high Cr# values (100*Cr/[Cr+Al] of 30–40) and negative oxygen fugacity (fO₂) values represent original spinel formed during mid-ocean ridge basalt (MORB) melt extraction. Other subhedral-anhedral and euhedral Cr-spinel with low Cr# values of &lt; 30 to 20 display a wide range of negative to positive fO₂ values, indicating interaction of peridotite with MORB magma. New zircon U-Pb dating results record ages of <i>ca.</i> 490 Ma for gabbro and <i>ca.</i> 486 Ma for diabase in the Hongliugou ophiolitic mélange. Whole-rock geochemical compositions suggest that the gabbro and diabase samples are tholeiitic and show MORB affinities or transitional affinities between MORB and island arc tholeiite. These mafic rocks might have been derived from a depleted MORB-source mantle that was variably metasomatized by subduction-induced fluids based on their variable Th/Yb, high La/Nb, low Th/Nb, and depleted isotopes (initial ⁸⁷Sr/⁸⁶Sr ratios of 0.703073 to 0.703385 and <i>ε</i>_Nd(<i>t</i>) values of +6.2 to +6.3). New results, when integrated with previous work, clarify that <i>ca.</i> 520 to 510 Ma ophiolites formed in the initial subduction setting and <i>ca.</i> 490 to 480 Ma ophiolites in a back-arc setting. Minor <i>ca.</i> 450 Ma ophiolite probably represents a late phase of ophiolite formation due to the presence of a very minor remnant ocean.

Early Cretaceous Plume–Ridge Interaction Recorded in the Band-e-Zeyarat Ophiolite (North Makran, Iran): New Constraints from Petrological, Mineral Chemistry, and Geochronological Data

The North Makran domain (southeast Iran) is part of the Makran accretionary wedge and consists of an imbricate stack of continental and Neo-Tethyan oceanic tectonic units. Among these, the Band-e-Zeyarat ophiolite consists of (from bottom to top): ultramafic cumulates, layered gabbros, isotropic gabbros, a sheeted dyke complex, and a volcanic sequence. Sheeted dykes and volcanic rocks are mainly represented by basalts and minor andesites and rhyolites showing either normal-type (N) or enriched-type (E) mid-ocean ridge basalt affinities (MORB). These conclusions are also supported by mineral chemistry data. In addition, E-MORBs can be subdivided in distinct subtypes based on slightly different but significant light rare earth elements, Th, Nb, TiO2, and Ta contents. These chemical differences point out for different partial melting conditions of their mantle sources, in terms of source composition, partial melting degrees, and melting depths. U-Pb geochronological data on zircons from intrusive rocks gave ages ranging from 122 to 129 Ma. We suggest that the Band-e-Zeyarat ophiolite represents an Early Cretaceous chemical composite oceanic crust formed in a mid-ocean ridge setting by partial melting of a depleted suboceanic mantle variably metasomatized by plume-type components. This ophiolite records, therefore, an Early Cretaceous plume–ridge interaction in the Makran Neo-Tethys.

Slab-controlled elemental–isotopic enrichments during subduction initiation magmatism and variations in forearc chemostratigraphy

Supra-subduction zone (SSZ) ophiolites formed in forearc settings and the Izu–Bonin–Mariana (IBM) in-situ forearc show conversions from normal mid-ocean ridge basalt (MORB) to boninitic affinities, which are widely interpreted as rules for the evolving chemical geodynamics of subduction initiation magmatism (SIM). Nevertheless, forearc sequence generated by subduction initiation need not always include boninitic rocks. Here we reconstruct a forearc chemostratigraphy without boninitic affinity based on the Central Palawan and Amnay (CPA) ophiolites of the west Philippines to show diversity in melt evolution of SIM. Zircon U–Pb dating on the MORB-type Amnay gabbros reveal a 10 m.y. of seafloor spreading from 33 to 23 Ma. These ages together with the progressively enriched zircon in-situ oxygen isotopes (δ18O= 1.62–10.1‰) and whole rock elemental and Nd–Hf isotopic signatures ([La/Yb]N= 0.19–3.45; Nb/Yb= 0.07–5.24; εNd= 2.1–10.2; εHf= 11.2–24.1) indicate that the depleted MORB mantle (DMM)-type source of the Amnay ophiolite was getting enriched by fluxing of aqueous slab fluid in the early stage followed by addition of adakitic and Nb-enriched slab melts in the later stage. This distinct mantle evolution together with higher εHf at a given εNd of the Amnay mafic samples than the contemporaneous South China Sea (SCS) oceanic samples do not support previous interpretation that the Amnay ophiolite was a fragment of the SCS. However, the 33–23 Ma Amnay ophiolite can be the younger part of the 34 Ma forearc-type Central Palawan ophiolite, and both (i.e. the CPA ophiolites) represent on-land fragments of the NW Sulu Sea. We propose that the 34–23 Ma CPA ophiolites and the NW Sulu Sea constitute forearc basin of the late Oligocene–middle Miocene island arc–backarc basin duo of the Cagayan Ridge and SE Sulu Sea, associated with subduction of the Cretaceous to Eocene Proto-SCS since 34 Ma. The CPA ophiolites lack boninite but show geochemical progression from normal to enriched MORB affinities, whose melt evolution differs from that of the IBM SIM. We attribute the differences in SIM and variations in forearc chemostratigraphy to the subduction of a younger slab with thick terrigenous sediment cover of the Proto-SCS versus an older slab with thin pelagic sediment cover of the Pacific Plate.

Newly discovered eclogites from the Bangong Meso–Tethyan suture zone (Gaize, central Tibet, western China): mineralogy, geochemistry, geochronology, and tectonic implications

ABSTRACTWe report the occurrence, mineralogy, whole-rock geochemistry, and geochronology of two types of eclogites newly discovered in the western Bangong Meso–Tethyan suture zone (Gaize, central Tibet, western China). Type 1 eclogites contain a peak metamorphic mineral assemblage of garnet + clinopyroxene whereas Type 2 ones are characterized by a peak metamorphic mineral assemblage of garnet + clinopyroxene + rutile. Type 2 clinopyroxene and garnet are relatively more enriched in Mg contents than Type 1 ones. Type 2 eclogites are significantly more enriched in TiO2, P2O5, and light rare-earth elements, but more depleted in Al2O3 contents and Mg numbers. P–T estimates for both types of eclogites are consistent with minimum pressures of 23–25 kbar at 830–920°C, with the retrograde amphibolite facies assemblage of 15.2–17.5 kbar and 500–732°C. Zircon U–Pb dating indicates that the protolith of Type 1 eclogites, with normal mid-ocean ridge basalt affinity, emplaced at ~260 Ma, whereas the protolith of Type 2 eclogites, geochemically familiar with oceanic-island basalt (OIB), erupted at ~242 Ma. 40Ar/39Ar analyses of hornblende from the retrograded eclogites indicate that the Gaize eclogite-bearing high-pressure rocks were exhumed at ~194 Ma. The occurrence of these high-pressure rocks implies the subduction of oceanic crust to a depth of ~85 km along the southern margin of the Qiangtang block during the Late Triassic.

Ophiolites in the Xing'an-Inner Mongolia accretionary belt of the CAOB: Implications for two cycles of seafloor spreading and accretionary orogenic events

The Xing'an-Inner Mongolia accretionary belt in the southeastern segment of the Central Asian Orogenic Belt (CAOB) was produced by the long-lived subduction and eventual closure of the Paleo-Asian Ocean and by the convergence between the North China Craton and the Mongolian microcontinent. Two ophiolite belts have been recognized: the northern Erenhot-Hegenshan-Xi-Ujimqin ophiolite belt and the southern Solonker-Linxi ophiolite belt. Most basalts in the northern ophiolite belt exhibit characteristics of normal-type to enriched-type mid-ocean ridge basalt affinities with depleted Nd isotopic composition (epsilon(Nd)(t)>+5), comparable to modern Eastern Pacific mid-ocean ridge basalts. Most basaltic rocks in the southern belt show clear geochemical features of suprasubduction zone-type oceanic crust, probably formed in an arc/back-arc environment. The inferred back-arc extension along the Solonker-Linxi belt started at circa 280 Ma. Statistics of all the available age data for the ophiolites indicates two cycles of seafloor spreading/subduction, which gave rise to two main epochs of magmatic activity at 500-410 Ma and 360-220 Ma, respectively, with a gap of similar to 50 million years (Myr). The spatial and temporal distribution of the ophiolites and concurrent igneous rocks favor bilateral subduction toward the two continental margins in the convergence history, with final collision at similar to 230-220 Ma. In the whole belt, signals of continental collision and Himalayan-style mountain building are lacking. We thus conclude that the Xing'an-Inner Mongolia segment of the CAOB experienced two cycles of seafloor subduction, back-arc extension, and final Appalachian-type soft collision.

Petrology, geochemistry and geochronology of gabbros from the Zhongcang ophiolitic mélange, central Tibet: Implications for an intra-oceanic subduction zone within the Neo-Tethys Ocean

In order to investigate the evolution of Shiquanhe-Yongzhu-Jiali ophiolitic melange belt, the gabbros from new discovered Zhongcang ophiolitic melange are studied through petrology, whole-rock geochemistry, zircon U-Pb dating and Lu-Hf isotope. The gabbros investigated in this paper contain cumulate gabbro and gabbro dike, and they have undergone greenschist-amphibolite facies metamorphism. The chondrite normalized rare earth element (REE) patterns of most of these rocks show flat types with slightly light REE (LREE) depletion and the N-MORB normalized incompatible elements diagrams indicate depletion in high field strength elements (HFSE) (Nb, Ta) and enrichment in large ion lithophile elements (LILE). These gabbros have island arc and mid-ocean ridge basalt affinities, suggesting that they were originated in an oceanic back arc basin. Whole rock geochemistry and high positive ɛ Nd(t) values show that these gabbros were derived from ∼30% partial melting of a spinel lherzolite mantle, which was enriched by interaction with slab-derived fluids and melts from sediment. U-Pb analyses of zircons from cumulate gabbro yield a weighted mean age of 114.3±1.4 Ma. Based on our data and previous studies, we propose that an intra-oceanic subduction system and back arc basin operated in the Neo-Tethy Ocean of central Tibet during Middle Jurassic and Early Cretaceous, resembling modern active intra-oceanic subduction systems in the western Pacific.

Depleted Basaltic Lavas from the Proto-Iceland Plume, Central East Greenland

New geochemical and isotopic data are presented for volumetrically minor, depleted low-Ti basalts that occur in the Plateau Basalt succession of central East Greenland (CEG), formed during the initial stages of opening of the North Atlantic at 55 Ma. The basalts have mid-ocean ridge basalt (MORB)-like geochemistry (e.g. depleted light rare earth elements) and are distinct from the high-Ti lavas that dominate the sequence. Rare earth element geochemistry implies derivation from a source more depleted than the typical MORB source, and suggests polybaric melting and contributions from both spinel- and garnet-facies mantle. The low-Ti basalts have Sr^Nd^ Pb^Hf isotopic characteristics that are similar to those of depleted magmas from Iceland (e.g. Theistareykir) and adjacent ridges (Kolbeinsey and Reykjanes) and distinct from global MORB (e.g. negative 207 Pb, and Hf and Nd isotope compositions that plot above the mantle reference line). Isotope and trace element data indicate the involvement of two depleted source components. One component has isotopic compositions similar to other depleted components identified in the North Atlantic and has high Rb/Zr and Ba/Nb. The second is isotopically less depleted with lower Rb/Zr and Ba/ Nb. Small degrees of crustal contamination (51%) by both amphibolitic and granulitic crust result in relatively large changes in isotopic composition (c .1 % lower for 206 Pb/ 204 Pb and 0·1% higher for 87 Sr/ 86 Sr depending on the contaminant). Negative Nb suggests a MORB affinity for the low-Ti magmas; however, they are distinguished from global normal (N)-MORB on the basis of vertical deviations from the Northern Hemisphere Reference Line (negative 207 Pb and positive 208 Pb), and relative enrichments in Ba, Sr and Pb. The isotopic compositions of the low-Ti CEG basalts suggest correlation with modern depleted components beneath Iceland and adjacent ridges, considered to be derived from upper mantle sources polluted by the Iceland plume. However, small positive Pb peaks when normalized to MORB, and lower Nb distinguish the CEG low-Ti basalts from depleted Icelandic compositions. The lower Nb (5 0) and 87 Sr/ 86 Sr, and suggestion of higher 206 Pb/ 204 Pb in crustally uncontaminated parental melts imply a closer affinity to compositions from the oceanic ridges surrounding Iceland (especially the Reykjanes Ridge), yet they are subtly distinct on the basis of available trace element data. We suggest that this depleted component was an integral part of the plume that melted primarily during the rapid lithospheric uplift and extension associated with continental break-up.

Depth variation of carbon and oxygen isotopes of calcites in Archean altered upperoceanic crust: Implications for the CO2 flux from ocean to oceanic crust in the Archean

Middle Archean greenstones with mid-ocean ridge basalt affinity and overlying bedded chert/banded iron formation (BIF) are exposed in the Cleaverville area, Pilbara Craton, Western Australia. On the basis of the hydrothermal carbonation of these Cleaverville greenstones, we estimated the potential CO2 flux from ocean to oceanic crust and the physical–chemical conditions of the subseafloor hydrothermal system for the middle Archean. The greenstones exhibit various extents of carbonation, and the igneous minerals contained in the greenstones are partly or completely replaced by calcite and other secondary minerals. The degreeof carbonation correlates with stratigraphy; the volume concentration of calcite in greenstones decreaseswith increasing depth below the chert/BIF horizon. Our results clearly indicate that the hydrothermal carbonation occurred along the axial zones of a middle Archean mid-ocean ridge. Both δ13C and δ18O values of calcite also change with the depth below the chert/BIF horizon. The δ18O value of calcite decreases with increasing depth, reflecting increasing temperature, while the δ13C value of calcite changes from positive to negative stratigraphically downward. A model reconstructing the δ13C of calcite suggests that the observed isotopic variation reflects the thermal structure and that the carbon source for calcite was derived from seawater. These mineralogical, geochemical, and geological features of calcite provide the amount of CO2 in the whole oceanic crust per seafloor unit area (1.2×107mol/m2). Based on this value, CO2 flux from the ocean to the oceanic crust in the middle Archean can be estimated to be 1.5×1014mol/yr when the spreading rate (m2/yr) of Archean oceanic crust is assumed to have been three times higher than it is today. The estimated CO2 flux into the oceanic crust is two orders of magnitude higher than the modern value, which points to the significance of sea-floor hydrothermal carbonation in the Archean carbon cycle.

The tectonomagmatic evolution of Scotland

ABSTRACTScotland has a magmatic record covering much of the period 3100–50 Ma. In this review, we pull together information on Scotland's igneous rocks into a continuous story, showing how magmatic activity has contributed to the country's structural development and assessing whether the effects of older magmatic events can be recognised in later episodes.The oldest igneous rocks are part of supracrustal sequences within the Lewisian Gneiss Complex, formed when Scotland was part of the supercontinent Kenorland. The supracrustal rocks were intruded between 3100 and 2800 Ma by granodiorites and tonalites, which were metamorphosed and deformed in a major tectonothermal event between 2700 and 2500 Ma. The break-up of Kenorland (2400–2200 Ma) was marked by the intrusion of mafic dyke swarms of tholeiitic affinity. The convergence of continental masses to form the supercontinent Columbia resulted, at ∼1900 Ma, in a series of subduction-related volcanic rocks and gabbro–anorthosite masses. Subsequent continent–continent collision formed a series of granite–pegmatite sheets at ∼1855 Ma and ∼1675 Ma and reworked much of the earlier rocks in the amphibolite facies. Columbia was breaking up by 1200 Ma, an event marked by remnants of basaltic magmatism in the NW of the country. Re-assembly of the continental fragments to form the supercontinent Rodinia resulted in the Grenville Orogeny, which in Scotland was marked by basement reworking but no confirmed magmatic activity. Early attempts to split Rodinia produced a rift-related, bimodal, mafic–felsic sequence in the Moine Supergroup of the Northern Highlands, at least some of the mafic rocks having mid-ocean ridge basalt affinities. Crustal thickening during a disputed orogenic event, the Knoydartian, may have caused regional migmatisation. The final break-up of Rodinia occurred in Scotland at ∼600 Ma, when very extensive tholeiitic magmatism characterised the later parts of the Dalradian Supergroup, while a series of granites intruded the Moine and Dalradian successions.Ordovician and Silurian times saw the closure of the Iapetus Ocean and the convergence of Laurentia, Avalonia and Baltica. The collision of a major arc system with Laurentia caused the Grampian event (480–465 Ma) of the Caledonian Orogeny, marked by ophiolite obduction, the generation of (largely) anatectic granites, volcanism in the Midland Valley and Southern Uplands, and intrusion of a major gabbro–granite suite in the NE. The late-Caledonian events (435–420 Ma) were largely post-collisional and were marked by the emplacement of alkaline igneous intrusions in the NW, calc-alkaline granitic intrusions over much of the country, widespread volcanic activity and regional dyke swarms. Laurentia, Avalonia and Baltica amalgamated to form the supercontinent Laurussia. Magmatic activity recommenced at 350 Ma, when intra-plate alkaline magmatism affected much of southern Scotland, in particular, through into Permian times. The alkaline magmatism was interrupted at ∼295 Ma by a short-lived event in which tholeiitic magmas were intruded as sills and dykes in a swarm ∼200 km wide. In the early Palaeogene, lithospheric attenuation related to proto-North Atlantic formation and the splitting of Pangaea was complemented by the arrival of the Iceland mantle plume. Huge volumes of mafic magma were emplaced as lava fields, central complexes and regional swarms, locally increasing crustal thickness by 30%

Magmatic evolution of the Mesozoic ophiolites in Austria

The Mesozoic ophiolites in the Eastern Alps are restricted to the Penninic windows: the Lower Engadin Window, Tauern Window and Rechnitz Window Group. The ophiolite sequence starts with serpentinites (harzburgitic chemistry) and some metamorphosed ultramafic cumulates which include rodingites and chloritites. The plutonic section comprises leucogabbros, ferrogabbros and rare plagiogranites. Pillow lavas, where recognizable, massive lava flows, and sometimes tuffs build up the extrusive series. Small bands of radiolarites, cherts and thick calc schists form the sedimentary cover. Ophicarbonates on top of the serpentinites are rare (Rechnitz Window Group). All ophiolites, apart from some oceanic alterations, have undergone a complex history of Alpine regional metamorphism of variable grade (pumpellyite-actinolite facies in the Lower Engadin Window, to greenschist to amphibolite facies in the Tauern Window). Geochemically, the plutonic section can be explained in terms of their Ti, V, Zr and Y concentration reflecting a fractional crystallization sequence with olivine, plagioclase, clinopyroxene, and later ilmenite and magnetite as cumulus phases. The volcanics show mid-ocean ridge basalt affinities. Five groups based on major-element variations can be observed: plag, ol and ilm-mag accumulations, and ol-plag-(cpx) and cpx-ol-plag fractionations. This grouping is also recognized in the trace elements. The variability in Zr/Y ratios within and between the groups can be explained by different degrees of partial melting and slight variation in the mantle source. This complex magmatic history indicates an oceanic ridge environment for the origin of the ophiolites with a structure similar to that of the North Atlantic and low spreading rates. Fracture zones may also play a certain role.

Geochemistry of basic dikes in the Lanzo massif (Western Alps): Petrogenetic and geodynamic implications

The Alpine peridotite massif of Lanzo (Italy) contains three generations of basic dikes (gabbros and basalts). The older gabbros are plagioclase-rich mantle segregates while the younger gabbro dikes are cumulates very similar in chemical composition to recent oceanic gabbros and gabbros from ophiolitic complexes. They both were derived from the N-type mid-ocean ridge basalt (MORB) magmas which were progressively more depleted in incompatible elements and were probably generated during a dynamic melting of a rising mantle diapir. The basaltic dikes are the N-type MORB and closely resemble the Alpine-Apennine ophiolitic basalts. They were derived from a different upper mantle source than the parental magmas of the gabbros. The source of the basalts was less depleted in light REE. The presence of basic magmas with N-type MORB affinities in the Lanzo massif is consistent with the close genetic relationship between the Alpine peridotite body and the ophiolites of the Liguro-Piemontese basin.

Deformation, geochemistry, and origin of massive sulfide deposits, Gossan Lead District, Virginia

The Gossan Lead district is a 28-km-long, northeast-trending belt of discontinuous massive sulfide deposits in the Blue Ridge province of southwestern Virginia. The deposits, hosted by the Ashe Formation of late Proterozoic age, consist of strata-bound lenses and layers of massive pyrrhotite, minor chalcopyrite, sphalerite, and pyrite, and rare arsenopyrite and galena. Deposits were mined principally in the Iron Ridge and Betty Baker segments, respectively, at the southwestern and northeastern ends of the belt. Detailed mapping of the Gossan Howard, Huey, and Bumbarger pits in the Iron Ridge segment indicates that the deposits occur at one horizon and have been variously folded and brecciated after sulfide deposition. The Gossan Howard consists of a single, gently dipping lens of sulfide. The Huey deposit is complexly folded and locally contains tectonically thickened ore. The Bumbarger deposit is a lens as much as 40 m thick--the thickest known in the district. This deposit contains abundant coarse breccia fragments of wall rock around which the massive sulfide has flowed (during deformation and metamorphism), probably thickening the original deposit significantly. In the northeastern part of the district, drill holes intersect several sulfide layers that possibly are structurally repeated.The Ashe Formation in the district is a sequence of metasedimentary rocks and local conformable lenses of amphibolite and actinolite-chlorite schist. The metasedimentary rocks include metapelite, quartz-feldspar granofels (metagraywacke), and minor quartzite and carbonaceous schist, and are interpreted as marine turbidites. The amphibolites and other mafic rocks have chemical compositions similar to low Ti tholeiitic basalt, with a high Y/Nb (>10) and high average contents of Co (40 ppm), Cr (403 ppm), Ni (211 ppm), and V (247 ppm). Immobile trace element signatures (Ti-Y-Zr; Th-Hf-Ta; Ti-Cr) suggest a magmatic affinity with midocean ridge basalt (MORB); rare earth elements (REE) have low abundance levels (10X-15X chondrite), broadly flat patterns [(La/Yb) N = 0.7-1.1], and a slight depletion in the light elements similar to midocean ridge basalts. An amphibolite from a much higher stratigraphic level, south of the district, differs significantly from the mafic rocks closer to the sulfide zone in having the chemical signature of a transitional, slightly alkalic tholeiite with high TiO 2 (3.87 wt %), Fe 2 O 3 (16.4 wt %), and P 2 O 5 (0.56 wt %), low Y/Nb (3.3), and a highly fractionated rare earth element distribution [(La/Yb) N = 3.9] similar to continental basalt.Some silicate wall rocks of the deposits are mineralogically and chemically unusual, and differ substantially from the clastic metasediments of the Ashe Formation. Such rocks are composed mainly or wholly of plagioclase feldspar, biotite, chlorite, muscovite, or spessartine-rich garnet. The unusual lithologies form local strata-bound lenses in the footwall and/or hanging wall of the deposits, typically within 10 m of massive sulfide. The plagioclase rocks (3.4-7.6 wt % Na 2 O) consist largely of granoblastic albite-oligoclase (Ab (sub 78-90) ) with minor quartz and biotite; rare earth elements are highly fractionated [(La/Yb) N = 6.8-7.1] and their patterns resemble those of the quartz-feldspar granofels (metagraywacke) from the district.The biotite schists, locally monomineralic, have FeO/(FeO + MgO) = 0.5 and contain high phosphorus (1 wt % P 2 O 5 ) and fluorine (0.5 wt % F), present in fluorapatite. The chlorite schist is essentially all ripidolite; rare earth elements are highly fractionated, and have a large negative Eu anomaly. The spessartine-rich rocks (6.3-8.9 wt % MnO) are in places interlayered with the other unusual wall rocks and consist of abundant Mn-rich garnet (Sp 50 Al 23 Gr 19 Py 8 ) and minor quartz, plagioclase, pyrrhotite, and biotite. The distinctive mineralogy and chemistry of these rocks suggest that they represent metamorphosed alteration zones and/or intermixed chemical and clastic sediments.The sulfide deposits are interpreted as syngenetic in origin but modified in form by deformation which accompanied metamorphism. The great length of the mineralized district parallel to the regional strike and the flyschoid (turbidite) nature of the host rocks suggest that sedimentation and initial sulfide deposition took place in a deep, elongate marine basin or graben overlying a crustal rift zone. A rift underlying the sedimentary pile is consistent with the occurrence of mafic metavolcanic rocks of midocean ridge basalt affinity and could also have served as the feeder system for a line of hydrothermal vents on the sea floor that generated the sulfide deposits.