Large Batholiths Research Articles

Geochemical characteristics of granitoids along the western margin of the Central Anatolian Crystalline Complex and their tectonic implications

AbstractThe closely related assemblage of igneous and metamorphic rocks that lie within a triangular area approximately bounded by the Tuzgölü Fault, the Ecemiş Fault and the Izmir‐Ankara‐Erzincan Suture and between the lines connecting Ankara, Sivas and Niǧde is called the Central Anatolian Crystalline Complex (CACC). The granitoids cropping out within the CACC can be divided areally into three groups: (1) a large number of individual small plutons which form an arcuate set and curve from NE‐SW to NW‐SE and extend from Sulakyurt to Niǧde along the western margin; (2) a relatively narrow and smaller set of disconnected plutons extending from Sivas to Çamardi along the eastern margin; and (3) a very large batholith along the northern margin exposed around Yozgat.This study deals only with the first group of rocks. Geochemical data have been used to classify the granitoids, to determine their chemical characteristics, and to estimate the possible source regions and tectonic environment of magma generation and emplacement. The geochemical data indicate that these western margin granitoids range in composition from monzonitic to granitic‐granodioritic varieties, belong to the alumina‐cafemic and cafemic associations and possess either a metaluminous or peraluminous character. All granitoid types display features which may indicate the presence of both S‐ and I‐type igneous rocks. Interpretation of the trace element data suggests a syncollisional to late/post‐collisional tectonic setting and a continental crustal source for the granitoids.

Some thermal and paleomagnetic consequences of tilting a batholith

If a rock body becomes fully magnetized and then is folded, faulted, or otherwise tilted, its vector of remanent magnetization will tilt with it. However, if a rock body is tilted while it is very hot, and then cools and becomes magnetized, its vector of magnetization will show little if any of the effects of tilt. It is shown, for a wide variety of circumstances, that tilting a large batholith should not be expected to result in equal tilt of that batholith's magnetic direction. This follows from several elementary considerations: (1) Batholiths cool slowly, and slow cooling lowers the blocking‐temperature range for acquisition of remanent magnetization. (2) If a tilted block is large (of the order of 25 km in width) and has been tilted through even a moderate angle (25° will do), its “lower” end originally must have been very hot; quite possibly too hot to retain a remanent magnetization. (3) Tilt entails uplift, and uplift rearranges the geothermal gradient. Rapid uplift accompanied by erosion is particularly effective in raising the temperature of the upper levels of the crust, thereby delaying acquisition of magnetization until tilting is nearly complete. Other thermal constraints on the tilting of remanence vectors in batholiths are discussed; in general, it appears that quite special circumstances are required to tilt a large pluton without leaving a clear magnetic record. Few, if any, Cordilleran batholiths studied to date show this kind of evidence of tilt.

Tectonic setting and origin of the Proterozoic rapakivi granites of southeastern Fennoscandia

ABSTRACTThe 1·65–1·54 Ga rapakivi granites of southeastern Fennoscandia represent the silicic members of a bimodal magmatic association in which the mafic members are tholeiitic diabase dykes and minor gabbroic-anorthositic bodies. They are metaluminous to slightly peraluminous A-type granites and occur as high-level batholiths and stocks in an E-W-trending belt extending from Soviet Karelia to southwestern Finland. The Soviet Karelian granites were emplaced into the contact zone between Archaean craton and Svecofennian juvenile 1·9Ga-old crust, while the Finnish granites were intruded into the Svecofennian crust. Deep seismic soundings show that the rapakivi granites and the contemporaneous, mainly WNW or NW-trending diabase dyke swarms are situated in a zone of relatively thin crust. Below the Wiborg Batholith there exists a domal structure in the lithosphere in which a transitional zone (mafic underplate) occurs between the crust and the mantle.The Nd isotopic evolution of the rapakivi granites (εNd(T) −3·1—−0·2) corresponds to the evolution of the 1·9Ga-old Svecofennian crust, as do their Pb isotopic compositions. This implies that the Finnish granites represent anatectic melts of the Svecofennian crust. In contrast, the Soviet Karelian granites show isotopic composition indicative of substantial incorporation of Archaean lower crust material. Petrochemical modelling of one of the Finnish batholiths shows that its parental magma could have been generated byc.20% melting of a granodioritic source and that fractional crystallisation was important during the subsequent evolution of this magma.The rapakivi granites are redefined as A-type granites that show the rapakivi texture at least in larger batholiths. The field, geochemical, and seismic data indicate that the classical Finnish rapakivi granites were generated in an anorogenic extensional regime by partial melting of the lower/middle crust. The melting, and possibly also the extensional tectonics, were related to upwellings of hot mantle material which led to intrusion of mafic magmas at the base and into the crust.

Deformation monitoring at Nevado del Ruiz, Colombia - October 1985 - March 1988

Deformation studies began at Nevado del Ruiz 23 days before the devastating 13 November 1985 eruption, at least 12 months after precursory seismicity and fumarolic activity began. The late start in geodetic monitoring, limited number of stations in the pre-eruption network, and inconsistent patterns in the observed deformation limit conclusions about intrusive activity in the months and weeks prior to the eruption. However, the data require that the magma source of the devastating eruption was either deeper than 7 km or, if shallow, recovered the same volume and position within one week of the eruption. Geodetic monitoring resumed 1 week after the eruption and, by April 1986, included 11 tilt-leveling stations, 38 EDM lines, and 7 short leveling lines - a network capable of detecting emplacement or movement of magma volumes as small as 3 MCM (3 × 10 6 m 3) to a depth of 2–3 km (using a point-source model), 10 MCM to 7 km, 50 MCM to 10 km, and 200 MCM to 15 km beneath Ruiz. In addition, 4 telemetered tiltmeters provided the capability of detecting, in real time, the fairly rapid ascent of much smaller magma bodies. Stations established to detect instability of the summit ice cap after the eruption were discontinued in early 1986. The data collected from the geodetic networks have higher than normal variance but demonstrate that little or no cumulative deformation of Ruiz occurred from October 1985 through March 1988. Thus, little, if any, magma intruded above 5 km beneath the summit during or after the 13 November 1985 eruption. This lack of significant intrusive activity agrees with the surprisingly low seismic energy release under Ruiz and makes direct degassing of a large batholith an improbable explanation of the large sulfur flux to date at Ruiz. Part of the variance in the geodetic data results from real but noncumulative deformation that may in part be pressure-buffered by a fairly large geothermal water-gas mixture for which abundant physical evidence exists. Part of the noncumulative deformation, some of the fairly dispersed and low-level seismicity under Ruiz, and some phreatic events appear to correlate with seasonal precipitation patterns. Hence rain/snow-loading and groundwater interaction may cause deformation events and possibly help trigger some phreatic explosions and seismic events at Ruiz and, as search of the literature reveals, at other volcanoes in metastable states.

The Hercynian-Indonesian collision type granites of west Yunnan and their tectonic significance

The Linchang Composite Granite Batholith (LCGB) of west Yunnan is an important constituent of the collision-related granite belt of a Permian-Triassic suture zone in this area. It consists of Pinhejie granite in the north and Linchang granite (LG) in the south, which are connected with each other at depth. The Pinhejie granite is N.N.W.-trending, 70 km long, 3–9 km wide and 450 km 2 in area. Its eastern part is Pinhejie gneissic granite (PGG), with an Rb-Sr isotopic age of 309 ± 27 Ma and an initial Sr ratio of 0.7153 ± 3. Its western part is Pinhejie massive granite (PMG), with an Rb-Sr isotopic age of 294 ± 6 Ma and an initial Sr ratio of 0.7205 ± 3. The LG is an extremely large batholith, with a total length of 370 km, a width of 18–36 km and a total area of some 10000 km 2. The Rb-Sr age of Rb-Sr isochron line for LG and PMG could be obtained with an age of 292 ± 15 Ma and an initial Sr ratio of 0.7216 ± 7. These data show that emplacement of LCGB took place approximately in the range of 292-275 Ma, i.e. in the Permian period. The modelling composition of source materials for LG is equivalent to a mixture consisting of 80% upper crustal Sr plus 20% lower crustal Sr. It suggests that the collision-related LCGB is a typical peraluminous S type granite in genesis.

Geochemistry of the Early Proterozoic granitoids in the Halls Creek Orogenic Subprovince, northern Australia

The Halls Creek Orogenic Subprovince contains various types of Early Proterozoic granitoids. Three suites of tonalite have been recognized in the central part of the Halls Creek Orogenic Domain which is an eastern wing of the subprovince. The oldest tonalite suite characteristically contains orthopyroxene, and includes trondhjemite. The youngest tonalite among the three is the Sally Downs Tonalite, and is a part of the Mabel Downs Granitoid. The tonalites and trondhjemite have low K 2O and Rb contents, fractionated REE patterns with HREE depletion. These geochemical characteristics suggest each of the tonalite melts is likely to have been derived from the partial melting of mafic rocks leaving a residue consisting of garnet or amphibole. It is inferred that subduction of an oceanic plate which had been present between the Kimberley and North Australian blocks was responsible for the generation of the tonalitic magma, melting either subducted oceanic crust or lower mafic crust of an island arc. The Bow River Granitoid Suite occurs in the western part of the Halls Creek Orogenic Domain, forming a large batholith. Granitoids of the suite are typically peraluminous and enriched in K 2O and Rb. The Sophie Downs Granitoid is an alkali-feldspar granite, and has A-type characteristics. The subduction and subsequent collision of the two blocks are considered to be responsible for the generation of those granitoids.

Neodymium and strontium isotopic constraints on the origin of the peraluminous granitoids of the South Mountain Batholith, Nova Scotia, Canada

Nd isotopic data from the South Mountain batholith show that its peraluminous granitic rocks have considerably higher ϵ t Nd ( t=372 Ma) values (−5.2 to −1.4) than the extensive flyschoid country rocks (−11.2 to −8.7). These isotopic data are combined with Sr isotopic measurements to estimate the involvement of the country rocks, either as source materials or as contaminants, in the batholith. Metasedimentary enclaves in the batholith generally have Nd and Sr isotopic compositions intermediate between those of the batholith and the country rocks, and provide evidence of contamination by the country rocks. Integrated with other geochemical data, the isotopic results show at least two possible origins for the South Mountain batholith: either this large batholith is a hybrid of mantle and crustal materials and contains a large proportion of mantle-derived Nd and Sr, or the magma is derived from deeper metasedimentary crustal material, with younger model ages or higher Sm/Nd ratios than the upper crust.

Some supracrustal (S-type) granites of the Lachlan Fold Belt

ABSTRACTS-type granites have properties that are a result of their derivation from sedimentary source rocks. Slightly more than half of the granites exposed in the Lachlan Fold Belt of southeastern Australia are of this type. These S-type rocks occur in all environments ranging from an association with migmatites and high grade regional metamorphic rocks, through an occurrence as large batholiths, to those occurring as related volcanic rocks. The association with high grade metamorphic rocks is uncommon. Most of the S-type granites were derived from deeper parts of the crust and emplaced at higher levels; hence their study provides insights into the nature of that deeper crust. Only source rocks that contain enough of the granite-forming elements (Si, Al, Na and K) to provide substantial quantities of melt can produce magmas and there is therefore a fertile window in the composition of these sedimentary rocks corresponding to feldspathic greywacke, from which granite magmas may be formed.In this paper, three contrasting S-type granite suites of the Lachlan Fold Belt are discussed. Firstly, the Cooma Granodiorite occurs within a regional metamorphic complex and is associated with migmatites. It has isotopic and chemical features matching those of the widespread Ordovician sediments that occur in the fold belt. Secondly, the S-type granites of the Bullenbalong Suite are found as voluminous contact-aureole and subvolcanic granites, with volcanic equivalents. These granites are all cordierite-bearing and have low Na2O, CaO and Sr, high Ni, strongly negative εNd and high 87Sr/86Sr, all indicative of S-type character. However, the values of these parameters are not as extreme as for the Cooma Granodiorite. Evidence is discussed to show that these granites were derived from a less mature, unexposed, deeper and older sedimentary source. Other hypotheses such as basalt mixing are discussed and can be ruled out. The Strathbogie Suite granites are more felsic but all are cordierite-bearing and have chemical and other features indicative of an immature sedimentary source. They are closely associated with cordierite-bearing volcanic rocks. The more felsic nature of the suite results in part from crystal fractionation. It is suggested that the magma may have entered this “crystal fractionation” stage of evolution because it was a slightly higher temperature magma produced from an even less mature sediment than the Bullenbalong Suite. The production of these S-type magmas is discussed in terms of vapour-absent melting of metagreywackes involving both muscovite and biotite. The production of a magma in this way is consistent with the low H2O contents and geological setting of S-type granites and volcanic rocks in the Lachlan Fold Belt.

Deformation around the Creede Caldera: A consequence of isostatic adjustment following Caldera Formation

The pattern of deformation around the Creede caldera (26.5 Ma), southwest Colorado, may provide clues to the physical mechanisms of caldera evolution, particularly resurgent doming. This paper considers the hypothesis that the deformation is due to isostatic relaxation of surface load (topography) following caldera formation, involving uplift of the caldera floor and subsidence of a surrounding tumescent dome. This hypothesis is tested by constructing finite difference models of relaxing topography which have as parameters the mechanical properties of rocks surrounding the caldera. The goal is to find models that predict spatial and temporal patterns of deformation matching those observed around the caldera, as inferred from: (1) the present structural form of the region as indicated by the surface distribution of stratigraphic units and (2) past structural forms as indicated by the locations, orientations, and ages of faults, which are presumed to be due to a combination of regional tension and stresses caused by bending at the surface. Successful models of the deformation have been found using a variable viscosity half‐space overlain by a thin elastic plate. In general, the nature of deformation at the surface is strongly dependent on the radial variation of viscosity at shallow depths beneath the elastic plate and is much less sensitive to variation of viscosity with depth. Models with relatively low viscosities beneath the caldera and relatively high viscosities in the surrounding region accurately predict uplift of the caldera floor which is rapid compared to the rate of marginal subsidence. The results indicate that the rate of deformation is governed by the flow of material with viscosities characteristic of wallrock (roughly 1019–1024 Pa s) rather than magma. A gap in faulting around the caldera at about one caldera radius outside the wall suggests the existence of a relatively low radial viscosity gradient in that position; higher viscosity gradients inside and outside of this low may reflect the margins of two nested magma bodies at depth (a central stock within a larger batholith). In order to match the observed pattern of faults around the caldera the orientation of the regional tension that is superimposed on the bending stresses must rotate clockwise with time. This change in stress direction is consistent in sense but earlier than that proposed by others for the western United States.

The depositional evolution of the Svecofennian supracrustal sequence in Finland and Sweden

On the basis of six new zircon ages from metavolcanic rocks, together with other isotopic, geophysical and geological data, the chronological and tectono-stratigraphic evolution of the early Proterozoic Svecofennian supracrustal rocks in Finland and Sweden are discussed. These data suggest that a NW-SE displacement from the Bothnian Bay to Lake Ladoga and a sinking of the southwestern block was initiated 1950–2000 Ma ago. Accumulation of pelitic sediments and greywackes on the subsiding Archaean basement continued until about 1860 Ma In south-central and northern Sweden, shallow submarine and subaerial volcanics were extruded 1880–1890 Ma ago. The thick sedimentary piles and volcanic rocks were subsequently intruded by plutonic rocks forming large batholiths 1860–1890 Ma ago.

RbSr geochronology and geochemistry of plutonic rocks from the Wadi Shuqub quadrangle, west-central Arabian Shield

New geochemical and RbSr isotopic age data are presented for three suites of plutonic rocks from the central and western parts of the Wadi Shuqub quadrangle. The syn-tectonic diorite-tonalite are the products of early arc magmatism and the post-tectonic biotite-granite is the result of intra-arc plutonism. These plutonic suites have an intrunsive relationship with the layered metavolcanic and metasedimentary rocks of the Baish and Bahah groups, which have contributed to the development of an oceanic crust. The syn-tectonic tonalite, which forms a large batholith of tonalite-quartz diorite in the western part of the Wadi Shuqub quadrangle, yields an age of 854 ± 10 Ma (2 σ) with an initial 87Sr/ 86Sr ratio of 0.70159 ± 0.00004 and an MSWD value of 1.97. The late-tectonic diorite-tonalite stock yields an age of 815 ± 13 Ma (2 σ) with an initial 87Sr/ 86Sr ratio of 0.7029 ± 0.00005 (2 σ) and an MSWD value of 1.81. The post-tectonic suite of biotite-granite from the Gabalat area defines a poorly fitted Rb-Sr isochron age of 552 ± 20 Ma with an initial 87Sr/ 86Sr ratio of 0.71327 ± 0.003 (2 σ) and an MSWD value of 4.47. The syn- to late-tectonic tonalite and diorite-tonalite are of low-K calc-alkaline affinity and are characterized by low abundances of K, Rb/Sr, and high Sr, CaO, MgO, TiO 2, and FeO t (total Fe as FeO is denoted as FeO t. These two suites with low initial 87Sr/ 86Sr ratios are likely to be derived from a mantle differentiation or partial melting of the basic rocks above the subduction zone. The post-tectonic biotite-granite is enriched in SiO 2, K, Rb, Rb/Sr and depleted in Sr, CaO, MgO and FeO t and is of dominantly high-K calc-alkaline affinity. This suite of biotite-granite with high initial 87Sr/ 86Sr ratio appears to have been derived from lower crustal material. Limited trace element data on diorite-tonalite and biotite-granite show that the biotite-granite is enriched in Rb, Pb, Zn, Sn, U, Th, Zr, and Y and depleted in Sr, Ba, Cu and Ni compared with diorite-tonalite.

Hydrogen isotope study of the granitic rocks of the Ryoke belt, Central Japan

Among many Cretaceous granitic masses in the Ryoke belt of central Japan, two discordant plutons, Kisokoma and Shinshiro, and the largest batholith, Inagawa, were investigated for the D/H ratio in coexisting biotite and hornblende. The relationships between δD- and X Fe- [= Fe/(Fe + Mg)] values for coexisting biotite and hornblende in the three granitic masses are markedly different. In the Kisokoma pluton, tielines between coexisting biotite and hornblende on the δ D— X Fe diagram are distributed in a narrow area and slopes of the tielines are essentially parallel to that derived from T. Suzuoki and S. Epstein's experimental work. In the Shinshiro pluton the tielines are distributed in two areas though the slopes are also almost parallel to that of Suzuoki and Epstein. In the Inagawa batholith, most of the tielines are nearly vertical with highly scattered δ D-values. The X Fe-values of the minerals are ∼ 0.5 in the Kisokoma and Shinshiro plutons, and ∼ 0.7 in the Inagawa batholith. Such a difference in the δ D— X Fe relationship is explained by the different geological conditions of intrusion of the three granitic masses. Namely, the Kisokoma and Shinshiro plutons are discordant, and the erosion level of the former is shallow and that of the latter is deep. The Inagawa batholith intruded during the geologic movement and interacted with the country rocks. On the basis of Suzuoki and Epstein's experimental work, the δ D-value of the water in a single granitic magma can be estimated to be as low as −60 to −70‰ in the initial stage, and as high as ∼ −30‰ in the later stage, because of the early crystallization of biotite and hornblende, whose δ D-values are usually lower than the value of water in the magma. When the experimental results for hornblende by C.M. Graham et al. and those for biotite by Suzuoki and Epstein are combined, the temperature of the hydrogen exchange equilibration between biotite and magmatic water is calculated to be from 400° to 630°C for the Kisokoma pluton, and from 650° to 800°C for the Shinshiro pluton. It is impossible to estimate the equilibration temperature for the Inagawa batholith, because there are no experimental data on hydrogen isotope fractionation between hydrous silicates, such as biotite and hornblende, with higher X Fe-values than 0.7 and coexisting water.

U–Pb zircon ages in supracrustal and plutonic rocks; North Spirit Lake area, Northwestern Ontario

The North Spirit Lake greenstone belt in the Sachigo Subprovince of the Superior Province comprises parts of three sequences of volcanic and sedimentary rocks; the main (youngest) is separated from the older sequences by an unconformity to disconformity. The belt is bounded by large granitoid batholiths and was metamorphosed under greenschist to, locally, hornblende–cordierite facies. U–Pb zircon dating was performed on volcanic, sedimentary, and plutonic rocks in order to establish an absolute chronology for the evolution of the area.A tuff breccia in the lowermost supracrustal sequence is dated at 3023 ± 2 Ma. Zircons from a tuff in the middle sequence show complex U–Pb relationships; although they do not allow a precise age determination, the data suggest that the tuff formed sometime between 2950 and 2800 Ma ago. Zircons from a quartz arenite, also in the middle sequence, yield a simple data pattern and define an age of [Formula: see text], suggesting derivation of the sediment from a uniform source of this age. A zircon analysis from a tonalitic clast in a conglomerate at the base of the upper sequence yields a minimum age of 2975 Ma. The clast may have had the same origin as the zircons in the quartz arenite. Another tonalitic clast from the same conglomerate yields a slightly older age of 3001 ± 3 Ma. No remnant of these tonalites can be recognized in the field, suggesting that they have been largely removed during subsequent erosional processes.Two subvolcanic intrusions from the upper sequence yield zircon ages of 2743 ± 2 and 2731 ± 2 Ma, respectively. A crystal tuff, also in the upper sequence, contains two generations of zircons: newly formed magmatic zircons, which date the extrusion of the tuff at 2735 ± 10 Ma, and older grains with a minimum age of 2862 Ma, which represent inherited zircons. This tuff was thus generated at least in part by anatectic melting of >2862 Ma crust.A quartz diorite from MacDowell Lake in the adjacent Berens River Subprovince yields an age of 2744 ± 2 Ma. A mafic inclusion in the quartz diorite contains amoeboidal, strongly fractured zircons, which point to an intercept age of about 2727 Ma. This age could either reflect a metamorphic event or represent a geologically meaningless mixed age.

Crustal evolution of Archean rocks in the Kakagi Lake area, Wabigoon Subprovince, Ontario, as interpreted from high-precision U–Pb geochronology

The evolution of the Archean volcanic–plutonic complex in the Kakagi Lake area occurred during a time interval of ca. 32 Ma. The earliest age is [Formula: see text] from analysis of zircon and baddeleyite in a gabbro intruding the lowermost Katimiagamak Volcanic Group. An age of 2723.2 ± 1.8 Ma on a tonalite gneiss from the interior of the underlying Sabaskong Batholith is indistinguishable from a previously dated massive border phase of the batholith and shows no evidence for inheritance from an older sialic component. An early tonalite phase from the adjacent Aulneau Batholith is dated at [Formula: see text], and the latest granodiorite phase is dated at [Formula: see text]. This defines a time span of about 7 Ma for intrusion of the bulk of the batholith and indicates that previously dated felsic volcanism from the uppermost sequence, above the Kakagi Lake Volcanic Group, is coeval with late plutonic activity in the Aulneau Batholith. The end of regional deformation in the area is given by the ages of two late-tectonic intrusions, the Heronry Lake pluton and the Stephen Lake pluton, dated at 2701.0 ± 1.2 and 2699.2 ± 1.9 Ma, respectively.U–Pb analyses of sphene were carried out on four of the samples in an effort to establish details of the post-folding thermal history of the area. Sphene is least reset in the Heronry Lake pluton (2699.2 ± 1.6 Ma), which is spatially most closely associated with the volcanic rocks and most reset in the Sabaskong gneiss (2673.7 ± 6.6 Ma), the sample most strongly affected by diapirism. The data indicate that regional deformation was a relatively late event, possibly caused by diapirism in the centres of the large batholiths and driven by a long-lived heat source in the mantle or lower crust.

Thermal metamorphism near Willi Willi, New South Wales

Broad igneous contact aureoles and isolated thermal highs are developed in deformed early Permian formations of the south‐eastern part of the New England Fold Belt, near Willi Willi, NSW. Typical resultant mineral assemblages of quartz‐biotite‐muscovite‐cordierite‐(pyrrhotite) in pelitic rocks and quartz‐diopside‐wollastonite‐grossular‐vesuvianite in calcareous rocks indicate a peak of static prograde metamorphism at upper hornblende hornfels facies, with 7 up to 550°C, P = 1 kb and XCO2 <0.10. Partial retrogression followed and at one locality a late stage hedenbergite‐fluorite‐stannian‐andradite skarn formed at the base of the Yessabah Limestone accompanied by Cu‐Ag‐Sn sulphide mineralization. The thermal metamorphism and mineralization are attributed to granitic plutons at shallow depth, belonging to a large batholith of which only highest levels are exposed.

U-Pb age and genetic significance of heterogeneous zircon populations in rocks from the Favourable Lake area, Northwestern Ontario

The study evaluates the relationships between measured U-Pb ages and zircon characteristics of five morphologically, texturally, and isotopically complex zircon populations and compares the zircon development stages to the orogenic evolution of the Favourable Lake area. Two distinct zircon types from a hornblendite xenolith in a granitoid batholith of the Sachigo subprovince of the Superior Province yield U-Pb ages of 2729.0±6.8 Ma and 2714.8 −6.4 +7.4 Ma, which date specific metamorphic phases coinciding with major plutonic pulses in the batholith. Zircons from a metamorphosed felsic dike, crosscutting the hornblendite, consist of an old zircon component with a minimum age of 2788 Ma possibly reflecting igneous crystallization ≧ 2950 Ma ago, and a younger component with an inferred age of 2725±15 Ma, probably reflecting metamorphism during batholith emplacement. In the Berens River subprovince to the south, granodiorite forms both a late tectonic phase in a large batholith and a post-tectonic pluton intruded into the batholith, yet zircons from these granodiorites have identical ages of 2697.3±1.7 Ma and 2696.2±1.2 Ma, respectively. The late tectonic granodiorite also contains inherited zircons with a minimum age of 2767 Ma which are indirect evidence for the presence of old sialic crust in this subprovince. Zircons from a sheared monzonite near the boundary fault between the two subprovinces yield an upper intercept age of 2769 −26 +63 Ma, which we interpret as the intrusion age of the monzonite. This rock is older than most dated units in the surrounding batholiths and suggests that the boundary is a long-lived Archean structure. A lower intercept age of about 1750 Ma for zircons of the hornblendite is the result of chemical alteration of the zircons. This, and a similar lower intercept age shown by the sheared monzonite zircons, are thought to reflect increased fluid activity and possibly shearing during the Early Proterozoic Hudsonian orogeny which occurred in the Churchill Province to the northwest. A later Pb-loss mainly from near-surface domains of the zircons is indicated by lower intercept ages of about 500–100 Ma.

Upper crustal structure of the Mount Hood, Oregon, region as revealed by time term analysis

Seismic refraction data with a dense areal distribution were collected to study the seismic structure of Mount Hood and the surrounding region. This area is typical of Cascade volcanoes and is geologically quite complex. The prime goals of this project were to search for velocity variations in the upper crustal rocks and to determine if the velocity of these rocks is anisotropic. A new system, including 100 remote recording units, was developed to facilitate the collection of data in this type of survey. The data collected in this study reveal a large variation in velocity and thickness of the uppermost crustal rocks that is probably typical of the High Cascade province. A regional structural pattern surrounding Mount Hood, where there is a marked thinning of low‐velocity near‐surface rocks, suggests that the present edifice of Mount Hood lies on top of a much larger structure, possibly the roof of a large batholith that was emplaced prior to the eruption of the volcanic rocks that form the modern mountain. A straightforward time term analysis of the data reveals this structure. When the time terms are used to correct the travel time observations for the variation in structure under each station, the remaining set of residuals indicates a variation of travel time with azimuth. While a systematic error in the time term reduction cannot be ruled out, this variation is probably the result of anisotropic velocity structure in the underlying refractor. The alignment of the direction of maximum velocity with the direction of maximum principal stress and the decrease in anisotropy with depth suggest that the anisotropic velocity structure is related to the opening and closing of fractures in the present stress field. The direction of maximum velocity is about N25°W, and the degree of anisotropy appears to vary with depth from about 3% near the surface to zero at depths below 8 km.

Geochronology and geochemistry of Wadi Shuqub granitic rocks, west-central Arabian shield

New geochemical and RbSr isotopic age data are presented for three suites of plutonic rocks from the central and western parts of the Wadi Shuqub quadrangle. The syn-tectonic diorite-tonalite are the products of early arc magmatism and the post-tectonic biotite-granite is the result of intra-arc plutonism. These plutonic suites have an intrunsive relationship with the layered metavolcanic and metasedimentary rocks of the Baish and Bahah groups, which have contributed to the development of an oceanic crust.The syn-tectonic tonalite, which forms a large batholith of tonalite-quartz diorite in the western part of the Wadi Shuqub quadrangle, yields an age of 854 ± 10 Ma (2σ) with an initial 87Sr/86Sr ratio of 0.70159 ± 0.00004 and an MSWD value of 1.97. The late-tectonic diorite-tonalite stock yields an age of 815 ± 13 Ma (2σ) with an initial 87Sr/86Sr ratio of 0.7029 ± 0.00005 (2σ) and an MSWD value of 1.81. The post-tectonic suite of biotite-granite from the Gabalat area defines a poorly fitted Rb-Sr isochron age of 552 ± 20 Ma with an initial 87Sr/86Sr ratio of 0.71327 ± 0.003 (2σ) and an MSWD value of 4.47.The syn- to late-tectonic tonalite and diorite-tonalite are of low-K calc-alkaline affinity and are characterized by low abundances of K, Rb/Sr, and high Sr, CaO, MgO, TiO2, and FeOt (total Fe as FeO is denoted as FeOt. These two suites with low initial 87Sr/86Sr ratios are likely to be derived from a mantle differentiation or partial melting of the basic rocks above the subduction zone. The post-tectonic biotite-granite is enriched in SiO2, K, Rb, Rb/Sr and depleted in Sr, CaO, MgO and FeOt and is of dominantly high-K calc-alkaline affinity. This suite of biotite-granite with high initial 87Sr/86Sr ratio appears to have been derived from lower crustal material. Limited trace element data on diorite-tonalite and biotite-granite show that the biotite-granite is enriched in Rb, Pb, Zn, Sn, U, Th, Zr, and Y and depleted in Sr, Ba, Cu and Ni compared with diorite-tonalite.

Omine acid rocks, Kii Peninsula - geology and major element chemistry.

Omine Acid Rocks are distributed in the central mountainland of Kii peninsula and made of several hypabyssal plutons which are scattered for the stretch of about 40Km in north-south direction. The acid rocks consist mainly of leucocractic and porphyritic granitic rocks and the present outcrops are closer to the ceiling of dome-shaped pluton. They have intruded two geological epochs, Paleozoic Chichibu Group and Mesozoic Hidaka Group, and give thermal and mechanical effects to country rocks. Omine Acid Rocks have not made a large batholith exposed intermittently but several independent plutons intruded successively. The general chemical feature of Omine Acid Rocks is characterized by SiO2-poor and K2O-rich as compared with other Shimanto granitic rocks and average Japanese granitic rocks (Aramaki, 1972). The chemical trend of the acid rocks on the diagram Q-Or-Ab is similar to that of Washinton's table rather than that of Japanese granitic rocks.

Mesozoic – early Cenozoic volcanism, plutonism, and mineralization in southern British Columbia: A plate tectonic synthesis

Three time–space profiles have been constructed using geologic data from British Columbia between 49° N and 56° N. They illustrate variations across the Cordillera, (1) in the stratigraphic and tectonic setting of volcanism, (2) in the age and modal type of granitoids, and (3) in the distribution and types of copper and lead deposits related to volcanic and plutonic rocks. These profiles provide the basis for a plate tectonic synthesis of the Mesozoic–Cenozoic geology, illustrated by six true-scale cross sections.The preferred model has, in the Triassic, two eastward-dipping subduction zones, giving rise to the copper-rich Karmutsen and Nicola–Takla volcanics respectively. After collision of the two volcanic belts by the Early Jurassic, a single eastward-dipping subduction zone remained active until the Eocene. Magmas produced by partial melting and fractionation of subducted lithosphere occurred across the western 300 km of the Cordillera, leading to thickening of the crust, and eventually to anatectic melting to generate large batholiths now containing pendants of volcanics. Jurassic and later geologic and metallogenic events across the eastern 500 km of the Cordillera are the results of an increased heat flux through inhomogeneous crust of varying thickness, comprised of relict ocean floor, continental margin sediments, older volcanics, and ancient cratonic basement. This results in patterns of metamorphism, volcanism, and plutonism which have no simple spatial relationship to the subduction zone.