Basic Igneous Complex Research Articles

Late Paleozoic granitoid magmatism of Eastern Kazakhstan and Western Transbaikalia: plume model test

We present results of a comparative study of Late Paleozoic granitoids of Eastern Kazakhstan and Western Transbaikalia composing the large Kalba-Narym and Angara-Vitim batholiths. We have established that despite the different geologic history of these regions, granitoid magmatism there proceeded nearly synchronously at the Carboniferous/Permian boundary (330–280Ma) and was accompanied by mantle magmatism. The regularities of its evolution are considered in terms of the plume model and different stages of interaction of mantle plumes with the lithosphere. The major principles of plume-lithosphere interaction in accretion-collision fold belts have been formulated: (1) Plume-lithosphere interaction results in large-scale melting of sublithospheric mantle, lower lithosphere, and crustal substrates warmed by the preceding orogenic processes; (2) The processes last 30 to 50 Myr and produce large volumes of igneous rocks, mostly granitoids; (3) The sequence of formation of granitoid and basic igneous complexes and the metallogenic specialization can be different and depend on the lithosphere structure and preceding geologic history of the region.

Peperitas en Costa Rica

El presente trabajo describe petrográfica y genéticamente, una serie de hallazgos de peperitas como ejem-plos de coexistencia espacial y temporal del magmatismo con la sedimentación en Costa Rica. Los casos se presentanen complejos ígneos básicos (ofiolitas) variando desde mezclas incompletas (megabloques de radiolaritas con diabasasen el Complejo de Nicoya) hasta mezclas completas en varios grados de calizas pelágicas del Maastrictiano-Paleoceno (Formación Golfito) o del Paleoceno con basaltos (Subcomplejo Quepos o Formación Tulín) para dar peperitas bre-choides y bulbosas. Dentro del arco Oligo-Mioceno, se tienen ejemplos a lo largo de la cordillera de Talamanca y susestribaciones, así como en las llanuras de Sarapiquí, representados por peperitas brechoides, fluidales y bulbosas encuerpos de lava latiandesíticos (Formación Pacacua) o chimeneas peperíticas brechoides en ignimbritas (MiembroMata de Limón), o posibles peperitas brechoides en los depósitos de flujos de lava del prospecto minero de Crucitas(Formación Cureña), o simples posibles peperitas brechoides en flujos de lava andesíticas (Formación Pacacua en Tala-manca), o como brechas hidroclásticas con fragmentos de composición dacítica de Carbonal en Guanacaste, justamenteentre las playas Cabuyal y Naranjo. Todas ellas serían los equivalentes proximales del vulcanismo explosivo, efusivoy subvolcánico (domos endógenos y criptodomos) y su concomitancia con la sedimentación fluvial. En el Cuaternario,un ejemplo se da en la base de la Formación Tiribí al contacto con el lacustre de Palmares. Se incluyen en el presentetrabajo, además, casos posibles o potenciales. Se espera que la descripción e interpretación de este tipo de rocas ayudea su identificación en otras regiones de Costa Rica y Centroamérica en general, así como una fuente potencial para laprospección de metales valiosos.

The LA-ICP-MS zircons U-Pb ages and geochemistry of the Baihua basic igneous complexes in Tianshui area of West Qinling

Baihua meta-igneous complex consists mainly of pyroxenite-gabbro(diorite)-diorite-quartz diorite. They form a complete comagmatic evolutionary series. The geochemical characteristics of basic-intermediate basic igneous rocks indicate that they belong to a tholeiite suite. The REE distribution pattern is nearly flat type and LREE is slightly enriched type, and their primitive mantle-normalized and MORB-normalized trace element spider diagrams are generally similar; the LIL elements (LILE) Cs, Ba, Sr, Th and U are enriched, but Rb, K and the HFSEs Nb, P, Zr, Sm, Ti and Y are relatively depleted. All these show comagmatic evolution and origin characteristics. The tectonics environment discrimination of trace element reveals that these igneous complexes formed in an island-arc setting. The LA-ICP-MS single-zircons U-Pb age of Baihua basic igneous complex is 434.6±1.5 Ma (MSWD = 1.3), which proves that the formation time of the island-arc type magmatite in the northern zone of West Qinling is Late Ordovician or Early Silurian, also reveals that the timing of subduction of paleo-ocean basin represented by the Guanzizhen ophiolite and resulting island-arc-type magmatic activities is probably Middle-Late Ordovician to Early Silurian.

Geochemical characteristics and zircon U-Pb isotopic ages of island-arc basic igneous complexes from the Tianshui area in West Qinling

The Liushuigou intermediate-basic meta-igneous complex at Guanzizhen, Tianshui area, is mainly composed of metagabbro, metagabbro diorite and metadiorite, while the Baihua basic meta-igneous complex consists mainly of pyroxenite, gabbro (gabbro diorite), diorite and quartz diorite. They form a relatively complete comagmatic evolutionary series. The geochemical characteristics of intermediate-basic igneous rocks indicate that they belong to a tholeiite suite. Their chondrite-normalized REE patterns are nearly flat and are LREE-slightly enriched type, and their primitive mantle-normalized and MORB-normalized trace element spider-grams are generally similar; the LILEs Cs, Ba, Sr, Th and U are enriched, while Rb and K and the HFSEs Nb, P, Zr, Sm, Ti and Y are depleted. All these show comagmatic evolutionary and genetic characteristics. The tectonic environment discrimination by trace element reveals that these igneous complexes formed in an island-arc setting. The Thermal Ionization Mass Spectrometry (TIMS) single-grain zircon U-Pb age for the Liushuigou intermediate-basic meta-igneous rocks in the Guanzizhen area is (507.5±3.0) Ma, representing the age of these igneous complexes, which indicates that island-arc-type magmatite rocks in the northern zone of West Qinling are Late Cambrian and also reveals that the timing of subduction of the paleo-ocean basin represented by the Guanzizhen ophiolite and resulting island-arc-type magmatic activity are probably Late Cambrian to Early Ordovician.

Petrology and geotectonic development of the Western Ecuadorian Andes: the Basic Igneous Complex

The Basic Igneous Complex (BIC) consists of several volcanic formations which form the Ecuadorian Western Cordillera and Coastal Cordillera. The rocks differ significantly from other Cordillerian successions and form an isolated tectnic block. The Upper Cretaceous to Lower Tertiary volcanic series of the Ecuadorian Basic Igneous Complex are of island-arc type according to their petrography and geochemistry. An outer tholeiitic island-arc series (parts of the Piñón Formation) can be distinguished from a central calc-alkalic series, depleted in potassium (Macuchi, Celica, and Silante Formations). The Chontal, Yunguilla, and Zapotillo Formations as well as the upper Cayo Formation (“Guayaquil cherts”) are interpreted to represent back-arc sediments and sediments of small intra-arc basins within a largely submarine island arc. The Cayo Formation consists almost entirely of volcaniclastic and epiclastic rocks deposited on the oceanward side of the island arc, which was situated close to the South American continent. Volcanic activity continued into early Eocene times as revealed by K/Ar-dating of the Cayo and Silante Formations (52 Ma). A small back-arc basin was filled with the sediments of the Yunguilla Formation. Similar to present-day volcanically active island arcs (e.g., Aleutians and Kuriles-Kamchatka) the BIC island arc was mostly underlain by oceanic crust, although parts of it built on continental crust (Celica Formation, southern Ecuador). In the Early Tertiary the island arc was accreted to the South American continent. Cessation of volcanic activity and accretion to the continent may be explained by increased drift velocities of the lithospheric plates accompanied by a change in the geometry and stress in the subduction zone.

The uplift magnetisation record in the central scandinavian caledonides: results from basic igneous complexes

To identify the palaeomagnetic record acquired during slow post-orogenic uplift and cooling in the central part of the Scandinavian Caledonides we have investigated four separated synorogenic basic complexes in the Middle and Upper Allochthons. A shallow N-S axis of magnetisation is common to three of these bodies and therefore postdates all major tectonism. It is defined in the Møsjoen Gabbro ( D/I = 7°/3°, N = 23 samples, α 95 = 5.0° ), in the Sulitjelma Gabbro ( D/I=20°/−9°, N = 22 samples, α 95 = 6.6° )and the Råna norite ( D/I = 9°/−16°, N=19 samples, α 95 = 7.9° ). No vestige of this uplift remanence is recognised in the Lyngen Ophiolite, where a characteristic remanence of D/I = 65°/43° ( N = 19 samples, α 95 = 10.6° ) is recorded. The possibility that the N-S shallow axis of magnetisation is of mid-Carboniferous age, is considered and rejected. Instead it is attributed to the interval of post-tectonic cooling of the Caledonian nappe pile between synorogenic events dated ca.. 420 Ma and the uplift to surface levels defined by later Devonian events. It corresponds closely to an axis of magnetisation extensively represented within the metamorphic Caledonides and Scotland and believed to record a post-tectonic thermochemical event. Subordinate components record Mesozoic overprinting events and relict Caledonian magnetisations which have escaped replacement during this thermo-tectonic event. The latter collectively imply a rapid movement from high to low palaeolatitudes in Late Ordovician and Early Silurian times.

Tectonic fabric of Sierra Leone, West Africa: implications for Mesozoic continental breakup

A lineament map interpreted from Landsat images has been integrated with available geological and geophysical data from both offshore and onshore regions of Sierra Leone. The lineament patterns are related to intraplate and plate marginal reaction of pre-existing structures during Mesozoic rifting events that resulted in strong tectonic controls on magmatism. Of four main lineament trends, the NNW-SSE to N–S, NNE–SSW, and ENE–WSW trends are directly relatable to Archaean fabric in the Leo Uplift. A NW-SE trend reflects coast-parallel late Mesozoic dykes that follow the Rokelide Pan–African fabric (reactivated Archaean NNW and N–S trending structures). NW–SE trending faults defining offshore basins are segmented and offset by ENE–WSW trending continental extensions of Ocean fracture zones. Both the Guinea and Sierra Leone Fracture Zones have nucleated on ENE–WSW trending sinistral Archaean shear zones in the Leo Uplift. The four lineament trends focused Mesozoic magmatic events through protracted reactivation, Spatial and geometric relationships between the magmatic provinces and tectonic fabric indicate that intraplate deformation occurs far inland during rifting events. The angular relationships between the Permo–Triassic NE trending Guinea Belt, the Jurassic–Triassic NW–SE trending coast-parallel dykes, and the ENE–WSW trending Ocean fracture zones centred on the eariy Jurassic Freetown basic igneous complex, suggest that the Sierra Leone–Liberia continental margin evolved as an obliquely-sheared, rift–rift–transform passive margin during Mesozoic continental breakup.

Geotectonic evolution of the Western Cordillera of Colombia: New aspects from geochemical data on volcanic rocks

The Western Cordillera of Colombia (WCC) is part of the Basic Igneous Complex (BIC), which is one of the world's largest ophiolitic complexes, extending from Costa Rica through Panama and Colombia to Ecuador. Major and trace element data on 32 volcanic rocks from the central and northern parts of the Western Cordillera are presented; no data have been available to date for volcanic rocks from the northern parts of the Western Cordillera. Petrographical and geochemical investigations show that the rocks are altered and have undergone low-grade metamorphism. The subalkaline rocks are represented by tholeiitic basalts, calc-alkaline basic andesites, andesites, and one dacite. It is concluded that a mature oceanic island arc existed in the Cretaceous, in what is now the northern part of the Western Cordillera. The tectonics of the region, particularly the intensive imbrication of the chain, indicates the presence of a paleo-subduction zone with an oceanic island arc that accreted on the old continental margin. These new data, combined with new and previous data from the central part of the BIC of Colombia, suggest that volcanic rocks of the Western Cordillera can be interpreted as allochthonous slabs. These slabs were imbricated with back-arc and fore-arc sediments and tonalitic bodies during the closing of a back-arc basin in northwestern South America and accretion of an oceanic island arc. Oblique subduction accreted these different areas to the continental margin during Late Cretaceous and early Tertiary times. Two plate-tectonic models are proposed: a) development of the calc-alkaline volcanic rocks in the northern parts of the Western Cordillera, separated by tholeiitic rocks, formed along a transform fault represented by the tholeiitic basalts of the central and southern parts of the Western Cordillera; or b) development of an oceanic island arc along the Cretaceous continental margin of northwestern South America. In the central and southern parts of this island arc, accretion took place early and therefore only an island-arc tholeiitic suite was formed.

Neue geochemisch-petrographische Untersuchungen im Basic Igneous Complex von Ecuador

Neue geochemisch-petrographische Untersuchungen im Basic Igneous Complex von Ecuador

Geochemistry and mineralogy of the Freetown layered basic igneous complex of Sierra Leone

The northern part of the Freetown tholeiitic layered complex is mapped. This lowermost exposed part of the intrusion is divided into units characterized by an olivine-rich and/or an ilmenite—magnetite-rich basal layer grading upward through hypersthene-and/or augite-rich zone to a plagioclase-rich top. Whole-rock analyses were made for major oxides and for Sr, Cu, Ni, Ba, Zn, Zr, Rb and Pb from 29 samples collected from base to top of seven units. One sample of an ilmenite—magnetite layer was also analyzed. The analyses showed a consistent depletion in Ni and Zn and enrichment in Sr upward in each layered unit. The abundances of trace elements which are concentrated in the cumulate minerals, as shown by close correlation of Ni and MgO, Sr and Al 2O 3, reflect modal variations related to stratigraphic height. Mineralogical investigations failed to indicate a consistent cryptic variation. The compositional variations — olivine Fo 71-63, hypersthene En 70-65, augite Ca 36−37Mo 47−45/1bFe 17−18, and plagioclase An 63−54 — are narrow and irregular in height in the sequence. An explanation for the apparent lack of Fe enrichment in the mafic minerals may be sought in the mode of occurrence of Fe/1bTi-oxide minerals. It is believed that magnetite and ilmenite crystallized from an early stage as a result of high oxygen fugacity and water content of the magma. Withdrawal of Fe/1bTi minerals from the residual liquid, by sinking, led to lack of enrichment in Fe in successive liquid fractions. The investigation reveals that the layered units are replicas produced by pulses from one parent-magma. Replenishment from the reservoir of magma would lead to a narrow and irregular trend in the composition of various mineral series that accumulated in the intrusion.

Geochemistry, petrology and origin of the Cenozoic volcanic rocks of the Northern Andes in Ecuador

The Cenozoic volcanic rocks of the northern Andes in Ecuador for which only little petrological and geochemical information was available until now, display a strong geochemical and petrological zoning from west to east. The magmas of the volcanoes of the Western Cordillera (WC) are unique to the Andes. They belong to andesite-plagidacite series which is characterized by low and fairly constant K 2O contents (1 ± 0.2%), low abundances of large cation elements (Rb, Sr, Ba), rare earth elements, highly charged cations (Zr, U, Th), and relatively high Ni and Cr abundances. The magmas of the volcanoes of the Eastern Cordillera (EC) belong to andesite andesitedacite-rhyolite series which is characterized by Rb, Sr, Ba, REE, Zr, U, and Th contents higher than in the rocks of the WC and varying with the K 2O content. They have Ni and Cr versus MgO correlations similar to the rocks of the Western Cordillera. Even higher contents of K 2O and the above-mentioned elements are encountered in the volcanic rocks of the Sub-Andean Area (SAA). All of the rocks have the same 37Sr/ 36Sr ratio of 0.704. The rocks are chemically more closely related to the Mesozoic to Tertiary rocks of the Basic Igneous Complex (BIC) of Ecuador and Colombia than to those of the Central Andes. It is suggested that the Cenozoic magmas of the volcanoes of the Western and Eastern Cordillera originated by hydrous partial melting of amphibolites and garnet amphibolites of the BIC-type in a subduction zone beneath the Cordilleras at pressures near 20 kbar which corresponds to a depth of about 70 km.

Precambrian and Tertiary geology between Kangerdlugssuaq and Angmagssalik, East Greenland

The preliminary results of a reconnaissance survey of the coast between Kangerdlugssuaq and Angmagssalik are summarized. The Archaean gneiss complex between Kangerdlugssuaq and Kap Japetus Steenstrup is fairly uniform and comprises granitoid gneisses with inclusions of supracrustal rocks, layered basic igneous complexes and older gneisses. Inland areas generally show low deformation states, large scale recumbent folds, and rocks are in granulite facies, whereas a belt of stronger deformation, steep foliation and amphibolite facies matamorphism extends alng the outer coast. To the south of Kap Japetus Steenstrup, Archaean rocks are strongly deformed in the Nagssugtoqidian mobile belt and granulite facies rocks are retrograded to amphibolite facies, except in pods of low deformation, between 2800--2600 m.y. ago. Large bodies of diorite and granodiorite (the Blokken gneisses) were emplaced and metamorphosed in amphibolite facies 2400 m.y. ago, and around Angmagssalik a complex of Ieuco-norite and charnockite was emplaced in granulite facies and caused coarse recrystallization of the adjacent gneisses. Post-tectonic igneous intrusions ranging from ultramafic to granite were intruded 1600 m.y. ago. Numerous Tertiary dykes and plutonic complexes ranging from gabbro to granite were emplaced along the coast between Tasilaq and Kangerdlugssuaq 60-35 m.y. ago.

Geochemistry of tholeiites of the Basic Igneous Complex of northwestern South America

Chemical analyses of basaltic rocks from the Basic Igneous Complex of the Pacific coastal areas of northern South America clearly demonstrate a consistent tholeiitic affinity for this belt and a geochemical similarity of samples from Ecuador, Colombia, and Panama. The rocks are more like ocean-ridge tholeiites than other varieties, but they have higher than expected K, Sr, and Rb contents. Most probably, alteration has caused these anomalies, but a firm conclusion has been frustrated by the lack of correlation of concentration of these elements with the degree of three types of microscopically observable secondary mineralogic changes in the rocks. The rocks form a bimodal geochemical population, about two-thirds basalt and one-third basaltic andesite; both groups are geographically widespread. The increased geochemical information confirms the correlation of this complex with similar rocks in the Cordillera Occidental of Colombia and Ecuador and the Nicoya Peninsula of Costa Rica.

207Pb/206Pb whole Rock Age of the Archaean Granulite Facies Metamorphic Event in West Greenland

IN the Archaean basement complex of West Greenland there are several widely separated granulite facies areas, up to 100 km across1. Rocks from three such areas, Sukkertoppen, Nord-land and Fiskenaesset (Fig. 1), were originally selected for this study on the assumption that the granulite facies metamorphism represented the oldest metamorphic event observable in West Greenland1. This idea has been shown to be ill founded by subsequent geological and geochronological work2,3. The granulite facies metamorphism is now recognized as a distinctive time marker which probably has the following chronological relationships with other principal events in the Archaean of West Greenland, particularly in the Godthaabsfjord area2,3: (1) Plutonic development ∼ 3,700-3,750 m.y. ago4,5 of a complex of granites, granodiorites and tonalites. Metamorphism, migmatization and deformation, with production of banded gneisses (Amitsoq gneisses). (2) Intrusion of a basic dyke swarm (Ameralik dykes) into the Amitsoq gneisses. (3) Eruption of basic volcanics and deposition of sediments (Malene supracrustals). (4) Emplacement of basic igneous complexes with prominent calcic anorthosites. Possibly broadly similar in age throughout the Archaean of West Greenland. (5) Tectonic interleaving of earlier rock units. (6) Intrusion of major suite of granites, granodiorites, tonalites and diorites at 3,040 ±50 m.y. ago6, the parent rocks of the Nuk gneisses. (7) Deformation and folding, accompanied by migmatization and metamorphism that culminated in amphibolite facies conditions in the Godthaabsfjord area, but reached granulite facies to the north and south (in the areas we discuss in this communication). Earlier felsic rocks converted to gneisses. (8) Emplacement of Qorqut granite and pegmatites, ˜ 2,600 m.y. ago (Oxford unpublished data). Intrusion of Precamb-rian dolerite dykes.

Chemical Composition and Age Determination of Tholeiitic Rocks in the Basic Igneous Complex, Ecuador

Two new chemical analyses show that tholeiitic lava on the coastal plain of Ecuador is similar to basalt on Nicoya Peninsula of Costa Rica and other basalt found along the Pacific Coast of northwestern South America. Harzburgite is found in the same group of mafic volcanic and intrusive rocks here called basic igneous complex. K:Ar age determinations of basaltic lava in this complex show that igneous activity continued at least until early Eocene, although most of the hypabyssal rocks could be Late Jurassic to Late Cretaceous. Discussion includes possible correlation of the basic igneous complex of Ecuador, the Nicoya Complex of Costa Rica, similar rocks along the Pacific Coast of Panama and Colombia, and the basaltic floor of Nazca plate.

Igneous structures and textures in the Majorqap qâva outcrop of the Fiskenæsset anorthosite complex

Primary structures and textures are extensively preserved in the large Måjorqap qâva outcrop of the early Precambrian basic igneous complex of Fiskenæsset. The outcrop is situated on the northern side of the head of Fiskenæsfjorden (63°16'N; 50°05'W) and is sub-rectangular in outline, 7 kilometres long and 5 kilometres wide (plate 1). Its form is basin-shaped and a section of 1000 metres is exposed through it from the summit of Majorqap qava to the floor of a narrow valley which cuts diagonally across the outcrop (plate 2).

Occurrence and Geological Significance of Layered Stratiform Intrusion in the Yilgarn Block, Western Australia

I WISH to announce the presence in the Yilgarn Block1, Western Australia, of a major stratiform basic igneous complex with many features similar to the well documented Bushveldt2, Skaergaard3 and Stillwater layered4 intrusions. There is a stratiform basic sill 64 km south-east of Mount Magnet within the complexly deformed Archaean greenstones of the Yilgarn Block5. The sill, here provisionally designated the Challa Intrusion, crops out over an area in excess of 400 km2, with a discontinuous strike length of more than 200 km in a north-westerly direction from Challa through Barrambie to Yarrabubba, 30 km south of Meekatharra (Fig. 1). The separate occurrences of vanadium-rich titaniferous magnetites that outcrop along the whole of the sill from Youanmi in the south through Wymandoo, Windimurra, Challa, Windsor and Barrambie to Yarrabubba in the north, have not been recognized before as part of the one deposit. The sill consists of alternating layers of gabbros, anorthosites, pyroxenites and magnetite layers and lenses; the principal rock type is a medium to coarse grained gabbro with an average grain size of approximately 4 mm.

The geochronology of the basic igneous complexes

Synopsis Rb-Sr whole-rock isochrons for the Upper Zone of the Insch mass and aureole schists of the Haddo House mass indicate crystallisation of the basic sheet of NE. Scotland at 486± 17 m.y. ago (t½ = 4.85 × 10 10 y. for Rb 87 ). It is argued that intrusion occurred into hot country rocks during, or very soon after, the migmatite overprint phase of the climax of regional metamorphism (M 3 ) in the Dalradian Series, so that this is also dated at about 486± 17 m.y. Small bodies of gneissose rocks in the Arnage mass give a whole-rock isochron age of 479± 12 m.y. and are interpreted as intrusions slightly younger than the Newer Gabbros. Post-metamorphic granites at Insch and Haddo House are dated at 459± 8 m.y. K-Ar mica ages from the gabbro complex are extremely uniform at 461 ± 5 m.y., whereas Rb-Sr ages on some of the biotites range from 471 ± 22 m.y. to 428 ± 9 m.y. Together, these ages preclude the hypothesis of overprinting by any discrete metamorphic event younger than the main climax. Instead, they are interpreted as indicating rapid post-orogenic uplift in NE. Scotland at about 460 m.y. ago.

Thermal Metamorphism of the Banded Iron Formation of Orissa, India

Effect of thermal metamorphism on the sedimentary iron formation of Orissa by basic igneous intrusives is described. Two mineral assemblages, quartz-cummingtonite-grunerite-magnetite with amosite and quartz-actinolite-zoisite-magnetite have been developed at the contact of the banded iron formation and the gabbro. The contact rocks show typical hornfelsic texture. The mineralogical assemblages indicate albite-epidote hornfels to hornblende hornfels facies of thermal metamorphism. The metamorphism took place under high water vapor pressure at a temperature of about 800° ± 50° C, which was confirmed by the decomposition temperature of amosite and also by the temperature of formation of cummingtonite. The banded iron formation consists of alternate bands of fine-grained iron oxide and cherty or cryptocrystalline silica. The basic igneous complex has been differentiated into pyroxenites, olivine gabbro, normal gabbro without olivine, anorthositic gabbro, norite, granodiorite and leucogranite. From the chemical analyses of the contact rocks and their mineralogical and textural studies, it has been established that the metamorphism of the iron-formation was accompanied by the introduction of Ca and Mg from gabbro to the iron formation. Cummingtonite was formed by the reaction of quartz and magnetite. Chemical analysis and X-ray powder photograph data of the mineral amosite are also given.

The Bigga House Basic Igneous Complex, New South Wales

AbstractThe complex described is composed of basic to ultrabasic rocks, ranging in composition from hornblendites, through meladiorites, to diorites. It is surrounded by the Bigga Granite, which has an intrusive relationship with the basic complex. The nomenclature of these rocks is discussed, and detailed petrographical descriptions, together with modal and chemical data, are given. It is suggested that the original rocks of this complex were probably of gabbroic composition, and that they subsequently assimilated and hybridized to give rise to the observed rock types.