Douglas A. Archibald

Intercalibration of 40Ar39Ar dating standards

A number of standards used for KAr and 40Ar39Ar dating have been intercalibrated. Multiple splits (∼ 15–40 mg each) of MMhb-1, SB-3 Biotite, LP-6 Biotite 40–60#, GA1550 Biotite, Fish Canyon Tuff-3 Biotite, Taylor Creek Rhyolite Sanidine, Bern 4 Muscovite, Bern 4 Biotite and Pennsylvania State University Orthoclase-1A were irradiated in capsules, fused and analyzed. In replicate analyses, most of these standards proved to be homogeneous in 40Ar★39ArK ratio (age) K, Ca, and 36Ar contents. MMhb-1 is inhomogeneous in age at the ∼ 15-mg level, indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle; the orthoclase sample is also inhomogeneous at the ∼ 30-mg level. Quoted relative to an age of 162.9 Ma for the SB-3 Biotite standard, total fusion ages are: LP-6 Biotite 40–60# = 128.1 Ma; GA1550 Biotite = 97.8 Ma; Fish Canyon Tuff-3 Biotite = 27.95 Ma; Taylor Creek Rhyolite Sanidine = 28.0 Ma; Bern 4 Muscovite = 18.51 Ma; Bern 4 Biotite = 17.21 Ma. These ages are in good agreement with the (absolute) KAr ages of these minerals. Different preparations of biotite and sanidine standards from a welded tuff such as the Fish Canyon Tuff may not be of identical ages. Tests on submilligram splits reveals that SB-3 Biotite, GA1550 Biotite, Fish Canyon Tuff-3 Biotite and Taylor Creek Rhyolite Sanidine are suitable for use as monitors in laser work; the sanidine specimen appears to be the most suitable for use as the primary monitor for 40Ar39Ar dating.

Paleozoic and Mesozoic high-pressure metamorphism at the margin of ancestral North America in central Yukon

Eclogite and blueschist in the Yukon-Tanana and Slide Mountain terranes in the Yukon and Alaska preserve a partial record of middle Paleozoic, late Paleozoic, and early Mesozoic subduction and subsequent unroofing. Near Faro, eclogites are commonly mylonitized and partially retrogressed. The analysis of Fe-Mg exchange between garnet and clinopyroxene suggests temperatures ( T ) in the 400–500 °C range for eclogite, and jadeite (∼40 mol%) + quartz suggest minimum pressures ( P ) of 11–13 kilobar at that temperature range; clinopyroxene-garnet-epidote-quartz suggest pressure near 15 kilobar ( P H 2 O = P S ). Eclogite near Last Peak, which contains brown hornblende, late-stage oligoclase, and biotite, yielded P-T estimates up to 625 °C at a minimum pressure of 14 kilobar. Eclogite near Ross River yielded temperatures near 525 °C and a minimum pressure near 13 kilobar. The previously proposed Permian-Triassic age of high-pressure tectonism for the entire Yukon-Tanana terrane in the Yukon was inferred from isotopic dating of one eclogite and one blueschist in separate localities. In this study, isotopic 40 Ar/ 39 Ar analysis of white mica from eclogite in the Simpson range yielded a date of 344 ± 1 Ma. Eclogite and blueschist from Ross River yielded white mica 40 Ar/ 39 Ar dates of 267 ± 3 Ma and 273 ± 3 Ma, respectively. White mica in eclogite from Faro was dated at 260 ± 3 Ma. White mica in two eclogites from Stewart Lake, about 2 km apart, yielded 40 Ar/ 39 Ar dates of 228 ± 1 Ma and 346 ± 3 Ma. White mica in Last Peak eclogite, for which a concordant U-Pb zircon date is 269 Ma, yielded an integrated date of 236 ± 1 Ma. Overprinting in the blueschist facies and final cooling in some localities predated eclogite metamorphism in others. The cooling ages indicate significant plate convergence as early as Carboniferous time and either the existence of more than one subduction zone or continuous convergence and episodic exhumation above a single zone. When this is considered together with the range of ages of other high-pressure occurrences, the North American Cordillera is seen to be characterized by multiepisodic and diachronous high-pressure metamorphism.

Middle Jurassic exhumation along the western flank of the Selkirk fan structure: Thermobarometric and thermochronometric constraints from the Illecillewaet synclinorium, southeastern British Columbia

New thermobarometric and thermochronometric data from the Illecillewaet synclinorium, a broad regional southwest-verging structure in the western flank of the Selkirk fan structure, indicate that accretion of the Intermontane superterrane resulted in deep burial (20–25 km) of the outer margin of Ancestral North America and that subsequent southwest-verging folding and thrusting were accompanied by rapid exhumation. Southwest-verging folding of lower Paleozoic rocks in the Illecillewaet synclinorium was initiated during regional peak-metamorphic conditions at 6–7 kbar. Crystallization of late-synkinematic granitoid plutons that intrude the Illecillewaet synclinorium also occurred at pressures of 6–7 kbar; however, equilibration of the contact aureoles of the plutons occurred at 3–3.5 kbar, during the latter stages of the southwest-verging deformation. Evidently, a decompression on the order of 3 kbar occurred concurrently with southwest-verging folding and thrusting in the Illecillewaet synclinorium. Thermochronometric analyses further indicate that plutonic rocks from the Illecillewaet synclinorium cooled rapidly between times of crystallization and times of Ar closure in biotite. Regional relationships indicate that the accretion of the Intermontane superterrane to the edge of the North American margin in southeastern British Columbia occurred between 187 and 173 Ma (Toarcian–Bajocian). Subsequent southwest-verging deformation occurred approximately between 173 and 168 Ma (Bajocian–Bathonian). Thus, the removal of at least 10 km of rocks from above the Illecillewaet synclinorium, inferred from thermobarometric data, occurred approximately between 173 and 168 Ma. Rapid denudation of the Selkirk fan structure was probably the result of synorogenic extension (or extensional collapse) contemporaneous with the development of southwest-verging structures at deeper levels of the orogen. The data presented here indicate that all significant deformation and most of the exhumation along the western flank of the Selkirk fan structure were complete by late Middle Jurassic time. After the Middle Jurassic, the Selkirk fan structure must have been transported passively northeastward along the basal decollement of the Cordilleran foreland thrust-and-fold belt, together with the Intermontane superterrane, as the fold-and-thrust belt tectonically overrode the margin of the North American craton.

Coherent French Range blueschist: Subduction to exhumation in <2.5 m.y.?

Coherent oceanic strata in the French Range belong to the exotic Cache Creek terrane of the Canadian Cordillera. They were metamorphosed to blueschist grade, tectonically extruded, eroded, and intruded by plutons—perhaps in <2.5 m.y. Sodic amphibole overprint chert as young as late Pliensbachian to Toarcian age (ca. 191 to ca. 177 Ma). Blueschist mineral assemblages defi ne the early metamorphic fabric along with phengite dated by 40 Ar/ 39 Ar as 173.7

Chronological constraints on the thermal and tilting history of the Sierra San Pedro Mártir pluton, Baja California, México, from U/Pb, 40Ar/39Ar, and fission-track geochronology

The tectonothermal history of the four major phases of the Sierra San Pedro Martir pluton and surrounding metamorphic rocks of the Mesozoic Peninsular Ranges batholith of Baja California is presented on the basis of U/Pb, 40 Ar/ 39 Ar step-heating, and fission-track dating, in combination with Al-in-hornblende geobarometry. A previous model proposed up to 90° of east-side-up tilting of the pluton, exposing >20 km of crustal section to account for its crescent shape, asymmetrical zoning, internal structure, the eastward younging of K-Ar dates across the intrusion and eastward increase in the metamorphic grade of the country rocks, from greenschist to amphibolite facies. The U/Pb data suggest that the different phases of the pluton were emplaced sequentially from west to east between 97.0 +4 / −1 Ma and 93.8 +1 / −1 Ma. All except one of the 105 40 Ar/ 39 Ar age spectra have well-defined plateaus and are interpreted as cooling ages. Samples from the pluton give hornblende and biotite 40 Ar/ 39 Ar plateau dates and apatite fission-track dates that young from west to east; thus, hornblende dates decrease from 95 to 91 Ma, biotite dates decrease from 94 to 88 Ma, and apatite dates decrease from 72 to 57 Ma. Muscovite, biotite, and plagioclase from the same rock sample collected at the easternmost phase of the pluton yield concordant 40 Ar/ 39 Ar dates of 88 Ma. The exposed part of the pluton underwent rapid cooling (approximately equal to 40 °C/Ma) down to ≈250 °C in the first 10 m.y. after intrusion. Modeling of track-length distribution in apatite is consistent with monotonic slow cooling from ca. 80 Ma to the present. The data do not support a history that includes major tilting of the pluton. Eastward younging of 40 Ar/ 39 Ar and fission-track dates may be explained by approximately equal to 15° of east-side-up tilting of the pluton at or after 88 Ma about a north-south horizontal axis. Furthermore, the fission-track data suggest that part or all of this tilting may have taken place at or after 57 Ma, and therefore may be a consequence of regional-scale crustal extension associated with the opening of the Gulf of California in Neogene time. Such tilting is in agreement with the Al-in-hornblende geobarometry for the hornblende-biotite intrusive phase that yields pressures of 5.2 ± 0.6 kbar. An approximately equal to 15° northeast-side-up tilt of the crustal block containing this pluton would explain the apparent paleomagnetic inclination discrepancies with cratonic North America and militates against large-scale northerly transport of Baja California.

Mesozoic igneous activity in the Maranha˜o province, northern Brazil: 40Ar/ 39Ar evidence for separate episodes of basaltic magmatism

Low- and high-Ti basaltic rocks of the Maranha˜o province in northern Brazil appear to be related to the opening of the Atlantic Ocean. 40Ar/39Ar dating was carried out on powdered whole-rock basalts, both washed and unwashed in dilute nitric acid, to document this relationship. Acid treatment does not alter the argon retention properties of retentive phases, but is effective in removing alteration phases (high Ca/K) degassed at low laboratory temperatures. Our results identify that at least two periods of volcanic activity formed the Maranha˜o province. Low-Ti basalts from the western sector are ∼ 200-190 Ma and are related to the opening of the North Atlantic. High-Ti basalts from the eastern sector are ∼ 129-124 Ma, slightly younger than compositionally similar rocks in the Serra Geral flood basalt province of southern Brazil, and are related to the opening of the South Atlantic. Other flood basalts, such as the Columbia River, Deccan, Rajmahal, Ferrar and Siberian provinces, were each predominantly formed in 1–2 Ma. The Maranha˜o basalts, remnants of separate igneous events spanning ∼ 70 Ma, should not be classed as a single flood basalt province. The temporal complexities noted in the Maranha˜o basaltic rocks appear to be related to the tracks of hotspots that led to the rifting of the Atlantic Ocean. Two sets of Mesozoic basaltic rocks occur in southern Africa, India and South America, resulting from events leading to the fragmentation and dispersal of sections of Gondwanaland; in each (sub)continent the two provinces were formed by events separated by ∼ 50–70 Ma. Of these pairs, only the Maranha˜o and Serra Geral show geochemical similarities, indicating compositionally distinct upper-mantle source regions under sections of Brazil separated by ∼ 3000 km.

Cooling pattern and mineralization history of the Saint Sylvestre and western Marche leucogranite pluton, French Massif Central: I. 40Ar/39Ar isotopic constraints

Abstract The Saint Sylvestre-western Marche Leucogranite Complex (LC) is a composite pluton emplaced 324 m.y. ago in the Variscan continental crust of the northwestern Limousin, French Massif Central. Petrographic, structural, and new isotopic data are used to constrain the cooling pattern of the pluton in the temperature range for Ar retention in muscovite (Part I) and the genetic relationships with the subsequent uranium mineralization postdating the time of intrusion by 40–50 m.y.(Part II). A suite of thirty-seven muscovite and nine biotite concentrates selected over the entire plutonic complex were analyzed by the 40 Ar/ 39 Ar technique. Muscovite is characterized by well-defined plateau ages ranging between 314 and 301 Ma, while biotite gives less well-behaved age spectra with apparent ages varying between 320 and 300 Ma. The muscovite age variations are systematic and clearly associated with regional trends at the scale of the pluton. Field, petrographic, gravimetric, and structural relationships indicate that the age pattern is controlled by the intrusion shape and the regional, post-cooling fault network cutting through the laccolith. When restored to their approximate pre-faulting geometry, the age variations are shown to correlate positively with sample elevation, with a top-to-base difference as large as 10–14 m.y. over a mean intrusion thickness of ∼3 km. This pattern is interpreted to reflect cooling driven by erosion and exhumation of the thickened Variscan crust during the Westphalian and is confirmed by two-dimensional thermal modelling (Part II). The data show that 40 Ar/ 39 Ar mica ages can postdate the time of crystallization of the host pluton by up to 23 Ma depending on the level sampled at the surface; they warn against interpreting K/Ar mica ages from similar, low-relief igneous settings as closely postdating intrusive events without a precise knowledge of the thermal structure, cooling mechanism, and post-cooling faulting of the pluton. In turn, provided that the pre-faulting geometry of the intrusion is properly assessed, the vertical cooling pattern can reveal fine-scale variations in the thermal structure of old eroded intrusions that may be extremely useful for recovering quantitative information on the mechanisms and rates of cooling in ancient orogens.

Diachronous deformation along the base of the Himalayan metamorphic core, west-central Nepal

Geologic mapping combined with microstructural and geochronologic analyses from the lower Himalayan metamorphic core in west-central Nepal record along-strike similarity in flow style despite variability in the timing of metamorphism and deformation. The Main Central thrust zone at the base of the Himalayan metamorphic core varies in thickness, tectonostratigraphy, and metamorphic gradient along the 250 km of strike length studied. In situ U-Th/Pb geochronology of monazite sampled from an along-strike transect at the top of the high-strain zone records Eocene−Oligocene prograde metamorphism followed by Miocene retrograde metamorphism. The timing of prograde and retrograde metamorphism and the muscovite 40 Ar/ 39 Ar dates gradually decrease along strike from northwest to southeast. This age trend is punctuated by an abrupt ∼3−8 m.y. decrease in the age of prograde and retrograde metamorphism and muscovite 40 Ar/ 39 Ar dates near the Marsyandi River in central Nepal. Quartz crystallographic preferred orientation fabrics from a parallel transect along the base of the high-strain zone document similar flow style at ∼440 °C in central Nepal. Muscovite 40 Ar/ 39 Ar ages, interpreted to approximate the age of deformation at this structural level, decrease from ca. 7 to 4 Ma along strike from northwest to southeast. Diachronous deformation and metamorphism along strike in west-central Nepal demonstrate the necessity of incorporating more than a single transect into tectonic models. Along-strike tectonometamorphic variability in west-central Nepal spatially corresponds to faults in the Indian basement bounding the subsurface Faizabad ridge, highlighting the possible influence of inherited basement faults on the geometry of the basal Himalayan detachment, the Main Himalayan thrust, as well as the tectonic evolution of the structurally overlying Himalayan metamorphic core. This study highlights the potential influence of inherited structures on the overlying orogenic wedge and the probability of along-strike diachroneity of deformation in the Himalaya.

Dating Hydrothermal Alteration Attending IOCG Mineralization Along a Terrane Bounding Fault Zone: The Copper Lake Deposit, Nova Scotia

The Copper Lake area of mainland Nova Scotia is one of several vein-controlled mineralized (Cu-Au-Co) systems associated with widespread carbonate and iron-oxide alteration proximal to the east-trending Cobequid-Chedabucto Fault System. Although this mineralization has been known for decades, its metallogenic affinity remains poorly defined, and in recent years an IOCG (iron oxide-copper-gold) model has been suggested. In order to determine the age of mineralization and provide an important time constraint for developing a metallogenic model, direct dating of the mineralization and associated alteration was undertaken. At Copper Lake, mineralization occurs in a set of sulphide-carbonate fissure veins hosted by fine-grained metasedimentary rocks of the Middle Devonian Guysborough Group. Dating of the sulphide-alteration (pyrite) and phyllic-alteration (muscovite) stages of the ore system utilized the Re-Os and 40 Ar/ 39 Ar methods, respectively. The two different chronometers yield ages of about 320 Ma and provide an absolute age for the mineralization. As part of this study additional geochronological data were obtained for detrital zircon (U-Pb age of 1634 ± 11.2 Ma) from the host sedimentary rocks, as well as timing of thermal events at ca. 370–380 Ma, 350 Ma and < 300 Ma based on whole rock 40 Ar/ 39 Ar and chemical Th-Pb dating of host rocks and monazite. The Th-Pb dating of monazite indicates that rare-earth element mobility accompanied mineralization. Collectively, the data indicate that the area experienced multiple thermal events, but hydrothermal activity related to mineralization is constrained to about 320 Ma and is tentatively interpreted to relate to structural focusing of fluids that may have been driven by a mid-crustal level mafic heat source. The mineralizing event coincides with regional Alleghanian deformation in this part of the Appalachian orogen and thus reflects larger-scale tectonothermal processes. RESUME Le secteur du lac Copper, dans la partie continentale de la Nouvelle-Ecosse, constitue l’un de plusieurs systemes mineralises (Cu-Au-Co) regis par des filons associes a une alteration etendue en oxyde de fer et en carbonates proximale du systeme de failles oriente vers l’est de Cobequid-Chedabucto. Meme si l’on connait cette mineralisation depuis des decennies, son affinite metallogenique demeure mal definie et des chercheurs ont avance ces dernieres annees un modele OFCO (oxyde de fer-cuivre-or). On a realise une datation directe de la mineralisation et de l’alteration connexe pour determiner l’âge de la mineralisation et etablir une delimitation chronologique importante pour l’etablissement d’un modele metallogenique. Dans le secteur du lac Copper, la mineralisation est presente dans un ensemble de filons de fissures remplies de sulfures-carbonates a l’interieur de roches metasedimentaires a grains fins du groupe du Devonien moyen de Guysborough. Les methodes Re-Os et 40 Ar/ 39 Ar, respectivement, ont permis la datation des stades de la sulfuration (pyrite) et de l’alteration phylliteuse (muscovite). Les deux differentes methodes chronometriques ont fourni des âges d’environ 320 Ma et conferent un âge absolu a la mineralisation. On a obtenu, dans le cadre de cette etude, des donnees geochronologiques supplementaires de zircon detritique (datation U-Pb de 1634 ± 11,2 Ma) des roches sedimentaires hotes ainsi que le moment des evenements thermiques, situes a environ 370-380 Ma, 350 Ma et < 300 Ma d’apres une datation de la roche totale 40 Ar/ 39 Ar et une datation chimique Th-Pb des roches hotes et de la monazite. La datation Th-Pb de la monazite revele qu’une mobilite des metaux des terres rares a accompagne la mineralisation. Les donnees signalent collectivement que le secteur a connu plusieurs evenements thermiques, mais que l’activite hydrothermale apparentee a la mineralisation est limitee a environ 320 Ma; on l’interprete provisoirement comme une activite liee a une concentration structurale des fluides qui pourrait avoir ete provoquee par une source de chaleur mafique mi-crustale. L’episode de mineralisation coincide avec la deformation alleghanienne regionale dans cette partie de l’orogene des Appalaches et correspond en consequence a des processus tectonothermaux a plus grande echelle. [Traduit par la redaction]

Mid-Miocene initiation of orogen-parallel extension, NW Nepal Himalaya

Geologic field mapping surveys integrated with structural, thermochronological, and geochronological analyses confirm the existence of an orogen-parallel strike-slip−dominated shear zone in the upper Karnali valley of northwestern Nepal. This shear zone obliquely cuts through the upper Greater Himalayan Sequence and is characterized by a S-dipping, high-strain foliation and intensely developed ESE-WNW−trending, shallow-plunging mineral elongation lineation. Monazite grains within the Greater Himalayan Sequence are deformed and transposed parallel to the orogen-parallel shear zone and ESE-WNW elongation lineations. In situ U-Th/Pb monazite geochronology constrains metamorphism between 19 and 15 Ma, which is consistent with the timing of Neohimalayan metamorphism and S-directed extrusion of the Greater Himalayan Sequence across the Himalaya, and it is therefore interpreted to have preceded orogen-parallel strike-slip deformation. Mineral deformation mechanisms and quartz c -axis patterns of orogen-parallel fabrics record a rapid increase in temperature of deformation from ∼350 °C along upper levels of the shear zone to greater than 630 °C at ∼2.5 km depth structurally below the shear zone. Symmetric quartz c -axis fabrics further suggest deformation included a significant component of pure shear. The 40 Ar/ 39 Ar thermochronology of foliation-defining muscovite indicates that orogen-parallel shearing was active in the area between ca. 13 and 10 Ma while temperatures cooled through the muscovite closure temperature for argon. By integrating these data with the current understanding of tectonic processes in the Himalaya, we interpret a transition from S-directed extrusion of the Greater Himalayan Sequence to orogen-parallel extension between ca. 15 and 13 Ma in the upper Karnali valley. Integration of our findings with chronological constraints from other migmatite-cored domes supports the growing recognition of a Himalayan-wide mid-Miocene initiation of orogen-parallel extension.