Ar Isotopic Ages Research Articles

Early Quaternary Age of the Final Stage of Volcanism in the Central Part of the Sredinny Range of Kamchatka (K–Ar Data)

A first set of K–Ar isotopic ages obtained, which allowed to estimate the age of the largest volcanoes of the Anaunsky Dol (3.2, 2.2 and 1.9 Ma) and eruptive centers of the post-caldera stage of Uksichan massif (1.2 and 0.8 Ma), located in the Sredinny Range of Kamchatka. Significant time intervals separating the individual phases of volcanism suggest either repeated activation of the fault zone to which the dated eruptive centers are confined, or the presence of several fault zones here.

Reassessing the age of Karpathos ophiolite (Dodecanese, Greece): consequences for Aegean correlations and Neotethys evolution

AbstractWhile the Neogene history of the Eastern Mediterranean region is now fairly well understood, our knowledge of older regional palaeogeographies is less accurate, especially the positions of blocks and nappes constituting the Aegean Islands prior to the Cenozoic. Our study focuses on the ophiolite exposed on the island of Karpathos (Dodecanese), which is located in the Aegean fore-arc at a pivotal position between the ‘western’ and ‘eastern’ ophiolites of the Mediterranean region and where conflicting Late Jurassic and Late Cretaceous ages have led to diverging tectonic and palaeogeographic interpretations. To test these ages, we targeted the radiolarian cherts that depositionally overlie the ophiolite and extracted diagnostic radiolarian assemblages of Aptian (∼125−113 Ma), early–middle Albian (∼113−105 Ma) and Turonian (∼93.9−89.8 Ma) ages. These results suggest that previous Late Cretaceous K–Ar isotopic ages (from 95.3 ± 4.2 Ma to 81.2 ± 1.6 Ma) may have been reset by Late Cretaceous metamorphism or affected by argon loss. Overall, the new Early Cretaceous ages show that the Karpathos ophiolite should not be correlated with the Pindos Nappes of Greece or the ophiolites of Cyprus or Syria but rather with the Lycian Nappes of Turkey and their root located in the Izmir–Ankara–Erzincan Suture Zone. Therefore, the Karpathos ophiolite represents a remnant of the Northern Neotethys, not the Pindos Ocean or the proto-Eastern Mediterranean Basin.

Using palaeomagnetic and isotopic data to investigate late to post-Caledonian tectonothermal processes within the Western Terrane of Svalbard

The analytical results of a total of 205 metabasic specimens from 10 palaeomagnetic sites collected from Oscar II Land in Western Spitsbergen are presented. Petrographic, structural and palaeomagnetic data all demonstrate that the pre-Caledonian ferromagnetic fabric of the metabasic rocks has been extensively reoriented and intensively remineralized. New in situ laser ablation inductively coupled plasma mass spectrometry 40 Ar/ 39 Ar age determinations suggest that the host rocks have been subject to three resetting events during the 426 – 380 Ma (Caledonian sensu lato ), 377 – 326 Ma and c. 300 Ma intervals. The latter two resetting events coincide in time with the Barents Shelf-wide rift-controlled subsidence events. The derived palaeomagnetic data do not fall on the expected apparent polar wander path of Laurussia for syn- to post-Caledonian time. Consequently, four models invoking palaeogeographical great and small circle rotations, regional tectonism involving thrusting and normal listric faulting have been investigated to account for this lack of correspondence. The palaeomagnetic data do not lend support to reconstructions linking Western Svalbard with Pearya but point instead to the importance of listric faulting related to the opening of the North Atlantic Ocean that modified the geometry of the West Spitsbergen Fold and Thrust Belt. Supplementary material: (1) Field characteristics of metabasic sites, (2) detailed description of applied rock magnetic and palaeomagnetic procedures, (3) microscopic images of investigated geochronological samples, (4) in situ LA-ICP-MS 40 Ar/ 39 Ar isotopic age determination results, (5) microscopic, SEM and BSE images of investigated metabasites, and (6) anisotropy of magnetic susceptibility are available at https://doi.org/10.6084/m9.figshare.c.3673924 .

Evolution of magmatic-hydrothermal system of the Kalaxiange’er porphyry copper belt and implications for ore formation (Xinjiang, China)

The Kalaxiange’er porphyry copper ore belt is situated in the eastern part of the southern Altai of the Central Asian Orogenic Belt and forms part of a broad zone of Cu porphyry mineralization in southern Mongolia, which includes the Oyu Tolgoi ore district and other copper–gold deposits. The copper ore bodies are spatially associated with porphyry intrusions of granodiorite, quartz diorite, quartz syenite, and quartz monzonite and have a polygenetic (polychromous) origin (magmatic porphyry, hydrothermal, and supergene). The mineralized porphyries are characterized by almost identical REE and trace element patterns. The Zr/Hf and Nb/Ta ratios are similar to those of normal granite produced through the evolution of mantle magma. The low initial Sr isotope ratio ISr, varying within a narrow range of values (0.703790–0.704218), corresponds to that of primitive mantle, whereas the εNd(T) value of porphyry varies from 5.8 to 8.4 and is similar to that of MORB. These data testify to the upper-mantle genesis of the parental magmas of ore-bearing porphyry, which were then contaminated with crustal material in an island-arc environment. The isotopic composition of sulfur (unimodal distribution of δ34S with peak values of −2 to −4‰) evidences its deep magmatic origin; the few lower negative δ34S values suggest that part of S was extracted from volcanic deposits later. The isotopic characteristics of Pb testify to its mixed crust–upper-mantle origin. According to SHRIMP U–Pb geochronological data for zircon from granite porphyry and granodiorite porphyry, mineralization at the Xiletekehalasu porphyry Cu deposit formed in two stages: (1) Hercynian “porphyry” stage (375.2 ± 8.7 Ma), expressed as the formation of porphyry with disseminated and vein–disseminated mineralization, and (2) Indosinian stage (217.9 ± 4.2 Ma), expressed as superposed hydrothermal mineralization. The Re–Os isotope data on molybdenite (376.9 ± 2.2 Ma) are the most consistent with the age of primary mineralization at the Xiletekehalasu porphyry Cu deposit, whereas the Ar–Ar isotopic age (230 ± 5 Ma) of K-feldspar–quartz vein corresponds to the stage of hydrothermal mineralization. The results show that mineralization at the Xiletekehalasu porphyry Cu deposit was a multistage process which resulted in the superposition of the Indosinian hydrothermal mineralization on the Hercynian porphyry Cu mineralization.

Late to post-Appalachian strain partitioning and extension in the Blue Ridge of Alabama and Georgia

Structural observations and U-Pb and 40 Ar/ 39 Ar isotopic age dates are reported for shear zones and metamorphic rocks in the southernmost Appalachian Blue Ridge. Two major mylonite zones, the Goodwater-Enitachopco and Alexander City fault zones, have retrograded peak amphibolite facies fabrics and assemblages in rocks of the ancient Laurentian margin. Both faults are within a zone of transition between Carboniferous (Alleghanian) west-directed thrusts in the foreland and synchronous strike-parallel dextral shear zones in the hinterland. The 40 Ar/ 39 Ar hornblende and muscovite dates record late Mississippian cooling and exhumation from the Late Devonian (Neoacadian orogeny, 380–340 Ma) peak. Retrograde mylonites of the Goodwater-Enitachopco fault are of two types. Earlier formed, type 1, upper greenschist to lower amphibolite facies shears are roughly coplanar with the dominant schistosity of the country rock, and show northwest-southeast stretching. Later formed, type 2 shears are discrete, steeply dipping, middle to upper greenschist facies shear zones that cut across the type 1 shears, displacing them in an oblique dextral and normal slip sense. A 366.5 ± 3.5 Ma U-Pb SHRIMP-RG (sensitive high-resolution ion microprobe–reverse geometry) date on zircon from a prekinematic trondhjemite dike that is cut by a type 2 shear zone places a maximum age on the time of movement along the Goodwater-Enitachopco fault. The 40 Ar/ 39 Ar cooling dates place a minimum on the timing of extensional movement along the type 1 shears of between ca. 334 and 327 Ma. The Goodwater-Enitachopco fault coincides at depth with a basement step up that has been interpreted as a Cambrian rift fault formed along the ancient Laurentian margin, possibly reflecting its reactivation during Mesozoic rifting of Pangea. The Alexander City fault zone is a middle greenschist facies, dextral strike-slip fault rather than a west-vergent thrust fault, as was previously thought. This fault zone is obliquely cut and extended by more east-trending, subvertical, cataclastic faults (Mesozoic?) characterized by intense quartz veining. These brittle faults resemble those in other parts of the Blue Ridge, Inner Piedmont, and Pine Mountain terrane and, together with the Goodwater-Enitachopco and Towaliga faults, they appear to form a broad graben-like structure across the entire piedmont. Shoulder rocks flanking the Alexander City fault zone contain earlier formed (peak to late peak metamorphic) dextral shear zones. A 369.4 ± 4.8 Ma U-Pb thermal ionization mass spectrometry date on zircon records the time of crystallization of a dike that cuts one of the shears bracketing the peak metamorphic fabric to between ca. 388 and 369 Ma and places a minimum age for right-slip shearing. Similar kinematics, geometries, tectonostratigraphic positions, and timing indicate that these Devonian shears are more southern counterparts to the system of Neoacadian dextral faults exposed in the North Carolina Blue Ridge. Kinematic analysis of the Goodwater-Enitachopco and Alexander City faults document that dextral strains in the Alabama and western Georgia Blue Ridge are partitioned much farther toward the foreland than is reported to the northeast, likely as a consequence of the southern Appalachian master decollement having passed obliquely across a several kilometer step up along the Cartersville transform. The top-to-the-south-southeast normal-slip component of movement along the Goodwater-Enitachopco fault is unusual, considering its position far toward the foreland. Loose timing constraints for this extensional event (late Carboniferous to Early Jurassic) leave room for several tectonic explanations, but we favor the following. (1) Late Pennsylvanian to Early Permian crustal thickening created a wedge of Blue Ridge rocks bound above by the Goodwater-Enitachopco, below by the decollement, and to the northwest (present-day direction) by a topographically steep mountain front. (2) Further convergence and crustal thickening caused this wedge to gravitationally collapse with southward-driven motion. (3) Mesozoic rifting reactivated some of the faults as the Gulf of Mexico began to open.

Geochemical and isotopic characteristics of volcanic rocks from the northern East China Sea shelf margin and the Okinawa Trough

Volcanic rocks both from the northern East China Sea (NECS) shelf margin and the northern Okinawa Trough are subalkaline less aluminous, and lower in High Field Strength Elements (HFSE). These rocks are higher in Large Ion Lithophile Elements (LILE), thorium and uranium contents, positive lead anomalies, negative Nb-Ta anomalies, and enrichment in Light Rare Earth Elements (LREE). Basalts from the NECS shelf margin are akin to Indian Ocean Mid-Ocean Ridge Basalt (MORB), and rhyolites from the northern Okinawa Trough have the highest (207)Pb/(208)Pb and (208)Pb/(204)Pb ratios. The NECS shelf margin basalts have lower (87)Sr/(86)Sr ratios, epsilon(Nd) and sigma(18)O than the northern Okinawa Trough silicic rocks. According to (40)K-(40)Ar isotopic ages of basalts from the NECS shelf margin, rifting of the Okinawa Trough may have been active since at least 3.65-3.86 Ma. The origin of the NECS shelf margin basalt can be explained by the interaction of melt derived from Indian Ocean MORB-like mantle with enriched subcontinental lithosphere. The basalts from both sides of the Okinawa Trough may have a similar origin during the initial rifting of the Okinawa Trough, and the formation of basaltic magmas closely relates to the thinning of continental crust. The source of the formation of the northern Okinawa Trough silicic rocks was different from that of the middle Okinawa Trough, which could have been generated by the interaction of basaltic melt with an enriched crustal component. From the Ryukyu island arc to East China, the Cenozoic basalts have apparently increasing trends of MgO contents and ratios of LREE to Heavy Rare Earth Elements (HREE), suggesting that the trace element variabilities of basalts may have been influenced by the subduction of the Philippine Sea plate, and that the effects of subduction of the Philippine Sea plate on the chemical composition of basaltic melts have had a decreasing effect from the Ryukyu island arc to East China.

New features from the Romblon Island Group: Key to understanding the arc–continent collision in Central Philippines

A complete ophiolite sequence and mélange are reported for the first time in the Romblon Island Group located at the central part of the Philippine archipelago. The study area comprises Tablas, Romblon and Sibuyan Islands built on amalgamated crust and mantle rocks that are typically fault-bounded and stratigraphically disrupted. The Sibuyan Ophiolite Complex (SOC) consists of harzburgite and dunite, layered clinopyroxenite, layered and isotropic gabbro, diabase dike swarm and basaltic–andesitic pillow lava and flow deposits. A Jurassic–Cretaceous age of formation and late Early Miocene age of emplacement are suggested for the SOC by available data. Emplacement of these crust–mantle units is believed to have been facilitated through displacements along east-verging thrust faults developed consequent to the Early Miocene arc–continent collision in Central Philippines. Along with recent K–Ar isotopic ages (12 Ma) obtained for some of the metamorphic rocks in the area, these lithologic units and structural features provide critical information regarding the arc–continent collision which is one of the significant events in the evolution of the Philippine island arc system. The usefulness of potential field geophysical methods to studying lithologic and structural relationships in an extremely disrupted terrane is evaluated in this study. Observed variations in gravity and magnetic values correlated with lithologic distributions and geologic structures. Higher relative Bouguer and magnetic anomalies are observed over members of the complete ophiolite sequence and younger igneous materials exposed in Tablas and Sibuyan Islands. Their geophysical characteristics contrast markedly with the less dense and magnetically inferior sedimentary rocks exposed only on Tablas Island. On the other hand, metamorphic units in Tablas, Romblon and Sibuyan Islands display variable magnetic and gravity signatures that reflect differences in composition and alteration or weathering. Thrust faults related to the emplacement of the ophiolite as a result of the collision are inferred from sudden changes in gravity and magnetic values.

Rb–Sr and K–Ar systems of biotite in surface environments regulated by weathering processes with implications for isotopic dating and hydrological cycles of Sr isotopes

Biotite is widely used for Rb–Sr and K–Ar isotopic dating and influences Sr isotope geochemistry of hydrological regimes. The isotopic system of biotite behaves diversely in response to surface weathering; i.e. the complete preservation of original Rb–Sr and K–Ar isotopic ages or dramatic reduction. In this study, we have explored the relation between the behavior of isotopic systems and complex weathering processes of biotites in the weathering profiles distributed on the Mesozoic granitoids in South Korea. In the lower parts of the profiles, biotite in the early stages of weathering was transformed into either oxidized biotite or hydrobiotite, with a mass release of 87Sr and 40Ar forced by the rapid oxidation of ferrous iron. During the transformation to oxidized biotite, 87Sr and 40Ar were preferentially released relative to Rb and K, respectively, via solid-state diffusion through the biotite lattice, resulting in a drastic reduction of original isotopic age. The reduction of Rb–Sr age was greater than that of K–Ar age because K was preferentially released over Rb whereas 87Sr and 40Ar were released proportionally to each other. However, during the transformation of biotite to hydrobiotite (i.e., to regularly interstratified biotite-vermiculite), 87Sr, Rb, 40Ar, and K were completely retained in the alternating biotite interlayer, and thus the original isotopic age can be preserved. In the upper parts of the profiles, where iron oxidation was almost completed, 87Sr, Rb, 40Ar, and K were gradually and proportionally released, with no further significant change in isotopic age during the gradual transformation of the early-formed oxidized biotite into hydrobiotite and vermiculite or during their final decomposition to kaolinite. The ratios and amounts of isotopes released from weathered biotites are dependent upon the degree of iron oxidation and the pathways of mineralogical transformation. Regional and local variations in isotopic systems affected by particular weathering processes should be considered when dating biotite or biotite-bearing rocks in weathering environments, modeling the transfer of Sr isotopes to hydrologic regimes, and tracking the provenance of sediments.

The 40Ar/39Ar geochronology constraint and geological significance of mylonites in Shangyi-Chicheng fault belt on the north of North China Craton

A dating of two biotite samples taken from the meso- and low-temperature mylonites within the Shangyi-Chicheng fault belt on the north of the North China Craton yields 40 Ar/ 39 Ar isotopic ages of (399±1) Ma and (263±2) Ma, respectively. These data reflect an Early Devonian deformation and a Late Carboniferous retrograde metamorphism event along the fault, suggesting that the tectonic activities of the North China Craton in Paleozoic should be reconsidered.

(40)Ar/(39)Ar isotopic ages of Qitianling granite and their geologic implications

Thie paper has reported the first application of 40Ar/39Ar dating to orthoclase from Qitianling granite. The resultant plateau ages yielded by three orthoclase specimens 2KL-17,99LQ-2 and 2KL-31 (Note:The last one was taken from the part of granite which had been attributed to Cailing super-unit of the Indosinian Period by the former researchers) collected from the said granite are (139.57±2.79) Ma, (140.55±2.81) Ma and (144.91±2.90) Ma respectively. The above-mentioned ages represent the closed 40Ar/39Ar age of the orthoclase. The consistency in age dating results, the similarity in geochemical characteristics and rock textures,and the NW-SE orientation of orthoclase phenocrysts almost throughout the granite, provide evidence for the intimate relationship between the Furong super-unit and the Cailing super-unit that form the main part of the granite,suggesting that they are products of comagmatic conjugate differentiation during the Late Jurassic. This paper also makes a comparison between the Qitianling granite and the Qianlishan granite.

40Ar/39 Ar isotopic ages of Qitianling granite and their geologic implications

This paper has reported the first application of 40 Ar/ 39 Ar dating to orthoclase from Qitianling granite. The resultant plateau ages yielded by three orthoclase specimens 2KL-17, 99LQ-2 and 2KL-31 (Note: The last one was taken from the part of granite which had been attributed to Cailing super-unit of the Indosinian Period by the former researchers) collected from the said granite are (139.57±2.79) Ma, (140.55±2.81) Ma and (144.91±2.90) Ma respectively. The above-mentioned ages represent the closed 40 Ar/ 39 Ar age of the orthoclase. The consistency in age dating results, the similarity in geochemical characteristics and rock textures, and the NW-SE orientation of orthoclase phenocrysts almost throughout the granite, provide evidence for the intimate relationship between the Furong super-unit and the Cailing super-unit that form the main part of the granite, suggesting that they are products of comagmatic conjugate differentiation during the Late Jurassic. This paper also makes a comparison between the Qitianling granite and the Qianlishan granite.

Comment on: ‘‘K–Ar evidence from illitic clays of a Late Devonian age for the 120 km diameter Woodleigh impact structure, Southern Carnarvon Basin, Western Australia’’, by I.T. Uysal, S.D. Golding, A.Y. Glikson, A.J. Mory and M. Glikson [Earth Planet. Sci. Lett. 192 (2001) 218–289

K–Ar isotopic ages presented by Uysal et al. for illitic clay minerals from drill core samples were interpreted to date the Woodleigh impact event at 359±4 Ma, allegedly implicating Woodleigh in the Late Devonian mass extinction. However, only very equivocal evidence is presented by Uysal et al. to support a link between clay mineral paragenesis and impact-related features, and the K–Ar ages reveal a distribution that is essentially a continuum between 308 and 364 Ma. The ‘age’ computed by Uysal et al. is based on an average of the five oldest ages within this group, which has no geological or statistical basis. The stratigraphic age constraints considered by Uysal et al. to be consistent with this age are much weaker than acknowledged, and the impact could have been much older than mid-Devonian. The size of the Woodleigh crater is poorly constrained (and the subject of an ongoing controversy); Uysal et al.’s suggestion of 120 km diameter is probably overestimated by a factor of two, in which case a link to any mass extinction is unlikely.

An age constraint on Gulf of California rifting from the Santa Rosalia basin, Baja California Sur, Mexico

Marine rocks of the Santa Rosalia basin, Baja California Sur, were sampled in a pilot study to determine their suitability for magnetostratigraphy and geochronology with the goal of providing an age constraint on Gulf of California rifting. Progressive demagnetization of samples from the Boleo Formation, the earliest marine sequence overlying the deeply eroded basement, reveals a high-coercivity characteristic remanent magnetization (ChRM) in addition to a low-coercivity overprint. The ChRM appears to be a primary magnetic remanence with stratigraphically bound normal- and reversed-polarity directions. A single ^(40)Ar/^(39)Ar isotopic age of 6.76 ± 0.90 Ma (2σ) was obtained for the cinta colorada, a tephra deposit of reversed paleomagnetic polarity within the Boleo Formation. The age of the cinta colorada is refined by calculating isotopic age probabilities for each of the reversed-polarity intervals of the geomagnetic polarity time scale (GPTS) in the ±2σ range 5.86–7.66 Ma. The interval with the highest probability is 6.57–6.94 Ma. In conjunction with the isotopic age, preliminary magnetostratigraphy of the Boleo Formation is correlated with the GPTS in order to further delineate the onset of marine sedimentation. The most likely correlation yields an age of 6.93–7.09 Ma (GPTS subchron C3Bn) for the base of the Boleo Formation and 6.14–6.27 Ma for the top. This correlation, indicating an average sedimentation rate of 28 ± 4 cm/k.y., could be significantly altered if a more thorough magnetostratigraphy proved the existence of additional polarity intervals in the Boleo Formation. However, even if the isotopic age of the cinta colorada is used as the only age constraint, the result is consistent with data from the northern Gulf of California and shows that rifting started much earlier than the ca. 3.6 Ma commencement of sea-floor spreading at the mouth of the Gulf of California. The deposition of the Boleo Formation was probably related to an early phase of gulf rifting caused by a change in Pacific–North American plate motions.

Sm–Nd, Rb–Sr and K–Ar geochronology of the Higo metamorphic terrane, west‐central Kyushu, Japan

The Higo metamorphic terrane situated in west‐central Kyushu island, southwest Japan, is composed of greenschist‐ to granulite‐facies metamorphic rocks. The southern part of the metamorphic terrane consists mainly of garnet–biotite gneiss and garnet–cordierite–biotite gneiss, and orthopyroxene or cordierite‐bearing S‐type tonalite with subordinate amounts of hornblende gabbro. Rb–Sr, Sm–Nd and K–Ar isotopic ages for these rocks have been determined here. The garnet–biotite gneiss gives an Sm–Nd age of 227.1 ± 4.9 Ma and a Rb–Sr age of 101.0 ± 1.0 Ma. The hornblende gabbro has an Sm–Nd age of 257.9 ± 2.5 Ma and a K–Ar age of 103.4 ± 1.1 Ma. These age differences of the same samples are due to the difference in the closure temperature for each system and minerals. The garnet‐cordierite–biotite gneiss contains coarse‐grained garnet with a zonal structure conspicuously distinguished in color difference (core: dark red; rim: pink). Sm–Nd internal isochrons of the garnet core and the rim give ages of 278.8 ± 4.9 Ma (initial 143Nd/144Nd ratio = 0.512311 ± 0.000005) and 226.1 ± 28.4 Ma (0.512277 ± 0.000038), respectively. These ages are close to formation of the garnet core and the rim. It has been previously suggested that the Higo metamorphic terrane belongs to the Ryoke metamorphic belt. But Sr and Nd isotopic features of the rocks from the former are different from those of the Ryoke metamorphic rocks, and are similar to those of the granulite xenoliths contained in the Ryoke younger granite.

Submicroscopic alteration of hornblende in the amphibolitic schists, northwestern Okchon metamorphic belt

Alteration phenomena in hornblende crystals of the northwestern Okchon metamorphic belt were investigated in detail using high-resolution transmission electron microscopy (HRTEM). HRTEM images of hornblende commonly show submicroscopic intergrowths of a variety of sheet silicates; chlorite is the most abundant sheet silicate within hornblende, and biotite is also frequently observed along with chlorite. Although the angle between (001) of chlorite and (100) of hornblende is variable, the (001) planes of both chlorite and biotite parallel the c axis of hornblende. Microstructural evidences indicate that both sheet silicates intergrown with hornblende are produced by retrogression of hornblende. Biotite apparently formed by early hydration reaction in the presence of fluids enriched with K+, and chlorite at a later stage of retrograde reaction when K+ was consumed. In addition, hornblende crystals contain numerous etch pits and poorly crystalline silicates as filling materials, which are the product of weathering. Such submicroscopic intergrowths of sheet silicates may result in somewhat high Al contents in the hornblende analysis, producing imprecise result in geothermobarometric calculations. Furthermore, submicroscopic alteration of hornblende deteriorates the Ar retentivity and consequently affects the K−Ar isotopic ages or the40Ar/39Ar age spectra of hornblende. It is thus evident that the HRTEM characterization of hornblende is prerequisite for the correct interpretation of P-T estimates and age data.

Karakoram fault zone rocks cool in two phases

The Karakoram fault zone of Ladakh, India, is a major strike-slip boundary along which the tectonic evolution of Tibet has been accommodated. In this paper 40 Ar/ 39 Ar isotopic age data are integrated with structural and metamorphic data to infer an exhumation history for granites and low- to intermediate-grade metamorphic rocks exposed adjacent to two strands of the fault zone. Near Tangtse, leucogranites which crystallized c . 17 Ma ago are cut by the Karakoram fault zone, indicating that fault movement was initiated subsequent to 17 Ma. The 40 Ar/ 39 Ar data give temperature-time histories which indicate that the <17 Ma metamorphic rocks within the fault zone were exhumed differentially relative to the adjacent (and already cooled to <150°C) Ladakh Batholith, while cooling progressively through amphibolite to below greenschist facies temperatures. Two phases of rapid cooling of Karakoram fault zone rocks are evident, one at c . 17 Ma to c . 13 Ma, and another, following an intervening period of relatively slow cooling, starting at c . 8 Ma and continuing until at least 7 Ma. Uplift since c . 17 Ma via dextral oblique thrusting has resulted in exhumation from amphibolite facies metamorphic conditions, possibly with a larger thrust component from 17 to 13 Ma, followed by a change to dominant strike-slip motion at about 13 Ma. Rapid cooling starting at c . 8 Ma at greenschist facies temperatures is probably the result of renewed oblique thrusting. It is notable that the two episodes of rapid cooling recorded in Karakoram fault zone rocks coincide with episodes of exhumation in the Pakistani Karakoram and also in southern Tibet.

New age constraints on the timing of volcanism and tectonism in the northern Main Ethiopian Rift–southern Afar transition zone (Ethiopia)

Forty new K-Ar and 40 Ar / 39 Ar isotopic ages from the northern Main Ethiopian Rift (MER)–southern Afar transition zone provide insights into the volcano-tectonic evolution of this portion of the East African Rift system. The earliest evidence of volcanic activity in this region is manifest as 24–23 Ma pre-rift flood basalts. Transition zone flood basalt activity renewed at approximately 10 Ma, and preceded the initiation of modern rift margin development. Bimodal basalt–rhyolite volcanism in the southern Afar rift floor began at approximately 7 Ma and continued into Recent times. In contrast, post-subsidence volcanic activity in the northern MER is dominated by Mio-Pliocene silicic products from centers now covered by Quaternary volcanic and sedimentary lithologies. Unlike other parts of the MER, Mio-Pliocene silicic volcanism in the MER–Afar transition zone is closely associated with fissural basaltic products. The presence of Pliocene age ignimbrites on the plateaus bounding the northern MER, whose sources are found in the present rift, indicates that subsidence of this region was gradual, and that it attained its present physiography with steep escarpments only in the Plio-Pleistocene. Large 7–5 Ma silicic centers along the southern Afar and northeastern MER margins apparently formed along an E–W-oriented regional structural feature parallel to the already established southern escarpment of the Afar. The Addis Ababa rift embayment and the growth of 4.5–3 Ma silicic centers in the Addis Ababa area are attributed to the formation of a major cross-rift structure and its intersection with the same regional E–W structural trend. This study illustrates the episodic nature of rift development and volcanic activity in the MER–Afar transition zone, and the link between this activity and regional structural and tectonic features.

Use of multifrequency, multipolarization shuttle imaging radar for volcano mapping in the Kunlun Mountains of Western China

A group of volcanoes northeast of Aksayqin Lake, in the western Kunlun Mountains, China, have been identified on multifrequency, multipolarization spaceborne imaging radar-C/X-band synthetic aperture radar (SIR-C/X-SAR) images. Field observations made on the volcanic morphology and terrain features are described in this paper. Analysis of single-band, single-polarization radar backscatter coefficients (σ°) shows that LHV best discriminates the two types of lava flows (pahoehoe and aa lavas), alluvium, and bedrock. The factors affecting the radar backscatter coefficient also are analyzed. Finally, this paper presents KAr isotopic ages of volcanic samples collected in the field and discusses the volcanism in the area.

Mineralogy and temporal relations of coexisting authigenic minerals in altered silicic tuffs and their utility as potential low-temperature dateable minerals

Coexisting fine-grained (0.1–20 μm) authigenic silicate minerals separated from altered tuffs in Miocene and Plio-Pleistocene lacustrine deposits were characterized petrographically and using X-ray powder diffraction. The authigenic minerals are dominated by clinoptilolite, erionite, phillipsite, K-feldspar, silica, calcite, smectite, and randomly interstratified illite/smectite. Minor accessories of opal-CT, cristobalite, and barite are present with the major alteration minerals. Authigenic minerals from altered tuffs were dated using the K Ar method to evaluate the utility of these minerals for determining the time of alteration in low-temperature diagenetic environments. The eruption ages of some of these zeolite-rich tuffs were determined using the 40Ar 39Ar method on single sanidine and plagioclase minerals. The K Ar isotopic ages of the fine-grained K-feldspar show minimal variation compared with results from the clinoptilolite separates. The isotopic ages from the authigenic K-feldspar (15-13.8 Ma) and some of the zeolites (16.-6.7 Ma) are similar to the eruption ages of the tuffs and indicate early alteration. Despite their open-framework structure, zeolites apparently can retain part or all of their radiogenic argon under favorable conditions (e.g., saturated environment). How much of the radiogenic argon is retained is estimated from the isotopic ages of other coexisting secondary minerals that are commonly dated by the K Ar method. Although zeolite isotopic ages should be interpreted with caution, they may be useful to constrain temporal relations of low-temperature diagenetic processes when used in conjunction with other dateable minerals.

Geochronological studies of Mesozoic igneous rocks from eastern India

A wide variety of Mesozoic basaltic and alkaline igneous rocks crop out on the eastern margin of the Indian shield. We present new KAr isotopic age data on whole rock samples and mineral separates from several igneous rock suites from the Meghalaya-Nagaland region of northeast India. The KAr isotopic ages of these samples lie between 149-107 Ma, i.e., Late Jurassic to Early Cretaceous. Published data on the tectonic setting and petrology of the basaltic and alkaline rocks suggest that their emplacement was controlled by major fault systems. Active mantle upwelling in an extensional tectonic régime related to the fragmentation of the indian plate from Australia-Antarctica in the Early Cretaceous, appears to have been the main trigger for the magmatic activity.