Alkali-feldspar Syenite Research Articles

A petrogenetic study of anorogenic felsic magmatism in the Cretaceous Paresis ring complex, Namibia: evidence for mixing of crust and mantle-derived components

Paresis is one of a group of Cretaceous ring complexes extending from the coast some 350 km NE across the Damara Belt. It consists of over 90% rhyolites and comendites, with subordinate intrusions of quartz syenite and alkali-feldspar syenite. These felsic units are accompanied by very minor amounts of silica-undersaturated basalt, phonolite and lamprophyre. Located near the edge of the Congo Craton, Paresis is the farthest inland of any felsic complex in the Cretaceous ring complex group. The other ring complexes in the vicinity consist of carbonatites and undersaturated alkaline rocks (e.g., Okorusu, Kalkfeld, Etaneno, Ondurakorume). Geochemical and isotopic data provide evidence for both mantle and crustal components in the sources of the Paresis magmas. The alkaline basalts, phonolite and lamprophyre have overlapping Sr and Nd isotopic initial ratios which plot within the mantle array close to bulk-earth values ( εNd=−0.9 to −2.8 and 87Sr/ 86Sr(i)=0.7042–0.7054) and may indicate a mantle plume component. The rhyolite units comprise metaluminous feldspar rhyolites and peraluminous, more differentiated quartz-feldspar rhyolites. Both units show prominent negative Nb and Ta anomalies on mantle-normalized multielement diagrams and have extremely nonradiogenic Nd ratios ( εNd=−21) and Sr initial ratios of 0.7117–0.7138. These isotopic values suggest a crustal origin from pre-Damara (Early Precambrian) gneisses and granitoids, which are exposed in the Congo craton and related inliers in northern Namibia. The comendites are peralkaline, highly differentiated rocks. In contrast to the rhyolites, comendites lack mantle-normalized Nb and Ta anomalies, have higher HFSE and LREE contents, extreme negative Eu anomalies and εNd values of −11. Like the comendites, alkali-feldspar syenites are peralkaline, lack Nb and Ta anomalies, have negative Eu anomalies and high HFSE concentrations. Their εNd values are −6.5 to −8. The quartz syenites overlap with the peralkaline units in isotopic composition but they are chemically very different. They are metaluminous, have a low degree of differentiation and have prominent negative mantle-normalized Nb and Ta anomalies like the rhyolites. The Sr–Nd isotopic composition and incompatible element ratios of the comendites, quartz syenites and alkali-feldspar syenites are intermediate between those of the rhyolites and basalts, precluding a simple origin by basalt fractionation or crustal melting. Instead, the data indicate that the comendite and syenites formed from hybrid magmas with both crustal and mantle-derived components. The metaluminous quartz syenites can be modelled by assimilation of pre-Damara crust by basaltic magma. The peralkaline nature and trace element characteristics of comendite and alkali-feldspar syenites, on the other hand, require an alkaline and HFSE-enriched basic endmember, whose fractionated equivalent may be the Paresis phonolite.

Structural setting, hydrothermal alteration, and gold mineralisation at the Archaean syenite-hosted Jupiter deposit, Yilgarn Craton, Western Australia

The Jupiter gold deposit in the northeastern Eastern Goldfields Province of the Yilgarn Craton of Western Australia is hosted in greenschist facies metamorphosed tholeiitic basalt, quartz–alkali-feldspar syenite, and quartz–feldspar porphyry. Syenite intrudes basalt as irregularly shaped dykes which radiate from a larger stock, whereas at least three E–W and NE–SW striking quartz–feldspar porphyries intrude both syenite and basalt. Brittle–ductile shear zones are shallow-dipping, NW to NE striking, or are steep-dipping to the south and west. Quartz ± carbonate veins that host gold at Jupiter occur in all lithologies and are divided into: (1) veins that are restricted to the shear zones, (2) discrete veins that are subparallel to shear zone-hosted veins, and (3) stockwork veins that form a network of randomly oriented microfractures in syenite wallrock proximal to shallow-dipping shear zones. The gold-bearing veins comprise mainly quartz, calcite, ankerite, and albite, with minor sericite, pyrite, chalcopyrite, galena, sphalerite, molybdenite, telluride minerals, and gold. Proximal hydrothermal alteration zones to the mineralised veins comprise quartz, calcite, ankerite, albite, and sericite. High gold grades (>2 g/t Au) occur mainly in syenite and in the hanging walls to shallow-dipping shear zones in syenite where there is a greater density of mineralised stockwork veins. The Jupiter deposit has structural and hydrothermal alteration styles that are similar to both granitoid-hosted, but post-magmatic Archaean lode-gold deposits in the Yilgarn Craton and intrusion-related, syn-magmatic, syenite-hosted gold deposits in the Superior Province of Canada. Based on field observations and petrologic data, the Jupiter deposit is considered to be a post-magmatic Archaean lode-gold deposit rather than a syn-intrusion deposit.

Cretaceous intrusions in the Commerce Mountain and adjacent areas of southeastern British Columbia and southwestern Alberta

An older oversaturated monzonitic and a younger undersaturated syenitic suite of Cretaceous igneous rocks are present at Commerce Mountain and adjacent areas of the southern Canadian Rockies. The undersaturated suite consists of foid-bearing alkali feldspar syenite and trachyte, with lesser amounts of phonolite and foid syenite. Normative data indicate a higher degree of undersaturation attributable to the presence of garnet. Sr and Nd isotopic data are consistent with generation of the magma by partial melting of an enriched source in the upper mantle or the lower continental crust; Pb isotopic data are more consistent with a source in the lower continental crust. High Sr and Ba concentrations in analcime-bearing alkali feldspar megacrysts support early crystallization at high pressure. Replacement of the analcime by muscovite suggests emplacement and continued crystallization at shallow depth under low pressures, at which the analcime was unstable. Prior to emplacement in the upper crust, the Commerce Mountain suite evolved in composition from mafic analcime-bearing perthite syenite to felsic analcime-nepheline-bearing perthite syenite to nepheline (micro)monzosyenite and to analcime (micro)syenite compositionally similar to analcime phonolites found in adjacent areas. Extensive metamorphic-metasomatic alteration suggests that Commerce Mountain was a volcanic center, possibly associated with the eruption of the Crowsnest Formation volcanic rocks.

Cretaceous intrusions in the Commerce Mountain and adjacent areas of southeastern British Columbia and southwestern Alberta

An older oversaturated monzonitic and a younger undersaturated syenitic suite of Cretaceous igneous rocks are present at Commerce Mountain and adjacent areas of the southern Canadian Rockies. The undersaturated suite consists of foid-bearing alkali feldspar syenite and trachyte, with lesser amounts of phonolite and foid syenite. Normative data indicate a higher degree of undersaturation attributable to the presence of garnet. Sr and Nd isotopic data are consistent with generation of the magma by partial melting of an enriched source in the upper mantle or the lower continental crust; Pb isotopic data are more consistent with a source in the lower continental crust. High Sr and Ba concentrations in analcime-bearing alkali feldspar megacrysts support early crystallization at high pressure. Replacement of the analcime by muscovite suggests emplacement and continued crystallization at shallow depth under low pressures, at which the analcime was unstable. Prior to emplacement in the upper crust, the Commerce Mountain suite evolved in composition from mafic analcime-bearing perthite syenite to felsic analcime-nepheline-bearing perthite syenite to nepheline (micro)monzosyenite and to analcime (micro)syenite compositionally similar to analcime phonolites found in adjacent areas. Extensive metamorphic-metasomatic alteration suggests that Commerce Mountain was a volcanic center, possibly associated with the eruption of the Crowsnest Formation volcanic rocks.

Les minéraux accessoires des granitoïdes de la suite taourirt, Hoggar (Algérie): conséquences pétrogénétiques

The post-Pan-African ‘taourirt’ suite of Hoggar (Algeria) is made up by themagmatic suite: G I monzogranite, G IIa monzogranite+syenogranite, G IIb subsolvus alkali feldspar granite (alaskite), GIII hypersolvus alkali feldspar syenite + granite. Silicates (zircon, thorite, allanite, chevkinite, titanite), oxides (magnetite, ilmenite, fergusonite) and phosphates (apatite, monazite), often abundant, constitute various accessory mineral associations. Crystal morphology, place in the sequence of crystallisation and mineral chemistry change as a function of parageneses. The constant titanite + magnetite + ilmenite assemblage in G I and G II rocks illustrates more oxidising conditions than in G III group, where hedenbergite + magnetite + ilmenite coexist. Two groups of accessory minerals are distinguished: (1) LREE rich (3×10 4 to 3×10 5 × chondrites) and Y rich (50 to 10 4 × chondrites), such as allanite, chevkinite and monazite, (2) LREE poor (100 to 10 4 × chondrites) and Y rich (1000 to 10 5 × chondrites), such as zircon, thorite, titanite and fergusonite. Shapes of chondrite-normalised patterns and evolutionary trends of REE, as well as of other HFSE, such as Nb, Zr, U and Th, in bulk rocks are dependent on relative abundances of the different accessory minerals.

Grenville Magmatism in West Texas: Petrology and Geochemistry of the Red Bluff Granitic Suite

zone of regional extension, as suggested by recent geophysical and The Red Bluff granitic suite (RBGS) underlies part of the Franklin petrological studies. Mountains in west Texas. It was emplaced into a Middle Proterozoic shelf sequence at 1120±35 Ma. The suite is predominantly metaluminous; it consists of early granitic to quartz syenitic sills, a main mass of alkali feldspar granite, small bodies of alkali feldspar syenite, leucogranitic dikes, and late-stage peralkaline

O magmatismo alcalino potássico Piracaia, SP (SE Brasil): aspectos composicionais e evolutivos

The Late Precambrian Piracaia massif (State of Sao Paulo, SE Brazil) com- prises a rock-association-of (quartz) diorites, (quartz) monzodiorites, (quartz) monzonites, (quartz) monzosyenites, (quartz) alkali-feldspar syenites and alkali-feldspar granites which result from the evolution of an alkaline transitional with (normative but not modal nepheline) high-K magma chemical features of alkaline, shoshonitic, withinplate and A magmatism, produced by of partial melting of an enriched sub-continental lithospheric mantle. During its initial evolution, under high-pressure conditions, the basic magma crosses the planes of criticai silica-undersaturation and silica-saturation. Simultaneously the magma reaches compositions compatible with the system Kp-Ne-Q and then its further evolution follows two paths: one part of the magma trends to syenitic compositions. cooling on/near the Or-Ab thermal barrier whereas another evolves along the thermal valley of the Or-Ab-Q subsystem producing quartz syenites and granites. Regarding the REE, the Piracaia massif comprises three main rock types: primitive, without Eu-anoma- lies; evolved, with negative Eu-anomalies; and complex rocks, with positive Eu-anomalies, wich result from the accumulation of variable amounts of fractionated minerais, mainly plagioclase. These rocks are also enriched in Y, Nb, Ga, etc..Some of the magmatic pulses which built up the Piracaia massif are characterized by high Gd values.

UPb geochronological constraints on the geological evolution of the Pinware terrane and adjacent areas, Grenville Province, southeast Labrador, Canada

UPb geochronological data show that the Pinware terrane and adjacent areas in the Grenville Province in southeast Labrador experienced three orogenic events: Labradorian, Pinwarian and Grenvillian. Labradorian (1710-1600 Ma) rocks in the Pinware terrane, previously only known from a dated felsic volcanic enclave in a younger granite, are now recognized to be widespread and include both supracrustal units and granitoid intrusions. One quartzite was deposited between ∼1805 and 1500 Ma, the ages of the youngest detrital zircon and subsequent metamorphism, respectively, and includes 1878 and 2720 Ma detrital zircon suggesting derivation of source material from pre-Labradorian Laurentia. A volcaniclastic(?) unit is interpreted to have an age of 1637 ± 8 Ma. Two Labradorian quartz monzonite intrusions yielded ages of 1650 +18 −9 Ma and 1649 ± 7 Ma. Coupled with previous information, these data indicate coeval plutonism, volcanism and sedimentation taking place at the same time as formation of the Trans-Labrador batholith farther north. The Pinwarian Orogeny is now extended to include events between 1510 and 1450 Ma. Granitoid rocks of the Upper Paradise River pluton, a large AMCG suite in the Mealy Mountains terrane, has been dated at 1495 ± 7 and 1501 ± 9 Ma, making it the only suite of this type and age known in North America. Migmatitic quartz monzonite, dated at 1450 +15 −21 Ma, provides the first proof of high-grade metamorphism in the Pinware terrane, and, in conjunction with recent geochronological data from other parts of the eastern Grenville Province, justify upgrading the Pinwarian to orogenic status. Grenvillian metamorphism throughout the Pinware terrane occurred between 1050 and 985 Ma, as indicated mainly on the basis of zircon lower intercepts, but including some monazite data and constraints imposed by dated younger rocks. This range of ages contrasts with the time span for Grenvillian metamorphism in the Lake Melville terrane, previously dated to between 1080 and 1000 Ma, and with which a newly determined age of 1047 ± 2 Ma from a granodiorite dyke at the Mealy Mountains-Lake Melville terrane boundary conforms. In the Pinware terrane, near the end of Grenvillian orogenesis, widespread alkalic magmatic activity occurred between 990 and 980 Ma. Units investigated include an aegerine-bearing alkali-feldspar syenite emplaced at 991 ± 5 Ma, a clinopyroxene-fayalite alkali-feldspar syenite having a probable age of 985 Ma, and alkalic mafic dykes emplaced at 985 ± 6 Ma. Similar (although amygdaloidal) alkalic mafic dykes in the Lake Melville terrane may be slightly younger, having an age of 974 ± 6 Ma. The short-lived alkalic activity was followed by late-tectonic magmatism in the Pinware terrane at 983 ± 3 Ma, which heralded post-tectonic granitoid plutons between 974 and 956 Ma. Cooling and uplift in the Pinware terrane is documented by titanite ages between 972 ± 5 Ma and 939 ± 5 Ma. It is inferred that cooling occurred 30 million years sooner in the southeast Pinware terrane than near its northwest margin.

Mineral Chemistry and Tectonic Significance of Neoproterozoic Ultrapotassic Plutonic Rocks from the Cachoeirinha-Salgueiro Fold Belt, Northeastern Brazil

The Terra Nova ultrapotassic igneous rocks of northeastern Brazil consist of two dike swarms (alkali-feldspar syenites to quartz syenites and alkali-feldspar granites) and one elongated E-W syenitic body (the Serra do Livramento pluton), which intruded metasediments of the Cachoeirinha-Salgueiro fold belt from 580 to 514 Ma. Mafic ultrapotassic syenite enclaves are recorded in the Serra do Livramento and Terra Nova shoshonitic plutons, both of which are cut by the dike swarms. Mineralogically, Terra Nova ultrapotassic hypabyssal rocks resemble shoshonitic lamprophyres. Pyroxene is present in all facies; the clinopyroxenes are zoned, SiO2 saturated, and Al2O3 poor (0.12 to 1.15%), and range from earlier diopside to late acmite. Amphiboles are characterized by high SiO2 and low Al2O3 (0.20 to 2.00%) and TiO2 (0.0 to 1.76%) contents; their compositions range from calcic to alkaline. The late amphiboles are riebeckite-arfvedsonites instead of K-richterites, as expected in ultrapotassic rocks, reflecting the e...

Large-scale silicate liquid immiscibility: a possible example from northeastern Brazil

Ultrapotassic, peralkalic silica-saturated plutons (580 Ma) are widespread in the Cachoeirinha-Salgueiro foldbelt, northeastern Brazil. They consist of alkali-feldspar syenites with pyroxenite as co-magmatic inclusions and syn-plutonic or late-stage dikes. Pyroxenite and syenite have the same mineral phases (aegirine-augite, microcline, sphene, apatite, blue amphibole, magnetite), but only in different proportions. Rare inclusions of a “mixed” rock (about 60% syenite+40% pyroxenite in an emulsion-like texture) are also present. Pyroxenes in the three units are all only slightly zoned, silica-saturated and extremely low in Al 2O 3 (0.2–1.4%). Amphiboles are mostly K-rich richterite, characterized by high SiO 2, low Al 2O 3 and TiO 2 contents and low Mg#. The three rock types have similar REE chondrite-normalized patterns, with negative slopes and lack of Eu anomaly, with the total REE in the pyroxenite greater than that of the syenite. Trace element patterns for the mixed rock are intermediate between those for the pyroxenite and syenite. Major element partitioning between pyroxenite and syenite has the same sense as that one observed between immiscible liquids in volcanic lavas and trace element partitioning is similar to the experimentally determined partition of immiscible liquid pairs. The rocks have similar high δ 18O values (avg. w.r.+ 8‰ SMOW, corrected from pyroxene), high initial 87 Sr 86 Sr ratios (about 0.710), and low 143 Nd 144 Nd (avg. 0.51104). Field and geochemical characteristics indicate chemical equilibrium among the three rock types and suggest liquid immiscibility between syenite and pyroxenite, the mixed rock representing the original magma composition.

Noble metal abundances and characteristics of six granitic magma series, Archean Abitibi Belt, Pontiac Subprovince; relationships to metallogeny and overprinting of mesothermal gold deposits

Noble metal abundances and characteristics of six granitic magma series, Archean Abitibi Belt, Pontiac Subprovince; relationships to metallogeny and overprinting of mesothermal gold deposits

Geodynamic evolution of the southern Abitibi and Pontiac terranes: evidence from geochemistry of granitoid magma series (2700–2630 Ma)

Four distinct granitic series developed during the evolution of the Archean Abitibi Southern Volcanic Zone (SVZ): (1) A minor synvolcanic tonalite–trondhjemite–granodiorite (TTG) series was emplaced at ~2700 Ma, which has extremely low K2O (<1.5 wt.%), Rb (<50 ppm), and Sr (<200 ppm), higher Nb, Y, and Sc, flat rare-earth-element (REE) patterns, and negative Eu anomalies. (2) A voluminous tonalite – granodiorite – granite – quartz monzonite (TGGM) series developed syntectonically at ~2695–2685 Ma, and displays low K2O (1–3 wt.%), Rb (10–50 ppm), Ba (<1000 ppm), and U, enhanced Sr, depletion of Ta, Nb, and Ti, and strongly fractionated REE patterns (La/Ybn = 49–21). (3) A late-tectonic quartz syenite – quartz monzonite – granite (SMG) series was emplaced from ~2685 to 2675 Ma, and is grossly similar to the TGGM but has lower CaO/(K2O + Na2O) and greater concentrations of Rb, Ba, Th, and U. (4) A late-tectonic to posttectonic alkali feldspar syenite – alkali feldspar quartz syenite (SS) series was emplaced from 2680 to 2670 Ma, and occurs along regional strike-slip structures. The primitive rocks (SiO2 ≤ 65 wt.%) exhibit coenrichment of large-ion lithophile elements (LILE) and mafic elements (Cr, Co, and Ni) and strongly fractionated REE patterns, whereas evolved phases (SiO2 ≥ 65 wt.%) display lower contents of compatible and incompatible elements stemming from differentiation.In the adjacent Pontiac Subprovince and the Lacorne block within the SVZ, two granitic series predominate: (1) A syntectonic to late-tectonic monzodiorite–monzonite–granodiorite–syenite (MMGS) series (2690–2670 Ma) is comparable to the Abitibi SMG and SS series in most major-element, LILE, and REE contents but is distinguished by high MgO contents, extremely high Ba/Th ratios, and coenrichment of Cr, Co, and Ni with light rare-earth elements (LREE), Li, and Cs. (2) A garnet–muscovite–granite (GMG) series (2644 ± 13 Ma) displays K2O/Na2O ≥ 1, restricted SiO2 range (69–75 wt.%), pronounced enrichments of Rb, Li, Cs, Ta, Nb, Th, and U, and moderately fractionated REE's (La/Ybn = 16–0.9), with prominent negative Eu anomalies.The synvolcanic TTG series is interpreted to have formed by differentiation of low-K mafic magmas of the Blake River Group type in suprasubduction-zone environments. Geochemical compositions of the TGGM, SGM, SS, and MMGS series resemble those of Phanerozoic granitoids in island-arc settings and reflect a transition from partial melting of the subducted or subcreted slab to melting of the metasomatized depleted mantle wedge assisted by LILE- and LREE-enriched fluids released from the slab. The GMG, which formed by partial melting of the Pontiac metasediments when the Pontiac Sub-province collided with and underthrust the Abitibi SVZ at ~2670 Ma, is similar to Himalayan collisional leucogranites.

Magmatic Oxidation in the Syenitic Murdock Creek Intrusion, Kirkland Lake, Ontario: Evidence from the Ferromagnesian Silicates

The syenitic Murdock Creek intrusion, of late Archean age, lies within one of the major lode gold camps of the world, at Kirkland Lake, Ontario. There is a strong spatial association between syenitic intrusions, gold mineralization, and carbonatization. Hydrothermal fluids that carried the gold had an unusually high oxygen fugacity ($$f_{o_{2}}$$). Notwithstanding a wide range in rock compositions within the intrusion (from clino-pyroxenite to alkali-feldspar syenite), clinopyroxene and biotite have consistently low Fe/(Fe + Mg) and high $$Fe^{3+}/(Fe^{2+} + Fe^{3+})$$ ratios. These compositional features indicate a deficiency of $$Fe^{2+}$$ in the magma during crystallization. Together with the presence of abundant, early-formed magnetite and titanite, this is interpreted to result from an intrinsically high magmatic $$f_{o_{2}}$$ that remained constant during pluton evolution ($$f_{o_{2}} \approx 10^{-12} bars$$). Mineral compositions, proportions of crystallizing mineral phases, and textural features indicate that the pluton formed from a single pulse of already-evolved syenitic magma that fractionally crystallized in situ under low water pressure at mid-crustal depths. The intrinsically oxidized nature of the pluton implies an oxidized source region such as metasomatized upper mantle where partial melting occurred. $$CO_{2}$$-rich fluid or melt was the likely agent for metasomatism. Dissolution and transport of gold is favored by high $$f_{o_{2}}$$ in the fluid phase. The high $$f_{o_{2}}$$ of the syenitic magma suggests a common genetic link between magma generation, $$CO_{2}$$ represented by "carbonatization, and gold-bearing fluids.

Multiple syenite intrusions at Kærven, Kangerdlugssuaq, East Greenland: Evidence from the 1986 field work.

Tue Kærven Syenite Complex is recognized as the earliest intrusive body in the Kangerdlugssuaq region associated with the opening of the proto-Atlantic Ocean in the Lower Tertiary. Earlier investigations of the geology of the Kærven Syenite Complex have interpreted the intrusion as a single igneous body, emplaced subsequent to the Kærven Gabbro and in turn cut by the Kangerdlugssuaq intrusion. Preliminary sampling on Kærven in 1977 revealed a relatively large range of rock types in the complex (syenite - alkali feldspar syenite - alkali feldspar granite). Tue field work conducted in 1986 has verified these findings and enabled the distinction of 8 intrusive events within the complex. Further, a preliminary geological map has been produced. An igneous breccia separates the complex from the earlier Kærven Gabbro along parts of the eastern and southern margins. The central part of the Kærven Syenite Complex include an hitherto unrecognized slice of Precambrian basement gneisses with numerous approximately N-S trending basaltic dykes. This is multiply intruded by syenite. Most notably in the eastern part of the complex, intimate associations of brecciated and remobilized basement gneiss and melts of syenitic through monzonitic to tonalitic composi­tions are found. The form of the intrusive bodies are more or less dyke-shaped and N-S trending. Toere is a general stratigraphic younging to the SW, which is matched by the tater adjacent Kangerdlugssuaq intrusion. Tue intrusive sequence and trend of the intrusions are thought to be closely related to an extentional tectonic regime present in this part of the fjord for a considerable period onwards from the earliest Tertiary magmatic activity in the area.

Fluid inclusion studies of the Rodeo de Los Molles REE and Th deposit, Las Chacras Batholith, Central Argentina

The Rodeo de Los Molles rare earth element (REE) and thorium deposit is located in granitic rocks of the Las Chacras-Piedras Coloradas Batholith, in the southern block of the Eastern Pampean Ranges, Central Argentina. Mineralization occurs within an elongate (2 km × 0.6 km) body of alkalifeldspar granite (alaskite) localized along the northeastern edge of a composite batholith. The surrounding lithology is predominantly a biotite monzogranite. Both the alaskite and localized areas of quartz alkalifeldspar syenite within the alaskite have been produced by hydrothermal alteration of a late-crystallizing phase of the monzogranite. REE minerals are primarily of the cerium group and include britholite and allanite, both partially replaced by bastnaesite or thorbastnaesite. These minerals occur as nodules with quartz, fluorite, aegirine-augite, sphene, and Fe-Ti oxides within aplitic to pegmatoidal quartz alkalifeldspar syenite. Uranothorite, along with a second generation of fluorite and minor amounts of MnBa oxides, occurs in the alaskite as nodules, or within quartz-lined miarolitic cavities, but is not found with the Ce-mineralization. Studies of fluid inclusions contained in quartz and fluorite indicate a complex history of open-system fluid migration and interaction with monzogranite host rocks. Fluids responsible for REE mineralization and quartz deposition, along with initial alteration of the monzogranite to alaskite and quartz alkalifeldspar syenite, were of relatively high temperature (T h of fluid inclusions in quartz = 356–535°C) and moderate salinity (15–25 eq. wt% NaCl). Mixed CO 2H 2O fluids (XCO 2 = .13–.07) found as both primary and secondary inclusions within fluorite are representative of fluids involved in the replacement of britholite-allanite by bastnaesite and sphene, aegirine-augite, and plagioclase by calcite. Minimum pressures of mineral deposition estimated from H 2OCO 2NaCl phase relations range from 1 to 2 kbars. Secondary aqueous fluid inclusions in quartz define a trend of low salinity-high temperature to high salinity-low temperature, thought to be a result of hydration reactions occurring in alaskite and quartz alkali-feldspar syenite. The highest salinity fluids (35–37 eq. wt% NaCl) detected in the area are associated with the formation of uranothorite and late fluorite. Multiple periods of hydrothermal fluid introduction are consistent with recent geological data that indicate that the batholith is composed of several stock-like bodies. The location of the mineralized area near the top of the magma chamber, the presence of numerous miarolitic cavities, and the bulk composition of inclusion fluids (Na ≥ K > Ca) suggest that the fluids responsible for REE and Th mineralization were of magmatic origin.

Early Paleozoic alkalic plutonism in east-central Idaho

U-Th-Pb analyses of zircon suites document the presence of three lower Paleozoic alkalic plutons in east-central Idaho. The igneous bodies range in composition from alkali-feldspar syenite to alkali-feldspar granite. The epizonal Arnett Creek pluton contains a morphologically bimodal suite of anhedral and euhedral zircon, which combined gives an upper intercept age on a concordia diagram of 492 ± 39 m.y. Zircon analyses from the Deep Creek pluton plot near the 500-m.y. locus on concordia, but the pattern suggests the presence of a small component of inherited radiogenic lead. Limited Rb-Sr whole-rock data for the Arnett Creek and Deep Creek plutons are compatible with an Early Ordovician age but are too imprecise to refine interpretations. Zircon from the epizonal Beaverhead pluton, for which the enigmatic Ordovician-Silurian K-Ar and Rb-Sr mineral ages have long been debated, yields an approximate age of 483 m.y. The tectonic setting of these intrusions is poorly known. The coincidence of early Paleozoic plutonism and independently documented syndepositional faulting with the coeval topographic high, known variously as the Lemhi or Salmon River Arch, tentatively suggests a mutual tectonic control. Further geochronologic, stratigraphic, and especially petrogenetic studies, however, are required to evaluate this possibility.

THE PERALKALlC MAGMATISM IN THE PRECAMBRIAN CACHOEIRINHA-SALGUEIRO FOLDBELT, NORTHEAST BRAZIL: GEOCHEMICAL ASPECTS

Peralkalic rocks of probable Brasiliano age intruded metasediments of the Precambrian Cachoeirinha and Salgueiro Groups and their basement, between the geographic coordinates 37oW and 40oW longitude and 7oS and 8°1S'S latitude, Pernambuco and Paraiba states. They enclose two groups of aegirine and/or riebeckite-arfvedsonite-bearing rocks: (i) silica-saturated quartz-alkali-feldspar grayish-green trachytes and alkali-feldspar syenite, and (ii) silica-oversaturated quartz-alkali-feldspar syenite to alkali-feldspar granite (fluorite and cassiterite-bearing). The first group forms a dike swarm at Manaira, Paraiba, and Terra Nova, Pernambuco, batholiths and dikes adjacent to the southern border of the Cachoeirinha-Salgueiro foldbelt, constituting a syenitoid line, besides small stocks piercing Cachoeirinha metasediments and the second one, dike swarms stocks and ring-dikes. The first group is characterized by normative nepheline and K 2 0 (from 9.7 to 12.8%) greater than Na 2 O. Agpaitic index varies from 1.156 to 1.280 and in the second group ranges from 0.840 to 1.339. The Σ REE are usualy low and patterns for the two groups are very fractionated, LREE-enriched, HREE-deplete and Eu anomaly is rather missing. A very duscrete negative Eu anomaly is seen in some of the saturated rocks, while the oversatured rocks exhibit, sometimes, a discrete positive Eu anomaly. Whole δ 18 O values are usually below 10 permilSMOW, except for the oversaturated subvolcanic dikes which intruded Cachoeirinha metasediments. Intense, early pyroxene fractionation from an alkali basic parental magma has been perhaps responsible for the generation of the Triunfo-syenite batholith, Pernambuco, and other syenites in the peralkalic syenitcid line. The peralkalinity of the Macacos hill oversaturated ring-dike next to Serrita, Pernambuco, may have resulted from Bowen's plagioclase effect in the crystalizaticn of a magma with trondhjemitic affinity.

Archaean basin development and crustal extension in the northeastern Yilgarn Block, western Australia

Abstract Supracrustal sequences in the northeastern Yilgarn Block, Western Australia, form two contrasting lithotectonic associations. Most sequences were deposited in subaqueous to subaerial environments in a basin 120 km in width. The preserved succession reaches a maximum of e. 10 km in thickness in the central part of the basin, although there is no record of the basement on which it was deposited. In the later stages of its development, the basin was intruded by calc-alkaline granitoids, that are in part comagmatic with calc-alkaline volcanic rocks in the upper part of the stratigraphic succession. Brittle fracture (normal strike-faults and conjugate cross-faults), open, upright folds and peak low-strain metamorphism were coincident with granitoid emplacement. The eastern and western margins of the basin were greatly modified during a period of increased crustal extension, marked initially by the generation of a mildly peralkaline volcanic-plutonic suite, the volcanic members of which were deposited in linear, faultbounded depressions. A major growth fault indicated by thin persistent units of oligomictic and polymictic conglomerate and localized intense metasomatism was probably contemporaneous with felsic volcanic activity. Subsequent sediments, derived largely from granitic source areas, were deposited in narrow grabens. The youngest felsic plutonic rocks in the area include small bodies of alkali feldspar syenite many of which are found near the margins of the late, sediment-filled grabens. Tectonism at the eastern basin margin extended into the Proterozoic. In contrast to sequences at the centre of the basin, those at its margin have undergone isoclinal folding, ductile deformation and high-strain metamorphism. The geological framework of the northeastern Yilgarn Block emphasizes the importance of crustal extension in the development of younger Archaean supracrustal sequences and provides valuable support for extension-dependent greenstone belt evolutionary models (e.g., Groves and Batt).

Cambrian nepheline syenite complex at Jabal Sawda, Midyan region, Kingdom of Saudi Arabia

The only nepheline syenite complex presently known in the Arabian Shield is at Jabal Sawda, about 30 km S of Haql in the extreme NW of Saudi Arabia. It is a post-tectonic, composite intrusion with a crudely concentric structure. A core of leuco-nepheline syenite, a partial ring of mela-nepheline syenite, and an almost complete outer ring of alkali-feldspar syenite are the main rock units. Several mega-inclusions of porphyritic nepheline syenite, nepheline monzosyenite, malignite and ijolite are present in the leuco-nepheline syenite. The chemical composition is notable for very high values of Al 2O 3, Na 2O, Ba, La, Nb, Sr and Zr. U Pb isotope dating indicates an emplacement age of 553 ± 4 Ma, one of an increasing number of reliable Cambrian isotope dates in the northern Red Sea region.

Jabal Hamra REE-mineralized silexite, Hijaz region, Kingdom of Saudi Arabia

The Jabal Hamra silexite, a crescent-shaped stock 300 m long by 100 m wide, averages over 6000 ppm combined REE and is the Kingdom's highest-grade resource of these elements. It is anomalously radioactive (total-count gamma radiation 1000–3000 cps), has high average contents of U (75 ppm) and Th (263 ppm) and is also enriched in Nb, Zr, Y, Sn and Ta. The silexite crystallized as a pressure-quenched rock resembling aplite, and was subsequently pervasively cataclased. It was derived by differentiation of a quartz alkali-feldspar syenite magma. Petrologic continuity can be demonstrated from quartz alkali-feldspar syenite through leucocratic and amphibole-bearing alkali-feldspar granite to silexite. Although the geochemical signature of the mineralization resembles that of mineralized Arabian alkali granites, the nature of the associated rocks and therefore the genesis of the deposit are significantly different.