Central Pluton Research Articles

Landsat 8 and Alos DEM geological mapping reveals the architecture of the giant Mesoproterozoic Kunene Complex anorthosite suite (Angola/Namibia)

The quasi-monomineralic composition and huge spatial extent of massif-type anorthosites make detecting lithological regions and boundaries challenging. We use processed Landsat 8 OLI multispectral images and ALOS digital elevation models integrated with field and petrographic observations to characterize the architecture of the ≥17,000 km2 Mesoproterozoic Kunene Complex (KC) anorthosite suite in Angola and Namibia. Images of false colour composite bands 6, 4 and 1 and band ratios 4/2 (ferric minerals), 6/5 (ferrous minerals) and 6/7 (OH-bearing minerals), as well as assessment of the PCA and MNF matrices of eigenvectors and eigenvalues from Landsat 8 data using available spectral libraries, have substantially improved the interpretation of the Kunene Complex rock types and structures. The dataset shows that the reflectance signature of KC anorthositic rocks is primarily a function of the degree of metasomatism, which is most significant in olivine-poor rock types. The weathering intensity of the olivine-bearing anorthosite substrate is another control on the remote sensing signal.Our remote sensing and field-based approach has enabled us to divide the KC anorthosite suite into six distinct spectral and architecture domains and at least four distinct magmatic plutons when considering available high-precision geochronological data. The northernmost pluton (ca. 1380 Ma) of massive, olivine-bearing anorthosite shows distinct remote sensing signatures in the band ratio and MNF images marked by the dominance of OH-bearing and subordinate ferric minerals. The central pluton (1412–1400 Ma) is composed of NNE- to N-striking steeply dipping interlayered olivine-bearing and olivine-poor anorthosite, which correspond to ridges of dark-coloured, low albedo rocks with subordinate slightly oxidised ferrous mineral spectral signatures, and valleys of low albedo and OH-bearing mineral spectral signatures. A NNE-striking tectonic zone along a linear belt of KC granite gneiss separates the northern and central plutons. To the south is a NNW- to NNE-striking layered pluton (ca. 1390 Ma), marked by olivine-bearing anorthosite that forms ridges and metasomatized olivine-poor and pyroxene-bearing anorthosite that forms valleys. This domain is separated from the similarly layered and E–W-striking Zebra Lobe–Oryeheke anorthosite by a top-to-the north thrust zone. A small domain of ca. 1438 Ma olivine-bearing anorthosite sensu lato is situated to the SE of the main Kunene Complex.This work highlights the variability in layered and massive textures, the extent of metasomatism, and the importance of internal tectonic boundaries in the architecture of the Kunene Complex anorthosite suite. Our approach can be applied in other anorthositic terrains and large igneous bodies elsewhere on Earth, especially those where outcrop is poor, or access is inhibited.

Granitic pegmatites in Sri Lanka: A concise review leading to insights and predictions

Abstract Granitic pegmatites can be a source of frustration and confusion. It is thus appropriate to begin with a quick review of basic concepts. Crystallization takes place in the presence of two fluid media; one is a viscous, polymerized silicate magma, and the other is a supercritical aqueous fluid. The two are largely but not completely immiscible. The proportion of aqueous fluid in the mixture progressively increases as the amount of melt diminishes. Pegmatites are of significant economic interest, as they are source rocks of industrial minerals like quartz and the feldspars. These minerals crystallized from the magma, but the feldspars generally become modified by the aqueous fluid. The same applies to accessory minerals that carry Nb, Ta, Zr, Sn and the rare earths. At a late stage, the magmatic fluid phase mixes with cooler, externally derived fluids that bring in elements like Ca and Mg, present in the latest generation of cavity minerals. Frustration arises because the question is recurrent concerning what is magmatic and what is post-magmatic. In terms of tectonic setting, granitic pegmatites may occur in contexts of crustal compression or crustal extension. The pegmatites of Sri Lanka, some of them syenitic and found in all four lithotectonic domains on the island, seem to be unrelated to central plutons. Rather, they are of anatectic origin and members of the NYF family. Some are deformed, but the largest ones are not. Mineralogical and geochemical indicators point to crystallization of the magmas largely in an environment of tectonic relaxation, as in the Grenville collision zone. Important questions remain about the timing of pegmatite emplacement, the interplay of anorogenic magmatism at the end of the Pan-African orogeny, and the role of the mantle in providing heat and fluids that led to localized anatexis of the granulite-grade metamorphic rocks .

Timescale of magma chamber processes revealed by U–Pb ages, trace element contents and morphology of zircons from the Ishizuchi caldera, Southwest Japan Arc

AbstractU–Pb geochronology and trace element chemistry of zircons in a microscale analysis were applied to the Ishizuchi caldera in the Outer Zone of Southwest Japan in order to estimate the timescale of the magma process, in particular, the magma differentiation. This caldera is composed mainly of ring fault complexes, major pyroclastic flow deposits, and felsic intrusion including central plutons. Using SHRIMP‐IIe, our new U–Pb zircon ages obtained from the major pyroclastic flow deposits (Tengudake pyroclastic flow deposits), granitic rocks from central plutons (Soushikei granodiorite and Teppoishigawa quartz monzonite), and rhyolite from the outer ring dike (Tenchuseki rhyolite) and the inner ring dike (Bansyodani rhyolite) are 14.80 ±0.11 Ma, 14.56 ±0.10 Ma, 14.53 ±0.12 Ma, 14.55 ±0.11 Ma and 14.21 ±0.19 Ma, respectively. Based on the U–Pb ages, the Hf contents and the REE patterns of the zircons, three stages are recognized in the evolutionary history of the magma chamber beneath the Ishizuchi caldera: (i) climactic Tengudake pyroclastic flow eruption; (ii) Tenchuseki rhyolite intrusion into the outer ring dike and central pluton intrusion; and (iii) Bansyodani rhyolite intrusion in the inner ring dike. These results indicate a magma evolution history of the Ishizuchi caldera system which took at least ca 600 kyr from the climatic caldera‐forming eruption to the post‐caldera intrusions. Our new geochronological data suggest that the Ishizuchi caldera formed as part of the voluminous and episodic magmatism that occurred in the wide zone along the Miocene forearc basin of Southwest Japan during the inception of the young Philippine Sea Plate subduction.

Contrasting magma emplacement mechanisms within the Rogart igneous complex, NW Scotland, record the switch from regional contraction to strike-slip during the Caledonian orogeny

AbstractThe Rogart igneous complex is unique within the northern Scottish Caledonides because it comprises an apparent continuum of magma types that records a progressive change in emplacement mechanisms related to large-scale tectonic controls. Syn-D2 leucogranites and late-D2 quartz monzodiorites were emplaced during crustal thickening and focused within the broad zone of ductile deformation associated with the Naver Thrust. In contrast, emplacement of the post-D2 composite central pluton was controlled by development of a steeply dipping dextral shear zone along the Loch Shin Line, interpreted as an anti-Riedel shear within the Great Glen Fault system. The mantle-derived nature of the late-to-post-D2 melts implies that the Naver Thrust and the Loch Shin Line were both crustal-scale structures along which magmas were channelled during deformation. A U–Pb zircon age of 425±1.5 Ma for the outer component of the central pluton provides an upper limit on regional deformation and metamorphism within host Moine rocks. These findings are consistent with the view that a fundamental change in tectonic regime occurred in the Scottish Caledonides at c. 425 Ma, corresponding to the switch from regional thrusting that resulted from the collision of Baltica and Laurentia, to the development of the orogen-parallel Great Glen Fault system.

Depth-Recursive Tomography of the Bohemian Massif at the CEL09 Transect—Part B: Interpretation

In the accompanying paper (Part A), depth-recursive tomography was applied to the CEL09 refraction data. A deblurred P-wave velocity image was obtained down to a depth of 20 km. This paper (Part B) is devoted to the interpretation of the upper- and middle-crustal structures of the Bohemian Massif imaged in the CEL09 section. Because of inherent ambiguity of the refraction method in imaging low-velocity zones, other well-known results based on other geophysical data sets are also used to independently verify the interpreted velocity features. Comparison with the density and velocity models previously obtained indicates that the presented P-wave velocity image has superior resolution revealing or verifying a number of geological features. The prominent lateral velocity changes encountered in the CEL09 pattern across the imaged crustal section were used to delineate the main terranes and deep regional fault zones such as the Krusne hory Fault, the SW continuation of the Litoměřice Fault Zone, the West and Central Bohemian Shear Zones, the Blanice-Rodl Fault, the Přibyslav-Vitis Fault and the Boskovice-Diendorf Fault. The 450-km-long CEL09 transect reveals seven major deeply rooted high-velocity (HV) anomalies identified as Variscan massifs intruded near or within these deep fault zones. They form buried ridges mostly parallel to the SW-NE trending Variscan strike. Their discovery allows new insights into a number of phenomena such as the West Bohemian earthquake swarms, the Saxothuringian paradox, the character of the Saxothuringian-Barrandian contact zone, the detachment surface due to the slab of the Saxothuringian crust subducting beneath the Tepla-Barrandian zone in the Devonian, the depth extent of the Marianske-Lazně Complex (MLC) as an equivalent unit of the Zone Erbendorf-Vohenstrauss (ZEV), the subsidence of the Barrandian syncline, the root zones of the Central and South Bohemian Plutons, the accretionary wedge formed along the Moravo-Moldanubian suture and its link with the Gfohl terrane, the Carpathian foreland relief and the subsidence observed in the Vienna Basin.

The North Pennine batholith (Weardale Granite) of northern England: new data on its age and form

SUMMARY A three-dimensional gravity model of the North Pennine batholith is interpreted to show that it comprises five contiguous steep-sided plutons. The North Pennine batholith was previously referred to as the Weardale Granite, but this term is now restricted to the largest, most central pluton, which was proved by the Rookhope Borehole in 1961. The surrounding plutons are the Tynehead and Scordale plutons in the west, and the Rowlands Gill and Cornsay plutons in the east. A new U–Pb zircon age of 399.3±0.7 Ma demonstrates an unequivocal Early Devonian (Emsian) age of emplacement for the Weardale Pluton. The similarity of the forms of the four unsampled plutons to the body beneath Rookhope, and to other Devonian granites in northern England, strongly suggests that they are also of Devonian age. Seismic reflection evidence supports a contrast between relatively deep-seated Devonian granites and tabular Ordovician granites in northern England. The most conspicuous magnetic anomalies over the North Pennine batholith are associated with the Early Permian Great Whin Sill, but there is also evidence of magnetized basement rocks or denser magnetic intrusive phases on the flanks of the non-magnetic, low density plutons. A long-wavelength magnetic low can be explained in part by the granite puncturing a deep magnetic basement, although the demagnetized zone may extend beneath the batholith. A spatial correlation between the vein systems in the Northern Pennine Orefield and the form of the batholith suggests that the granite masses played a significant role in focussing mineralizing fluids into the overlying Carboniferous rocks. The highest temperature mineralization occurs over the Weardale and Tynehead plutons, where there is also the clearest evidence for the channelling of mineralizing fluids through the batholith. The other plutons are generally associated with a lower intensity of mineralization, which occurs on their flanks rather than above their roofs. The Rookhope and Tynehead areas may therefore mark the location of convective ‘chimneys’ which were important in focussing the fluid circulation responsible for the mineralization. The coincidence of the ‘chimney’ zones with the minimum of the long wavelength magnetic low suggests that the hydrothermal system may also have played a role in modifying crustal magnetization.

Petrogenesis of A-type granites and origin of vertical zoning in the Katharina pluton, Gebel Mussa (Mt. Moses) area, Sinai, Egypt

The central pluton within the Neoproterozoic Katharina Ring Complex (area of Gebel Mussa, traditionally believed to be the biblical Mt. Sinai) shows a vertical compositional zoning: syenogranite makes up the bulk of the pluton and grades upwards to alkali-feldspar granites. The latters form two horizontal subzones, an albite–alkali feldspar (Ab–Afs) granite and an uppermost perthite granite. These two varieties are chemically indistinguishable. Syenogranite, as compared with alkali-feldspar granites, is richer in Ca, Sr, K, Ba and contains less SiO 2, Rb, Y, Nb and U; Eu/Eu* values are 0.22–0.33 for syenogranite and 0.08–0.02 for alkali-feldspar granites. The δ 18O (Qtz) is rather homogeneous throughout the pluton, 8.03–8.55‰. The δ 18O (Afs) values in the syenogranite are appreciably lower relative to those in the alkali–feldspar granites: 7.59–8.75‰ vs. 8.31–9.12‰. A Rb–Sr isochron ( n = 9) yields an age of 593 ± 16 Ma for the Katharina Ring Complex (granite pluton and ring dikes). The alkali–feldspar granites were generated mainly by fractional crystallization of syenogranite magma. The model for residual melt extraction and accumulation is based on the estimated extent of crystallization (∼ 50 wt.%), which approximates the rigid percolation threshold for silicic melts. The fluid-rich residual melt could be separated efficiently by its upward flow through the rigid clusters of crystal phase. Crystallization of the evolved melt started with formation of hypersolvus granite immediately under the roof. Fluid influx from the inner part of the pluton to its apical zone persisted and caused increase of P H2O in the magma below the perthite granite zone. Owing to the presence of F and Ca in the melt, P H2O of only slightly more than 1 kbar allows crystallization of subsolvus Ab–Afs granite. Abundance of turbid alkali feldspars and their 18O/ 16O enrichment suggest that crystallization of alkali-feldspar granites was followed by subsolvus fluid–rock interaction; the δ 18O (Fsp) values point to magmatic origin of fluids. The stable and radiogenic isotope data [ δ 18O (Zrn) = 5.82 ± 0.06‰, I Sr = 0.7022 ± 0.0064, ε Nd ( T) values are + 3.6 and + 3.9] indicate that the granite magma was generated from a ‘juvenile’ source, which is typical of the rocks making up most of the Arabian–Nubian shield.

Timing and Duration of Hydrothermal in the Oligocene Hamad Cauldron, SW Japan: Evidence from K‐Ar Ages of Sericites

Abstract: Crystallinity, chemical compositions and K‐Ar ages of sericites in highly‐sericitized granites and associated fissure‐filling veins were examined to delineate the timing and duration of the hydrothermal activity in the Oligocene Hamada cauldron in the San‐in district, SW Japan. Sericite separates (>2 μm) from the highly‐sericitized granites consist mainly of 2M1 polytype having high crystallinity and low Kübler indices of 0.22–0.35, while those in the fissure‐filling veins have lower 2M1/1Md ratios and crystallinity, and high Kübler indices of 0.29–0.35. This suggests that the sericites in highlysericitized granites were formed at a higher temperature than the vein sericites.Sericites from the highly‐sericitized granites of the Kumogi pluton give K‐Ar ages of 30.0±0.7, 30.4±0.7, 30.6±0.7, 30.6±0.7, 32.1±0.7, 32.3±0.7 and 33.0±0.7 Ma (1s̀), while those of the central plutons, 33.8±0.7 and 33.8±0.7 Ma. Sericites in the fissure‐filling veins of the Kumogi granite give K‐Ar ages of 31.0±0.7, 31.5±0.7, 31.6±0.7, 31.7±0.7 and 32.3±0.7 Ma. Biotite separates from the fresh Kumogi granite give K‐Ar ages of 31.7±0.8, 32.0±0.8, 32.7±0.7 and 33.5±0.7 Ma. The K‐Ar age data revealed that the hydrothermal alteration began at about 33 Ma and ended by about 30 Ma and that the period of sericite alteration was nearly synchronous with the cooling of the granite intrusions in the Hamada cauldron.Despite intense hydrothermal alteration, the Oligocene granitoids have not accompanied with any economic base metal mineralization. The bulk chemical analyses of sericite separates in the veins indicate that the post‐magmatic fluids were originally barren in heavy metals.

Geochemical and isotopic (Nd, O) data from Ordovician felsic plutonic and volcanic rocks of the Miramichi Highlands: petrogenetic and metallogenic implications for the Bathurst Mining Camp

Middle Ordovician felsic magmatism contemporaneous with Bathurst Camp Pb-Zn volcanogenic massive sulphide(VMS) deposits consists of strongly altered volcanic to subvolcanic rocks, belonging to the Tetagouche Group, and relativelyunaltered granitoid plutons, which are divided into northern, central, and southern groups within the Miramichi Highlands.Calc-alkalic felsic volcanic rocks and northern plus central plutons have EpsilonNd(T) values ranging from -8.2 to -1.9 and -4.0 to +0.3, respectively. They exhibit within-plate-type volcanic and transitional I- to A-type granite geochemical characteristics.Granitoid rock Delta18O values range from +8.0 to +10.1‰. Published granitoid rock Pb isotopic compositions overlapunpublished galena data from Bathurst VMS deposits. Field, geochemical, and isotopic evidence indicate that these volcanicand granitoids rocks are consanguineous and mainly derived from Proterozoic orolder infracrustal sources. Alkalic felsic volcanic rocks, and associated alkaline basaltic rocks, are more juvenile (EpsilonNd(T) = +3.2 to +4.2) and were possibly derivedfrom slightly enriched mantle sources. Southern plutons exhibit continental arc-type features. The felsic magmatism and VMS deposits likely formed in an Okinawa-type back-arc basin developed from rifting the Early Ordovician Popelogan continentalarc, of which the southern plutons are remnants. Correlations between pluton groups and volcanic formations indicate that felsic magmatism was erupted through and onto the Miramichi Group. As most felsic volcanic formations lack plutonicequivalents, the Tetagouche Group probably does not represent disrupted slices of an originally conformable stratigraphic section. This supports a model in which thrust slices juxtapose remnants of volcanic centres erupted at different locationswithin a back-arc basin.

Isotopic and chemical constraints on the petrogenesis of Blackburn Hills volcanic field, western Alaska

The Blackburn Hills volcanic field is one of several Late Cretaceous and early Tertiary (75–50 Ma) volcanic fields in western Alaska that comprise a vast magmatic province extending from the Arctic Circle to Bristol Bay. It consists of andesite flows, rhyolite domes, a central granodiorite to quartz monzonite pluton, and small intrusive rhyolite porphyries, overlain by basalt and alkali-rhyolites. Most of the field consists of andesite flows which can be divided into two groups on the basis of elemental and isotopic composition: a group having lower (87Sr86Sr)i, higher (143Nd144Nd)i, and moderate LREE and HREE contents (group 1), and a group having higher (87Sr86Sr)i, lower (143Sr144Sr)i, and lower HREE contents. Basalts are restricted to the top of the stratigraphic section, comprise the most primitive part of group 1 [(87Sr86Sr)i = 0.7033; (143Nd144Nd)i = 0.5129], and have trace-element ratios that are similar to those of oceanic island basalts (OIBs). In contrast to the basalts, group 1 andesites have higher (87Sr86Sr)i and lower (143Nd144Nd)i, and represent interaction of mantle-derived magmas with the lower crust of Koyukuk terrane. Group 2 andesites have (87Sr86Sr)i and (143Nd144Nd)i that are near bulk-earth values and probably formed by partial melting of the lower crust of Koyukuk terrane. The central pluton and rhyolite porphyries are isotopically uniform (87Sr86Sr)i ≈ 0.704, (143Nd144Nd)i ≈ 0.51275, and are interpreted to have formed by melting of young mafic to intermediate crustal rocks or by fractionation of group 1 andesites. The rhyolite domes have an isotopic range similar to that of the basalts and andesites [(87Sr86Sr)i = 0.70355-0.70499; (143Nd144Nd)i = 0.51263–0.51292], which suggests they formed by fractionation of the and site and basalt magmas. Although some workers have suggested that the volcanic field is underlain by old continental crust, none of the data require the presence of Paleozoic or Precambrian continental middle or upper crust under this part of the volcanic field. However, the ultimate source of some of the rocks in the Yukon-Koyukuk province that have high 87Sr86Sr and low 143Nd144Nd ratios may be old sub-continental mantle and/or lower crust, which was previously subducted beneath the Yukon-Koyukuk province during Early Cretaceous arc-continent collision.

The Butler Hill Caldera: a mid-Proterozoic ignimbrite-granite complex

Abstract Uplift along rift-related faults in the eastern St. Francois igneous terrane of southeastern Missouri exposes a high-level anorogenic igneous complex consisting of 1.5 km of intracaldera ignimbrites, a zoned central pluton, and late-stage ring plutons. A collapse structure, named the Butler Hill Caldera (BHC), formed in response to eruption of comenditic ignimbrites of the Lake Killarney Formation which attains an intracaldera thickness of 400 m. Continued peralkaline volcanism resulted in an additional 1000 m of Grassy Mountain Ignimbrite within the caldera and a thin, but widespread, outflow sheet. Following degassing of the magma chamber, the central pluton forcefully intruded the intracaldera volcanic sequence producing arcuate folds concentric to the caldera axis. Remnants of the original magma chamber roof are not preserved. The final igneous event related to the BHC involved intrusion of amphibole-granitoids in an annular pattern within, and concentric to, the caldera walls. The latter bodies probably merge, with depth, into a single ring pluton. The petrographic zoning exhibited by the mildly peraluminous central pluton of the BHC consists of: (1) a roof-zone of fine-grained, brick-red, hypersolvus granophyre with abundant miarolitic cavities which is nearly devoid of hydrous minerals; (2) a transitional zone of pink fine- to medium-grained seriate granite in which granophyric intergrowths are common and plagioclase and biotite are present in trace amounts only; and (3) a border zone of medium- to coarse-grained pink to grey subsolvus biotite granite with rapakivi texture. The paradoxical association of peralkaline and peraluminous suites with hypersolvus and subsolvus petrographic traits at the same structural level is recognized in other high-level anorogenic complexes. The common factor in these terranes, regardless of their age, appears to be that initial magmas were hot, relatively dry, low in CaO and high in F. These compositional features favor ascent of hypersolvus magmas to subvolcanic levels where saturation is accomplished by influx of meteoric water. Field, chemical, isotopic, and petrographic evidence indicate that BHC rocks have experienced at least two periods of subsolidus metasomatism. The resulting metasomatic features are typical of anorogenic ignimbrites and high-level granitoids elsewhere and must be quantified before existing chemical data can be applied to questions of igneous petrogenesis.

Tertiary Ishizuchi Cauldron, southwestern Japan Arc: Formation by ring fracture subsidence

The Tertiary Ishizuchi Cauldron, in the Setouchi volcanic belt of Miocene age in northwestern Shikoku, is 7–8 km in diameter and includes outer and inner ring fractures, inward dipping welded tuffs, central plutons, and ring fault complexes. Andesitic to dacitic welded tuffs within the central subsided block demonstrate repeated explosive activity during evolution of the cauldron. Inward dips of ring fractures surrounding the cauldron indicate that the bounding faults have the form of an upward opening cone. The cone shape is substantiated by peripheral upturning of intracaldera welded tuffs downdropped against the encircling ring faults. The ring fault complexes consist of rhyolitic to andesitic composite dikes, as well as intrusive tuffs and breccias from which some of the adjacent pyroclastic volcanic deposits were probably erupted. Central plutons of porphyritic granodiorite and fine‐grained adamellite were intruded about 14 Ma ago, have walls dipping outward and flat roofs, and may have formed concave‐upward tabular bodies which pass downward into ring dikes and upward into resurgent domes. Many Valles‐type calderas including Ishizuchi Cauldron are located in the area where the direction of the maximum horizontal compression of regional origin is parallel to arc structure in the middle Miocene southwestern Japan arc.

Crystallization history of a Permian composite monzonite-alkali syenite pluton in the Sande cauldron, Oslo rift, southern Norway

Monzonitic rocks ( larvikite) occur as megarafts embedded in younger alkali syenite in the Sande central pluton. Mineral equilibrium data are presented, in order to establish a crystallization history for the members of the complex. The larvikites (in part orthopyroxene-bearing) have followed cooling paths close to the QFM buffer. During intrusion of the alkali syenite ( nordmarkite) the older intrusives were reheated to the solidus, with sencondary, buffered cooling taking place at higher oxygen fugacity levels (roughly equivalent to the NNO buffer), which are within the ƒ O 2 range estimated for the nordmarkite magma. Gradual transitions between larvikite and nordmarkite are results of the secondary reheating.

Granitic plutonism as an indicator of microplates in the Palaeozoic of central and eastern Maine

Oxygen and strontium isotope ratios have been used to characterize source regions for granitic magmas for a transect across the northern Appalachian orogen in central and eastern Maine. The northwestern plutons (Katahdin and Seboeis) have δ 18O values of 10.3–13.3 and initial 87Sr/ 86Sr ratios of 0.7083 and 0.7066, respectively. The central plutons (Bottle Lake and Center Pond) have lower δ 18O values (8.2–9.9) and initial 87Sr/ 86Sr ratios (0.7043–0.7055). The southeastern plutons (Lucerne and Deblois) have δ 18O values (9.0–11.0) but initial 87Sr/ 86Sr ratios (0.7077 and 0.7041, respectively) which are intermediate between the northwestern and central plutons. Source models derived from these results and other petrological and geochemical data reflect the juxtaposition of discrete source regions by transcurrent faulting, which may be related to oblique plate motions. This model illustrates the importance of microplate accretion in the Palaeozoic history of the northern Appalachian orogen.

Uranium–lead dates from the Central Gneiss Complex and Ecstall pluton, Prince Rupert map area, British Columbia

Uranium–lead dates for nine zircon fractions from six rock samples of Central Gneiss complex (CGC) are concordant between 65 and 85 Ma. Two fractions from a sample near the eastern edge of the CGC give discordant U–Pb dates between 108 and 171 Ma, but analytical uncertainties and geological complexities prohibit meaningful interpretation. Two fractions from two samples from the Ecstall pluton give concordant U–Pb dates of about 98 ± 4 Ma and date the emplacement of the Ecstall pluton. The concordant CGC dates indicate Late Cretaceous to early Tertiary peak metamorphism (granulite and amphibolite facies) of the CGC east of Work Channel lineament. This contrasts with a slightly older (mid-Cretaceous) peak metamorphism west of Work Channel lineament inferred from the synmetamorphic nature of the Ecstall pluton.

An 18O/16O and D/H study of Tertiary hydrothermal systems in the southern half of the Idaho batholith

During Eocene time, 37 to 49 m.y. ago, a series of large hydrothermal systems was developed around a group of epizonal granite plutons in the Idaho batholith. These systems involved deep and extensive circulation of fluids derived from low-δ 18 O (∼−16) and low-δD (∼−120) meteoric waters. Water-rock interactions occurred at temperatures of 150 to 400 °C, lowering the 18 O/ 16 O and D/H ratios in the surrounding Mesozoic rocks (tonalite, granodiorite, and granite), such that the feldspar δ 18 O and biotite δD values became as low as −8.2 and −176, respectively. These values contrast markedly with the primary isotopic compositions of +9.3 ± 1.5 and −70 ± 5, respectively. Widespread propylitization of the Mesozoic plutonic rocks accompanied these isotopic exchange effects. Systematic mapping shows that anomalous δD and δ 18 O values occur over more than 15,000 km 2 , indicating the extensive lateral dimensions of the ancient circulating systems. The former zones of intense hydrothermal activity are marked by low- 18 O zones, which were mapped in the vicinity of the margins of several Eocene plutons (for example, at Rocky Bar) and also within a giant (5- to 20-km wide, 60- to 40-km diam) ring zone that surrounds the Sawtooth Mountains. The latter anomaly is coincident with the high-permeability ring fracture zone of an Eocene caldera system. Most of the ore deposits in the southern half of the Idaho batholith are epithermal and mesothermal Au-Ag veins that are located near the periphery of the low- 18 O zones (that is, near the outermost δ 18 O = 8 isopleth). This association links these deposits with the Tertiary hydrothermal activity and has great potential as an exploration tool in the heavily forested region. Evidence is presented that the Eocene ground-water circulation pattern was affected over large lateral distances (25 to 50 km) and great depths (5 to 7 km). These conclusions, together with the indications that large amounts of water (> 7,000 km 3 ) were involved in some systems and that the circulation patterns probably are related to caldera ring structures, may be of particular importance in geothermal exploration and exploitation of analogous modern systems. For example, the “fossil” hydrothermal activity mapped in the Idaho batholith may be characteristic of deep-level fluid circulation in geothermal systems such as Yellowstone National Park, Wyoming. In such regions, the major zones of hydrothermal activity seem to be principally associated with either (1) the caldera ring zones or (2) the central plutons (resurgent domes).

Some mezo- and microforms of granite weathering and erosion in the Novobystřická vrchovina (highland)

The highland of Novobystřická vrchovina (Southern Bohemia, the Czech Socialist Republic) is a part of the Central granite pluton (Moldanubicum). The author describes forms of granite weathering in that area as tors, oastle coppies, gigantic blocks, mushroom rocks and balanced boulders, as well as rock honeycombes, weather pits, tafoni and psedolapies. The cryogenic forms occur also in this area. Some of mentioned phenomena deserve to be protected.

Granitic ring complexes and Precambrian hot-spot activity in the St. Francois terrane, Midcontinent region, United States

Surface and subsurface geologic data have led to the recognition of granitic ring complexes in the 1.5-b.y.-old St. Francois terrane of southeastern Missouri. The distinctive rock association of the ring complexes comprises alkaline silicic rocks with minor quartz monzonite, trachyte, and basalt. Three periods of granite emplacement are recognized: sub-volcanic granite massifs were emplaced below comagmatic rhyolites and ash-flow tuffs, quartz monzonite and associated hypabyssal rocks were intruded along ring fractures in cauldron-subsidence structures, and central plutons were emplaced in resurgent cauldrons. Similar magmatic complexes in the buried Precambrian basement of the Midcontinent indicate a period of widespread anorogenic magmatism that may have been induced by intraplate hot-spot activity. The post-Precambrian geologic record of the region suggests that recurrent, but localized and less intense, hot-spot activity was responsible for emplacements of alkaline ultramafic rocks, for tectonic adjustments, and for increased geothermal gradients that created favorable ore-concentrating environments in the sedimentary rocks.

Neutron activation analysis of gold in Archean igneous and metamorphic rocks of the Rice lake-Beresford lake area, southeastern Manitoba

Gold abundances determined by the method of neutron activation are reported for 26 replicate analyses of a variety of igneous and metamorphic rocks. Samples analyzed come from scattered locations in and adjacent to an Archean greenstone belt containing numerous low-grade gold-quartz veins. Most of the richer veins are enclosed by mafic intrusions. The average gold content of each of the rock types is as follows:Rock Type No. of analyses Au (ppb)Northern Granites; quartz diorite: 4 1.9 + or - 1.2Manigotagan Gneissic Belt; paragneiss: 2 0.6 + or - 0.4associated quartz monzonite: 2 0.3 + or - 0.1Rice Lake Group; metavolcanics: 3 0.3 + or - 0.2Mafic intrusives; gabbro, diabase: 2 18.2 + or - 1.5Central plutons; quartz diorite: 5 1.1 + or - 0.6related felsic porphyry dykes: 2 6.1, 1.1Altered diabase wall-rock and vein zone: 6 0.5-325Gold abundances for the quartz diorite, quartz monzonite and metavolcanic groups are fairly typical of younger rocks of similar composition. Explanations are put forward to account for the unexpectedly low gold content of the paragneiss samples and relative gold enrichment in one felsic dyke sample.There is evidence to indicate that the mafic intrusions may be the source rocks of the gold. Thus, in addition to the spatial relationship of the richer veins to the mafic intrusions the latter also contain an order of magnitude more background gold than other rocks in the area. Furthermore, gold is apparently depleted in altered diabase wall-rock adjacent to a vein in the San Antonio gold mine.Non-repeatable replicate analyses observed in some mafic samples may be due to inhomogeneous distribution of gold between powder aliquots resulting from an erratic distribution of gold-rich host minerals.