Hornblende Syenite Research Articles

Enrichment of Incompatible Elements in Alkaline Syenites in Large Igneous Provinces Due to Magma Replenishment and Reactive Porous Flow in a Mush Reservoir

AbstractUnderstanding the petrogenesis of alkaline syenites is important for constraining the mechanisms of rare earth element (REE) and rare-metal mineralization. Here we report a detailed petrological and geochemical study of early Permian syenitic rocks from the southwestern Tarim large igneous province (TLIP) in the Wajilitag area (China). We use these data to investigate the complex magmatic processes responsible for the enrichment of incompatible elements (e.g. REEs and rare metals) in these rocks. The Wajilitag syenitic rocks comprise early hornblende syenite (281 Ma) and later nepheline syenite (278 Ma), both of which are spatially and temporally associated with mafic intrusions (i.e. gabbro and diabase). These syenitic rocks show continuous variations in major elements with the mafic rocks. They also have similar Sr–Nd–Hf isotopic compositions with the nearby mafic rocks, denoting that the Wajilitag syenitic rocks should be derived from the melts represented by these mafic rocks. However, these syenitic rocks, particularly the nepheline syenites, are typically characterized by extreme enrichment of incompatible elements, which is hard to be accounted by simple fractional crystallization of mafic magmas. Some clinopyroxene phenocrysts in the Wajilitag nepheline syenites show oscillatory zoning with strikingly increasing and then gradually decreasing MgO contents, which recorded replenishment of mafic magma at the interval. Low-MgO clinopyroxene mantles and/or rims have much higher incompatible element contents (e.g. Nb, Ce, and Zr) and ratios (e.g. Ce/Y) than their cores, which could reflect melt injection controlled by reactive porous flow in a mush reservoir of a crustal magma chamber. Such processes may also cause the high Ce and/or Nb contents of the mantles and/or rims of zoned titanite and apatite phenocrysts. The injection of reactive porous flow melts is the key process that produces the extreme enrichment of incompatible elements in the alkaline syenites from the western TLIP, as well as other plume-related alkaline syenites that host world-class REE and rare-metal deposits.

Petrogenesis of Alkaline Complex of the Longbaoshan Rare Earth Element Deposit in the Luxi Block, North China Craton, China

The alkaline complex in the southwest region of Luxi Terrane of the North China Craton is spatially correlated with the newly discovered Longbaoshan REE deposit. Its petrogenesis, however, remains ambiguous. In this study, we present an integrated petrology, whole-rock geochemistry, sphene U-Pb and rare earth element data from the Longbaoshan alkaline complex to investigate the petrogenesis, magma source and tectonic evolution. The Longbaoshan alkaline complex consists of mafic to intermediate rocks of hornblende diorite and alkaline hornblende syenite porphyry, biotite monzonite porphyry and aegirine diorite porphyrite. The hornblende diorites show a composition of low SiO2, high MgO, Fe2O3 and moderate Na2O, CaO and are metaluminous and medium-to-high-K calc-alkaline. The hornblende syenite porphyries, biotite monzonites and argirine diorite porphyrites display a relatively higher content of SiO2, Na2O, K2O and Al2O3 and lower contents of MgO, Fe2O3 and CaO and are metaluminous, peralkaline, high-K calcic-alkaline and shoshonite. The sphene U-Pb data shows that the parent magma of the hornblende diorite was emplaced at ca. 120 Ma. All these samples show a common depletion in Th, Nb-Ta and Zr-Hf and enrichment in large ion lithophile elements (e.g., Pb, Ba, Sr) and Light Rare Earth Elements. The magma may have experienced fractionation of pyroxene, amphibole, sphene, apatite and zircon during its evolution. The variable La content, La/Sm, Rb/Sr and (Ta/Th) N ratios indicate that the parent magma may produce by partial melting of a mantle source that was interacted with sediment-derived melts in a subduction setting. Therefore, we propose that the parent magma of the Longbaoshan alkaline complex was derived from a lithospheric mantle which was metasomatized by sediment-derived melt in a prior subduction process. The enriched magma was emplaced through an extension process and experienced subsequent fractionation and assimilation with the continental crust during the rollback of the Paleo Pacific Ocean plate.

Chemical Composition of the Mesozoic Alkaline Rocks of the Medvedev Massif (South Yakutia, Aldan-Stanovoy Shield, Leglier Ore Cluster)

The study shows chemical composition of the Mesozoic igneous rocks of the Medvedev massif. The massif is a part of the Central Aldan ore region. It is spatially located in the central part of the Nimnyr Block and is a part of the Leglier ore cluster. The massif is a multiphase structure of the most productive stage of the territory development – the Mesozoic tectonic-magmatic activation of the Aldan-Stanovoy shield. According to the materials of previous researchers, the rocks of the massif are represented by the augite hornblende and hornblende syenite porphyry. As a result of field work, we found that the massif has a zonal structure and is represented by three phases of intrusion. The rocks of the selected phases differ in their structural and compositional characteristics. The main objective of the work was to establish compositional characteristics by studying the chemical compositions of potential gold mineralization rocks of the Medvedev massif. In the course of studies and interpretation of chemical analysis of the massif Mesozoic magmatic rocks, we have identified the following characteristics. According to various classification data the following groups of rocks in Medvedev massif are identified – quartz syenites (Phase I), quartz monzodiorites of shoshonite series (Phase II) and monzonites of high-potassium calc-alkaline series (Phase III). The total alkali content (Na2O+K2O)>5% in them corresponds to alkaline rocks. All rocks are characterized by high Al2O3%>14 and low TiO2<1%. The potassium type of alkalinity Na2O/K2O<1 is typical for quartz syenites of the massif, while for quartz monzodiorites this indicator is intermediate, and in monzonites of the massif this parameter Na2O/K2O>1 corresponds to rocks of the alkaline potassium-sodium series. The massif rocks characteristic feature is the decrease of rocks alumina (al’) values from quartz syenites to quartz monzodiorites followed by the increase to monzonites, while the value of the rocks AG coefficient is significantly reduced in the series. Based on these facts, it was suggested that there are two sources of the mantle and mantle-crustal nature. Also, based on the study of the material composition of the Medvedev massif magmatic rocks and the similarity of the geological development with the Mesozoic magmatism of the Samolazovsky and Ryabinovy deposits, it was concluded that gold manifestations can be associated with such multiphase massifs within the Leglier ore cluster.

广东从化石岭碱性杂岩体角闪石正长岩LA-ICP-MS锆石U-Pb定年

广东从化石岭碱性杂岩体角闪石正长岩LA-ICP-MS锆石U-Pb定年

The Hydro-Thermal Stability of Hornblende Syenite in NaOH Solution

The microcline powder, which collected from the hornblende syenite from Anhui, China, was treated by NaOH solution at 250 °C for 2-6h. The solid samples were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The XRD shows that the main solid sample is hydroxycancrinite. The images observed by SEM indicate that the crystal morphology of the hydroxycancrinite can be presented as hexagonal cylindrical when it reacted with 4.17M NaOH at 250 °C for 6h.

U–Pb dating of hydrothermal zircon from the Dongping gold deposit in North China: Constraints on the mineralization processes

The Dongping gold deposit is a large deposit with total gold reserves of >100tons. It is located at the northern margin of the North China Craton, northwestern Hebei province, China. The ore-bodies are hosted by the Devonian Shuiquangou syenite complex and consist mainly of auriferous quartz veins and disseminated ore in the altered and silicified syenite. U–Pb dating of zircon from hornblende syenite on the western margin of the complex yields a crystallization age of 400±3.5Ma (MSWD=0.018). Morphology, cathodoluminescence imaging and geochemical classifications of zircon from the first stage of disseminated ore and gray auriferous quartz veins, and from later stage, low grade quartz veins point to their newly crystallized hydrothermal origin. The hydrothermal zircon from the disseminated ore and auriferous gray quartz vein are dated at 389±1.0Ma and 385±5.7Ma, respectively, which are detectably younger than but close to the crystallization age of the syenite complex and might have been formed by post-magmatic hydrothermal processes. Both types of ore are dominant in the ore deposit, and we propose that the pervasive, post-magmatic hydrothermal alteration is the main ore forming stage. Hydrothermal zircon from a low grade auriferous quartz vein yields a U–Pb age of ~140Ma, interpreted as forming during a younger period of superimposed Yanshanian hydrothermal mineralization. Thus, the ore of the Dongping gold mine represents a post-magmatic Devonian hydrothermal ore deposit with Jurassic–Cretaceous hydrothermal overprinting.

Geochemistry and Sr–Nd–Pb–Hf isotopes of the Mesozoic Dadian alkaline intrusive complex in the Sulu orogenic belt, eastern China: Implications for crust–mantle interaction

The Dadian alkaline intrusive complex is located within the Sulu orogenic belt, and includes hornblende syenite, syenite, quartz syenite and syenite porphyry. All the rocks from the complex show high SiO 2, K 2O + Na 2O and LREE and low CaO, FeO T, MgO and HFSE (Nb, Ta, P, Ti) concentrations. The Sr, Nd, and Hf isotopic compositions (( 87Sr/ 86Sr) i = 0.708505–0.70876, ε Nd( t) = − 16.5 to − 17.9, ε Hf( t) = − 21.5 to − 16.4) of the rocks fall within the compositional field of mafic dykes occurring nearby, suggesting that the Dadian alkaline complex was mainly derived from an enriched lithospheric mantle. The initial Pb compositions (( 206Pb/ 204Pb) i = 16.706–16.779, ( 207Pb/ 204Pb) i = 15.427–15.450, ( 208Pb/ 204Pb) i = 37.266–37.352) of the Dadian rocks are lower than those of the Yangtze lithospheric mantle, albeit similar to that reported from beneath the North China Craton, implying that the mantle source of the Dadian alkaline complex most likely belong to the North China Craton. The relatively high SiO 2 and HREE contents and low Nb/Ta ratios as well as strongly negative Eu and Sr anomalies of some of the rocks from the Dadian complex suggest that crustal material was also involved in the formation of this complex and that the crustal component was probably derived through partial melting at low pressure. The high crystallization temperatures revealed by zircon saturation thermometry and low pressures suggested by Al-in-hornblende barometry indicate that the Dadian alkaline complex crystallized at high temperature and intruded into shallow crustal level. This intrusive complex does not show any relationship with post-orogenic process in the region, and its petrogenesis is inferred to be associated with the Paleo-Pacific subduction tectonics.

Multiple crust–mantle interactions for the destruction of the North China Craton: Geochemical and Sr–Nd–Pb–Hf isotopic evidence from the Longbaoshan alkaline complex

In situ zircon U–Pb ages and Hf isotopic data, major and trace elements, and Sr–Nd–Pb isotopic compositions are reported for the Longbaoshan alkaline intrusive complex in the western Shandong Province (Luxi Block), southeastern North China Craton. The Longbaoshan complex, which consists of quartz syenite, aegirine–augite syenite, hornblende syenite, monzonite, and syenodiorite, was emplaced at 129.4–131.7Ma. The complex is characterized by high concentrations of SiO2, K2O+Na2O, Al2O3, LILEs (e.g., Sr and Ba), and LREEs, and low concentrations of CaO, Fe2O3, MgO, and HFSEs (e.g., Nb, Ta, P and Ti). The Sr, Nd, Pb, and Hf isotopic values of this complex are similar to the nearby mafic rocks, which were derived from EM2-type lithospheric mantle, indicating that the Longbaoshan complex was mainly derived from partial melting of the EM2 source. Combined with geochemical and isotopic features, the high Nb/Ta ratios (average 19.2) of the Longbaoshan complex suggest that the EM2 source was induced by modification of crust-derived melts, which were in equilibrium with rutile-bearing eclogite. This was attributed to the subduction of the Yangtze continental crust. The presence of inherited zircons (2.51–2.64Ga) with positive εHf(t) values (0.2–6.2) suggest that the ancient crust of the North China Craton was also involved in the formation of the Longbaoshan complex. Magma mixing modelling shows that those 10–35% crustal materials were assimilated in the complex. This complex may have experienced hornblende, apatite/monazite, and titanite crystal fractionation before emplacement in the shallow crust level. Two stages of crust–mantle interaction can be identified through the formation of the Longbaoshan complex, including (1) the lithospheric mantle was metasomatized by the melts/fluids derived from the subducted crust and then transformed into an EM2 counterpart, and (2) magma derived from the EM2 source underplated the ancient crust and assimilated the crust-derived materials. We thus propose that multiple crust–mantle interactions are the essential mechanism for the destruction of the NCC. The first intensive crust–mantle interaction was aroused by crustal subduction and was responsible for the lithospheric mantle destruction, and the second major crust–mantle interaction was induced by mantle-derived magma underplating and was responsible for the crustal activation.

Intrusion mechanisms in a turbidite sequence: the Voetspoor and Doros plutons in NW Namibia

Two syntectonic plutons of Cambrian age intruded Neoproterozoic metaturbidites in Namibia at the junction of the NS trending Kaoko and EW trending Damara belts. Sinistral transpression in the Kaoko Belt produced km-scale upright D1 folds overprinted by minor D2 folds. D3 is associated with N–S shortening in the Damara Belt. The plutons show two main pulses of intrusion: hornblende syenite intruded late during D1 or during D2 and biotite granite during D3. Each tectonic event produced a strain shadow defined by the shape of folds and the foliation trend around the plutons. The internal igneous fabric and the arrangement of wall rock xenoliths that locally make up 50% of the intrusion mass suggest that the plutons have a disk or wedge shape. A marginal shear zone indicates that one of the syenite intrusions descended during emplacement with respect to the wall rock. Emplacement is therefore inferred to be at least partly accommodated by descent of the intrusion floor. The biotite granite intruded into mega-strain shadows and tension gashes alongside and in the syenite during D3. The plutons show evidence of sinistral solid state rotation with respect to the wall rocks in response to D1–D2 transpression.

Petrogenesis and significance of the Hongshan syenitic pluton, South Taihang: zircon SHRIMP U-Pb age, chemical compositions and Sr-Nd isotopes *

Abstract The Honghsan syenitic pluton (South Taihang) is mainly made up of hornblende syenite and minor granite. SHRIMP ziron dating yields a U-Pb age of 135±2.7 Ma for the emplacement of the pluton, while biotite Rb-Sr isotopic data give an isochron of 120.3±2.4 Ma. Hongshan syenites show low silica, high alkalis and LILE such as Sr, and exhibit negligible Eu anomalies or slightly positive anomalies in the REE patterns. The syenites show quite enriched isotopic compositions with Isr from 0.7052 to 0.7102 and eNd(t) from −7.5 to −11.1. Petrogenesis of the pluton can be that partial melting of an enriched lithospheric mantle gave birth to an alkali basaltic magma, which subsequently underplated in the lower crust and experienced a coupled fractionation of ferromagnesian phases like pyroxene and hornblende and minor (<10percnt;) contamination of lower continental crust, producing the Hongshan syenites that partially evolved into granite through combined fractionation of hornblende and feldspar. Our data are...

Jurassic to Miocene magmatism and metamorphism in the Mogok metamorphic belt and the India‐Eurasia collision in Myanmar

Situated south of the eastern Himalayan syntaxis at the western margin of the Shan‐Thai terrane the high‐grade Mogok metamorphic belt (MMB) in Myanmar occupies a key position in the tectonic evolution of Southeast Asia. The first sensitive high‐resolution ion microprobe U‐Pb in zircon geochronology for the MMB shows that strongly deformed granitic orthogneisses near Mandalay contain Jurassic (∼170 Ma) zircons that have partly recrystallized during ∼43 Ma high‐grade metamorphism. A hornblende syenite from Mandalay Hill also contains Jurassic zircons with evidence of Eocene metamorphic recrystallization rimmed by thin zones of 30.9 ± 0.7 Ma magmatic zircon. The relative abundance of Jurassic zircons in these rocks is consistent with suggestions that southern Eurasia had an Andean‐type margin at that time. Mid‐Cretaceous to earliest Eocene (120 to 50 Ma) I‐type granitoids in the MMB, Myeik Archipelago, and Western Myanmar confirm that prior to the collision of India, an up to 200 km wide magmatic belt extended along the Eurasian margin from Pakistan to Sumatra. Metamorphic overgrowths to zircons in the orthogneiss near Mandalay date a period of Eocene (∼43 Ma) high‐grade metamorphism possibly during crustal thickening related to the initial collision between India and Eurasia (at 65 to 55 Ma). This was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 Ma. Similar syntectonic syenites and leucogranites intruded the Ailao Shan‐Red River shear belt in southern China and Vietnam during the Eocene‐Oligocene to Miocene, and the Wang Chao and Three Pagodas faults in northern Thailand (that most likely link with the MMB) were also active at this time. The complex history of Eocene to early Miocene metamorphism, deformation, and magmatism in the MMB provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of India and resulting extrusion or rotation of Indochina.

Sm‐Nd Geochemistry and U‐Pb Geochronology of the Mont Rigaud Stock, Quebec, Canada: A Late Magmatic Event Associated with the Formation of the Iapetus Rift

The Mont Rigaud Intrusion is a syeno‐granitic stock located in the St. Lawrence River–Ottawa River Graben and surrounded by sedimentary rocks of the St. Lawrence Lowlands. The intrusion is circular and divided into three units: an outer ring of hornblende syenite, an inner ring of feldspar‐hornblende syenite, and an inner core of quartz syenite and granite. Geochemical, U‐Pb, and Nd isotopic data gathered from the stock indicate that all three units are comagmatic and were intruded within a short period of time. Rock samples from the intrusion have a geochemical profile typical of within‐plate A‐type granites. The pluton is enriched in high field strength elements and rare earth elements and is characterized by negative Eu anomalies in the more fractionated units. The εNd values from the two syenitic units range from +1.5 to +2.2, while the granite yields values of about +0.5. The Nd isotopic data lie below depleted mantle values but above those of the underlying Grenville crust (−2.5). Thus, the initial syenitic magma probably did not form by crustal melting but may be a mantle‐derived melt that has undergone an assimilation–fractional crystallization process. U‐Pb zircon studies of the granite unit yielded an age of 564 +10/−8 Ma, consistent with the ages of other rift‐related intrusions. Magmatism associated with the St. Lawrence River–Ottawa River Rift falls into two major periods. The first, from 615 to 588 Ma, is characterized by intrusion of mafic dike swarms of tholeiitic affinity. The second, from 577 to 554 Ma, is mostly characterized by alkaline, silica‐undersaturated intrusions with minor oversaturated syenitic intrusions, such as the Mont Rigaud Stock. Rift‐related magmatic activity ended with the extrusion of the Tibbit Hill volcanics in the Mont Sutton area of southwestern Quebec.

The Rb-Sr age and geological implication of alkaline syenitic complex from Liangtun-Kuangdonggou at Gaixian, eastern Liaoning Province

Liangtun-Kuangdonggou complex, 30 km southeast of Gaixian Town, Liaoning Province, is composed of aegirine-augite hornblende syenite, aegirine-augite syenite, hornblende syenite, pyroxene syenite and syeno-aegirinolite. Rb-Sr isotopic data to the complex are reported and used for discussion of chronology and geological implication. They show that the complex occurred in early Proterozoic and is a tectonic sign of extension setting. Moreover, the date indicates that the Liaohe Group, which is metamorphic wall rock of the complex, formed over 1 900 Ma ago.

A Petrochemical Study of the Proterozoic Alkaline Complex of Uppalapadu, Prakasam Province, Andhra Pradesh, India

Abstract The Proterozoic Uppalapadu alkaIine complex (1348±41 Ma) in the Prakasam province of Andhra Pradesh is predominantly composed of nepheline syenite, hornblende syenite and quartz syenite, all of which represent an alkaline series. The pluton is closely associated with tholeiitic gabbro and ferrosyenite representing a sub-alkaline series. Geochemical characters suggest that the rocks of the alkaline series were formed by the differentiation of the hydrous alkaline basaltic magma under high PH2O, whereas those of the sub-alkaline series were formed by differentiation of an anhydrous olivine tholeiitic magma under low fO2 (reducing) conditions. Quartz syenite (an offshoot in the alkaline series) is formed either by fractionation of K-feldspar, ferrohastingsite and/or biotite under high PH2O conditions or by crustal contamination of critically saturated syenitic liquids.

Mineralogy and Textural Features of Settupalle Syenites, Prakasm District, Andhra Pradesh

Abstract The Settupalle igneous complex is characterised by syenitic variants with distinct mineralogic assemblages. The variable perthitic patterns, nature of plagioclase, fayalite-quartz assemblage in Settupalle syenites clearly suggest that the Settupalle igneous complex has evolved under fluctuating pressure and temperature conditions. The mineral chemistry of K-feldspar and plagioclase suggests that K-feldspar assemblage in these rocks indicates their subsolvus nature. The probable crystallization sequence of rocks is fayalite clinopyroxene syenite (FC-Syenite) - fayalite quartz syenite (FQ-Syenite)-hornblende syenite - quartz syenite - nepheline syenite in that order.

The Prakasam Alkaline Province in Andbra Pradesh, India

Abstract The Prakasam alkaline province is essentially composed of three major nepheline syenite plutons at Elchuru, Purimetla and Uppalapadu, and one silica-oversaturated (syenite and quartz syenite) pluton at Settupalle. These four post-orogenic plutons, all occurring in the Prakasam District of Andhra Pradesh, display dissimilar internal structures and variable rock associations; they are almost exclusively salic and generally show evidence of passive emplacement. The gross petrographic characteristics of the individual plutons are presented, and a generalised discussion on the petrogenetic aspects is included. The undersaturated and oversaturated syenites of the province are supposed to have formed from a critically undersaturated hornblende syenite magma by branching differentiation mechanism. The gabbros represent tholeiitic magmatism preceding the emplacement of alkaline rocks, and the ferrosyenites represent the final product of a closed system fractionation (with Jow fO2) of magma that gave rise to gabbro-anorthosite complexes. The geological setting of the plutons is portrayed and the tectonic significance of the province, especially in its regional context, is highlighted. The feldspathoidal plutons of the province are conspicuously confined to an extremely narrow linear belt which is close to the known basement fracture zone, with which the major Bouguer gravity axis runs in close paralJelism ; this belt lies at the junction between two (granitegreenstone and gneiss-granulite) terrains. The localisation of feldspathoidaI plutons at the western margin of the middle Proterozoic mobile belt and their absence in the cratonic region is significant. The over-all development of the mobile belt is speculated in the light of currently fashionable tectonic models. Crustal shortening and thickening of the mobile belt due to collision tectonics, predate the emplacement of the alkaline plutons; the lithosphere controls the character of the magmatism, and pre-existing abyssal faults determine the location of alkaline magmatism.

Paleomagnetism of Archean rocks from northwestern Ontario: V. Poohbah Lake alkaline complex, Quetico Subprovince

The Poohbah Lake alkaline complex is a late synkinematic Kenoran pluton in the Quetico gneiss belt of the western Superior Province. Three units of the complex, porphyritic syenite (PS), malignite (M: a nepheline–clinopyroxene–K-feldspar rock), and hornblende syenite (HS), as well as baked Archean schists near the intrusive contact, have a predominantly reverse-polarity R magnetization with mean direction D = 198° I = −22.5° (k = 62, α95 = 5°, N = 13 sites) and a paleopole at 60°E, 50.5°N. Pyrrhotite and coarse primary magnetite are carriers of the R remanence. PS, M, and biotite pyroxenite (BP) exhibit also a predominantly normal N magnetization carried by fine-grained, probably secondary magnetite. N is systematically steeper than R: its mean direction is D = 359.5° I = +55.5° (k = 28, α95 = 8°, N = 13 sites) with a paleopole at 90°E, 77.5°N. R and N do not record an asymmetric field reversal, since reverse-polarity N vectors and normal-polarity R vectors are occasionally found. R resembles in polarity and direction the natural remanent magnetization (NRM) of the 2630 Ma Matachewan diabase. It is probably the primary NRM of the Poohbah Lake pluton, with an age of about 2650 Ma in approximate agreement with the K/Ar isochron age of 2700 ± 25 Ma. N resembles in polarity and direction NRM's from the 2580 Ma Shelley Lake granite and the late Archean Burchell Lake granite. It is probably a secondary NRM about 2550 Ma in age, as suggested by updated K/Ar mica ages. The characteristic NRM of HS samples and secondary magnetizations in other rock types have poles on the Grenville Track of the polar wander path but there is no evidence for Grenvillian-age events in the area.

Differentiation Trends of Spessartite Dikes, Sandia Mountains, New Mexico

Hornblende spessartite dikes of the Sandia Mountains, New Mexico, contain small leucocratic dikelets that appear to be differentiated from the spessartite by fractional crystallization. Mineralogically the leucocratic dikelets are hornblende syenite, but chemically they more closely resemble Nockolds's (1954) average monzonite. During differentiation $$SiO_{2}, Al_{2}O_{3}, Na_{2}O, and K_{2}O$$ increased; $$TiO_{2}$$, FeO, MnO, MgO, $$H_{2}O$$, and $$P_{2}O_{5}$$ decreased markedly; and $$Fe_{2}O_{3}$$ and CaO decreased slightly. Textural relationships suggest unobstructed crystallization of hornblende and possibly some plagioclase in a $$H_{2}O$$-rich fluid of low viscosity with late crystallization of primary epidote and chlorite.

Dikes of the McClure mountain — Iron mountain alkalic complex, Fremont county, Colorado, U.S.A.

The McClure Mountain — Iron Mountain alkalic complex, the largest of three Precambrian alkalic complexes in south-central Colorado, is an irregularly ovoid body about 8×6.5 kilometers in plan. It contains major petrologic units (in age sequence): 1) peridotite and ilmenite-magnetite lenses, 2) gabbro, 3) hornblende syenite, 4) ijolite and 5) nepheline syenite, whose internal arrangement is irregular. Dikes are numerous and varied both within the complex and in the surrounding Precambrian metasediments and meta-volcanics (Idaho Springs formation), granitoids and migmatites to a distance of 27 kilometers from the complex. Fenitization has only locally affected these wall rocks at their contacts with the complex. Dikes within the complex are chiefly light colored and of syenitic or nepheline syenitic composition. Texturally they range from aphanitic, through porphyritic with an aphanitic to fine-grained matrix, to uniformly medium-grained. Most of the intracomplex dikes (save the few carbonatites) are represented by compositionally equivalent major units within the complex. In contrast, the extracomplex dikes are chiefly lamprophyres and carbonatites, none of which has a compositionally similar unit within the complex. The carbonatite dikes are younger than the lamprophyres; where the two types co-occupy the same fracture, the lamprophyre has been carbonatized. The lamprophyres are represented by an extraordinary number of textural varieties, involving phenocrysts and phenocryst combinations of augite, barkevikite, olivine and plagioclase in varying matrix combinations of olivine, augite, barkevikite, biotite, magnetite, apatite, plagioclase and orthoclase. Many of the dikes can be classed as camptonites or olivine camptonites. The Goldie lamprophyres contain local marginal phases spectacularly marked by concentrically zoned ovoids. The Cabin dike is studded by megaphenocrysts of augite. The carbonatites are of three main types: Many carbonatites and Th-RE veins emit, when broken, a fetid gas consisting of a mixture of fluorinated hydrocarbons of the C5 and C6 types along with F2, HF, and F2O. The fluorine has been derived from structurally degraded radioactive fluorite; the hydrocarbon gases appear to represent primary inclusions. Within the dike halo are two small breccia pipes consisting of fenitized, locally derived Precambrian gneiss blocks with minor fragments of transported biotitized lamprophyre in a subordinate calcitic matrix also containing aegirine, crocidolite, potash feldspar and hematite. The abundance of the lamprophyres, the great width of the dike halo, the lack of ring structure within the complex, the restriction of carbonatite to dikes, and the subordinate fenitization effects all combine to suggest that the McClure Mountain — Iron Mountain complex is now exposed at a relatively deep niveau, possibly near the mesozone — catazone boundary.

Gravity Study of a Hypersthene Syenite in the Laramie Anorthosite Complex, Wyoming

On the northwest border of the Laramie anorthosite complex, 236 gravity stations were located approximately 1 per square km over a hypersthene syenite body and country rock. The hypersthene syenite has a maximum width of about 6 km, extends about 18 km, and is bordered principally by anorthosite, hornblende syenite, and granite gneiss. Three 15 to 18-mgal anomalies with maximum gradients of about 4 mgal/km are centered over the hypersthene syenite. Bouguer anomaly values range from a mean of about —150 mgal in the anorthosite to a high of —128 mgal over the hypersthene syenite to Bouguer mean values of —150 mgal over granite gneisses to the west. Strikingly, isoanomaly lines locally crosscut geologic contacts. Computed minimum density contrasts of about δσ = .23 indicate that a subsurface mafic mass, possibly norite, is the principal disturbing mass and not the hypersthene syenite. Models indicate that this mafic mass is within .5 km of the surface in places and extends to a depth of about 5 km. The geometry of the hypersthene syenite is difficult to ascertain.