Diorite Suite Research Articles

An overview on geochemistry of Proterozoic massif-type anorthosites and associated rocks

A critical study of 311 published WR chemical analyses, isotopic and mineral chemistry of anorthosites and associated rocks from eight Proterozoic massif anorthosite complexes of India, North America and Norway indicates marked similarities in mineralogy and chemistry among similar rock types. The anorthosite and mafic-leucomafic rocks (e.g., leuconorite, leucogabbro, leucotroctolite, anorthositic gabbro, gabbroic anorthosite, etc.) constituting the major part of the massifs are characterized by higher Na2O + K2O, Al2O3, SiO2, Mg# and Sr contents, low in plagioclase incompatible elements and REE with positive Eu anomalies. Their δ 18O‰ (5.7–7.5), initial 87Sr/86Sr (0.7034–0.7066) and ɛ Nd values (+1.14 to +5.5) suggest a depleted mantle origin. The Fe-rich dioritic rocks occurring at the margin of massifs have isotopic, chemical and mineral composition more close to anorthosite-mafic-leucomafic rocks. However, there is a gradual decrease in plagioclase content, An content of plagioclase and XMg of orthopyroxene, and an increase in mafic silicates, oxide minerals content, plagioclase incompatible elements and REE from anorthosite-mafic-leucomafic rocks to Fe-rich dioritic rocks. The Fe-rich dioritic rocks are interpreted as residual melt from mantle derived high-Al gabbro melt, which produced the anorthosite and mafic-leucomafic rocks. Mineralogically and chemically, the K-rich felsic rocks are distinct from anorthosite-mafic-leucomafic-Fe-rich dioritic suite. They have higher δ 18O values (6.8–10.8‰) and initial 87Sr/86Sr (0.7067–0.7104). By contrast, the K-rich felsic suites are products of melting of crustal precursors.

Geochemistry and magmatic setting of Wadi El-Markh island-arc gabbro–diorite suite, central Eastern Desert, Egypt

Wadi El-Markh gabbro–diorite complex is composed of pyroxene hornblende gabbros, hornblende gabbros, diorites and quartz diorites. According to their bulk rock geochemistry and mineral chemistry, the gabbroic and dioritic rocks represent fractionates along a single line of descent and crystallized from a calc-alkaline mafic magma. When compared to the primitive mantle, all members of the gabbroic–dioritic rock suite are enriched in the large ion lithophile elements relative to the high field strength elements and display distinctive negative Nb and P 2O 5 anomalies. This signals an arc setting. Fractionation modeling involving the major elements reveals that the hornblende gabbros were generated from the parent pyroxene hornblende gabbros by 61.86% fractional crystallization. The diorites were produced from the hornblende gabbros by fractional crystallization with a 58.97% residual liquid, whereas the quartz diorites were formed from the diorites by 26.58% fractional crystallization. According to geothermobarometry based on amphibole mineral chemistry, the most primitive pyroxene hornblende gabbros crystallized at ∼830 °C/∼5 kbar. The crystallization conditions of the quartz diorites were estimated at ∼570 °C/∼2 kbar. In consequence the Wadi El-Markh gabbro–diorite complex represents a single magmatic suite of which fractionates crystallized in progressively shallower levels of an arc crust.

The Urumieh plutonic complex (NW Iran): A record of the geodynamic evolution of the Sanandaj–Sirjan zone during Cretaceous times – Part I: Petrogenesis and K/Ar dating

The Urumieh plutonic complex is located in the northwestern part of the Sanandaj–Sirjan zone (Iran). The Urumieh plutonic complex comprises three components, i.e. a dioritic suite, a granitic suite consisting of biotite-bearing granites, and younger stocks, i.e. an alkaline suite (alkali syenite and alkali granite) and a muscovite-bearing granite. Based on K–Ar chronology of separated minerals (amphibole, biotite, muscovite and feldspar) and field observations, the Urumieh complex was built during two distinct episodes: the diorite and granite suites were emplaced between 100 and 92 Ma, whereas the younger suites were emplaced between 82 and 80 Ma. The dioritic suite has SiO 2 contents in-between 56–59 wt% and major and trace elements characteristic of mafic I-type granitoids (A/CKN molar ratio = 0.81–0.88). This suite is characterized by relatively high Mg# (>49), low Na 2O content (<3.5 wt%) and a relatively high abundance of (LILE) incompatible elements. Their chondrite-normalized REE patterns are relatively flat ([La/Yb] N = 7 in average) and show small or no negative Eu anomalies (Eu/Eu ∗ = 0.9 in average). Some diorites show mingling and mixing textures that may support local crustal contamination of the dioritic melt. Geochemical features suggest that diorites formed in a subduction setting by partial melting of metasomatized mantle. The coeval granites are peraluminous (1.04 < molar A/CKN < 1.37), with SiO 2 contents between 66–69 wt%. Their chondrite–normalized REE patterns are slightly enriched in LREE relative to HREE ([La/Yb] N = 11 in average) with negative Eu anomalies (Eu/Eu ∗ = 0.4 in average). These granites have biotite as the main mafic mineral, and locally contain restitic xenoliths. They originated by partial melting of metasediments. The younger alkaline syenite and granite are only slightly peraluminous (molar A/CNK = 1.04 in average). They show enrichment in HFSE, high contents of REE, and relatively high FeO/MgO and Ga/Al ratios, typical of A-type magmas. The other younger granite pluton (Dourbeh granite) has peculiar mineralogical and geochemical features. None of these late syenite and granites were intruded by mafic dikes and they do not contain any comagmatic mafic enclaves. They seem to have been derived only from crustal melting. Changing of magmatic sources during the 100–80 Ma time interval is discussed with respect to the geodynamic context. The first magmatic episode (100–92 Ma) is regarded as subduction-related, whereas the second one (at ca 80 Ma) likely ocurred during the collision between the Arabian margin and the Sanandaj–Sirjan zone.

Cretaceous reduced granitoids in the Goodpaster Mining District, east central Alaska

The &gt;5 Moz (1 oz (troy) = 31.103 g) gold veins of the Liese Zone and nearby prospects in the Goodpaster Mining District of east central Alaska are spatially and temporally associated with late-Early to Late Cretaceous reduced granitoids that are divided into a granite suite, tonalite suite, and diorite suite in decreasing age. Synkinematic to postkinematic biotite ± hornblende granite, granodiorite, pegmatite, and two-mica granite with accessory garnet compose the granite suite (109–107 Ma). The tonalite suite (107–103 Ma) forms small to large bodies of postkinematic hornblende–biotite granodiorite to tonalite with rare granite. Intruding the granite and parts of the tonalite suites are aplite and pegmatite that grade through sugary pegmatoidal or aplitic quartz veins and finally into quartz veins along strike. The diorite suite (95.4–93.7 Ma) consists of small stocks of diorite to tonalite that intrude the older suites and are inferred to intrude the shallowly dipping auriferous quartz veins. Limited data indicate granitoid emplacement at 5–9 km depths, consistent with formation of the auriferous quartz veins based upon published fluid inclusion data. The weakly peraluminous granite and tonalite suites are distinguished by variable amounts of monazite and zircon whereas the diorite suite is metaluminous, contains &lt;5% magnetite, and lacks monazite. All suites are subalkalic, calc-alkaline and have low magnetic susceptibilities, high large-ion lithophile element/high field-strength element (LILE/HFSE), and depleted Nb and Ti. The granite suite has higher206Pb/204Pb values (19.4–19.6) than the diorite (19.1). Overall the granite and tonalite suites likely represent melts generated late during crustal thickening that intruded along shallowly dipping faults during exhumation, whereas the diorite suite represents postdeformation melts that underwent less interaction with the old silicic crust.

The Nordre Strømfjord shear zone and the Arfersiorfik quartz diorite in Arfersiorfik, the Nagssugtoqidian orogen, West Greenland

The Nordre Strømfjord shear zone in the fjord Arfersiorfik, central West Greenland, consists of alternating panels of supracrustal rocks and orthogneisses which together form a vertical zone up to 7 km wide with sinistral transcurrent, ductile deformation, which occurred under middle amphibolite facies conditions. The pelitic and metavolcanic schists and paragneisses are all highly deformed, while the orthogneisses appear more variably deformed, with increasing deformation evident towards the supracrustal units. The c. 1.92 Ga Arfersiorfik quartz diorite is traceable for a distance of at least 35 km from the Inland Ice towards the west-south-west. Towards its northern contact with an intensely deformed schist unit it shows a similar pattern of increasing strain, which is accompanied by chemical and mineralogical changes. The metasomatic changes associated with the shear zone deformation are superimposed on a wide range of original chemical compositions, which reflect magmatic olivine and/or pyroxene as well as hornblende fractionation trends. The chemistry of the Arfersiorfik quartz diorite suite as a whole is comparable to that of Phanerozoic plutonic and volcanic rocks of calc-alkaline affinity.

Archaean boninite-like rocks in an intracratonic setting

Boninites and high-Mg norites show considerable compositional overlap. While the petrogenesis of both may involve large ion lithophile element (LILE) replenishment of a mantle source, boninites appear restricted to convergent margin settings, while many high-Mg norite suites intrude intracratonic rifts. Rocks with boninitic compositions are extremely rare in Archaean terranes, where they are also interpreted to have formed at convergent margins. High-Mg, LILE-enriched basaltic rocks form a ca. 2950 Ma sub-volcanic sill within the 3010–2935 Ma Mallina Basin, of the Pilbara Craton, northwest Australia. They are spatially associated with low-Ti tholeiitic intrusions and a high-Mg diorite (sanukitoid) suite. They have a strong boninitic affinity, with high SiO 2 (52–54 wt%), Mg # (62–67) and Al 2O 3/TiO 2 (62–73), low TiO 2 (0.24–0.28 wt%) and strong LILE enrichments. Primitive-mantle normalised La/Gd and Gd/Yb ratios of ∼4.5 and ∼0.6, respectively, produce prominent ‘U-shaped’ normalised trace element patterns. Despite such close compositional affinity to boninite, these rocks probably formed more than 40 Myr after regional magmatism and 60 Myr after the last regional event linked to subduction. Geological evidence suggests that the Mallina Basin formed in an intracontinental setting, not at a convergent margin, and so high-Mg norite provides a more realistic analogy for the Mallina boninite-like rocks in terms of tectonic setting. In old terranes, the tectonic setting of rocks with boninite-like compositions must be interpreted with caution. Trace element modelling shows that contamination by felsic crust is a plausible explanation for LILE enrichments in the Mallina boninite-like rocks, but the scarcity of suitable contaminants makes this explanation unlikely. Petrogenesis of the boninite-like rocks was closely associated with that of the high-Mg diorite (sanukitoid) suite. LILE enrichments in high-Mg diorite require that the mantle beneath the entire Mallina Basin was variably metasomatically enriched by an earlier subduction event at, or before, ca. 3015 Ma. The refractory source for the boninite-like rocks may have also been enriched during this event.

The Archaean High-Mg Diorite Suite: Links to Tonalite-Trondhjemite-Granodiorite Magmatism and Implications for Early Archaean Crustal Growth

The 2·95 Ga Pilbara high-Mg diorite suite intrudes the central part of the Archaean granite–greenstone terrain of the Pilbara Craton, Western Australia, and shows many features typical of high-Mg diorite (sanukitoid) suites from other late Archaean terrains. Such suites form a minor component of Archaean felsic crust. They are typically emplaced in late- to post-kinematic settings, sometimes in association with felsic alkaline magmatism, and are either unaccompanied by, or post-date, tonalite–trondhjemite–granodiorite (TTG) magmatism, which comprises a much greater proportion of Archaean felsic crust. The TTG series comprises sodic, Sr-rich rocks with high La/Yb and Sr/Y ratios, thought to result from partial melting of eclogite facies basaltic crust. High-Mg diorite shares these characteristics but has significantly higher mg-number (∼60), and Cr and Ni concentrations, suggesting a mantle source. Many compositional features of TTGs are also shared by Cenozoic felsic magmas called adakites. Adakites form by melting of a young, hot, subducting slab and provide an a priori reason to invoke a subduction origin for TTG. During ascent through the mantle wedge, adakite commonly assimilates, or is contaminated by, peridotite, and the resulting ‘wedge-modified adakite’ bears strong compositional similarity to Archaean high-Mg diorite. Nevertheless, the latter are not simply an Archaean analogue of ‘wedge-modified adakite’ (i.e. ‘wedge-modified TTG’) because their intrusion is post-tectonic and unaccompanied by TTG magmatism. The petrogenesis of the Pilbara high-Mg diorite suite requires remelting of a mantle source, extensively metasomatized by addition of about 40% TTG-like melt. However, although the generation of this metasomatized source appears to require a subduction environment, many Archaean TTG suites show no clear chemical evidence of having interacted with a mantle wedge, and on that basis are more likely to represent partial melts of basaltic lower crust rather than of subducted slab. High-Mg diorite suites appear to concentrate in the Late Archaean, suggesting that subduction may have become an important process only after ∼3·0 Ga.

Les granitoïdes paléoprotérozoïques des boutonnières du Bas Drâa et de la Tagragra d'Akka (Anti-Atlas occidental, Maroc): en élément du puzzle géodynamique du craton ouest-africain (Palæoproterozoic granitoids from the Bas Drâa and Tagragra d'Akka Inliers [western Anti-Atlas, Morocco]: part of the jigsaw puzzle concerning the Western African Craton)

Two Palæoproterozoic plutonic units are recognised in the Bas Drâa and Tagragra d'Akka Inliers (western Anti-Atlas, Morocco) using mineralogical and chemical data. The first one consists of a calc-alkaline suite of diorites, monzogabbrodiorites, granodiorites and granites (50.9 < SiO 2 < 70.9%). Its mineralogy (oligoclase-andesine, amphibole, biotite, titanite ± magnetite ± epidote) and their chemical features are those of calc-alkaline I-type granites. Available isotopic data and rare earth element patterns suggest a lower crustal, or mantle, origin with variable contamination by crustal material. The second one corresponds to peraluminous granodiorites, granites and leucogranites (61.7 < SiO 2 < 74.8%). Its mineralogy (oligoclase, biotite, muscovite ± garnet ± tourmaline) and peralaminous composition reflects a crustal source. The nature and chemical signature of these two coeval plutonic units imply interaction between mantellic magmas and a continental crust. The siliciclastic nature of their sedimentary host rocks (Palæoproterozoic in age) corroborates the existence of an older continental domain of probable Archæan age. The lack of a volcaniclastic series in the two inliers precludes any volcanic-arc at that time. A back-arc geodynamic context, similar to that described in the Palæozoic of Argentina, is proposed. A comparison with the Reguibat and the Leo Rises shows the existence of two large Palæoproterozoic domains in the West African Craton: (i) an older, recycled Archæan crust to the west and north (Man, Tiris and Anti-Atlas); and (ii) a Palæoproterozoic juvenile domain located to the southeast and northeast (Baoulé-Mossi Block and eastern Reguibat Rise).

Petrogenese kontrastierender Granitplutone in Westb�hmen (Tschechien)

Late-Variscan granitoid plutons in western Bohemia (Bor, Waidhaus-Rozvadov) have distinct petrographic, geochemical and isotopic features that suggest different magmatic evolutions. The Bor pluton comprises a suite of metaluminous tonalites and quartz diorites (Bor I), weakly peraluminous (monzo-)granites and granodiorites (Bor II) and medium-aluminous, late vein-forming leucomonzogranites (Bor III). The Waidhaus-Rozvadov pluton is strongly peraluminous, comprising a cordierite-biotite granitoid (CBG), the Rozvadov granite (ROG), the Barnau granite (BAG) and the subordinate, highly evolved Kreuzstein (Křižový kamen) granite (KG). Geochemical parameters and initial87Sr/86Sr ratios straddle the boundary between I- and S-type granites in the Bor pluton and are characteristic of purely S-type granites in the Waidhaus-Rozvadov pluton. The Bor II granitoids have been dated by the Rb-Sr whole-rock method at 341±17 Ma (ISr = 0.70724±0.00060). K-Ar biotite and muscovite ages of all units of the Bor pluton are mainly in the range 321-315 Ma. The K-Ar mineral ages are in good agreement with recently published U-Pb zircon data of these rocks. The different units of the Waidhaus-Rozvadov pluton have yielded less well-constrained Rb-Sr whole-rock ages, ranging from 313 to 300 Ma. However, the intrusion sequence is constrained by K-Ar muscovite ages (312-302 Ma), which define a systematic decrease towards the chemically more evolved granite types. Taken as a whole, it seems likely that the new radiometric ages characterize two temporally distinct periods of late-Variscan granitoid intrusion. The regional significance of these periods is emphasized by contemporaneous ages previously found in the adjacent northeastern Bavarian granitoids. The initial Sr and Nd isotope systematics indicate that the Bor and the WaidhausRozvadov plutons were derived from different source rocks. The Bor granitoids reflect the influence of less evolved crustal material which may have been similar to paragneisses of the Tepla-Barrandian region, including the Zone of ErbendorfVohenstraus (ZEV). The Waidhaus-Rozvadov granitoids probably resulted from anatexis of rocks resembling surrounding Moldanubian paragneisses or metapelites. In addition, the two plutons exhibit poorly defined, opposite trends of eNd(T) variation which are ascribed to assimilation processes.

Petrogenesis of the Hercynian Tichka plutonic complex (Western High Atlas, Morocco): Trace element and Rb&z.sbnd;Sr and Sm&z.sbnd;Nd isotopic constraints

The late tectonic Hercynian Tichka plutonic complex in Morocco was emplaced in the lower Cambrian volcanosedimentary sequence of the Western High Atlas and displays an exceptional acid-basic association which consists of five well-defined plutonic intrusions: gabbros, diorites, granodiorites-tonalites, monzogranites and two types of leucogranites, biotite ± amphibole leucogranites and two-mica leucogranites. With the exception of the late two-mica leucogranites, the plutonic suites present field relationships clearly indicating an almost synchronous magmatic emplacement. The monzogranites and biotite ± amphibole leucogranites plot along the same Rb&z.sbnd;Sr isochron and most of them also plot along the same Sm&z.sbnd;Nd isochron, yielding, respectively, ages of 291 ± 5 Ma and 283 ± 24 Ma with initial 87Sr/ 86Sr and 143Nd/ 144Nd ratios of 0.70348 ± 4 and 0.51243 ± 4. Major and trace element data together with the Rb&z.sbnd;Sr and Sm&z.sbnd;Nd isotopic results rule out any genetic process involving a unique homogeneous source. The Sm&z.sbnd;Nd isotopic heterogeneity, even at the scale of one unit, implies the existence of contrasting source rocks and the combination of different mechanisms. The gabbro and diorite magmas are assumed to be derived from the melting, to different degrees, of a depleted upper mantle source in a within-plate geotectonic environment, whereas a combined wallrock assimilation-fractional crystallisation model is proposed for the evolution of the diorite suite. In contrast, the granitoids are mainly the result of the anatexis of a dominant basic to intermediate igneous heterogeneous continental crust.

The Ballachulish Igneous Complex, Scotland: Petrography, Mineral Chemistry, and Order of Crystallization in the Monzodiorite-Quartz Diorite Suite and in the Granite

The Ballachulish Igneous Complex, Scotland: Petrography, Mineral Chemistry, and Order of Crystallization in the Monzodiorite-Quartz Diorite Suite and in the Granite

Diorites and associated rocks in the Anglem Complex at The Neck, northeastern Stewart Island, New Zealand: an example of magma mingling

Exposures of the Anglem Complex (Cretaceous) at The Neck, northeastern Stewart Island consist dominantly of diorite, microdiorite, tonalite and granite. Dyke-rocks include microgranite, microdiorite, andesite and microgranodiorite. A Diorite Suite composed of hornblende gabbro, diorite and quartz-bearing diorite forms a complex association of basic to intermediate lithologies with highly variable grainsize. The Heterogeneous Suite formed in part by mingling of diorite and tonalite intrudes the Diorite Suite. Enclaves within this suite show highly variable margins, from sharp sometimes crenulated and apparently chilled contacts, through to diffuse contacts. Prior to, and during intrusion of this heterogeneous magma, significant textural modifications of dioritic enclaves occurred, and some important controls on this modification process are discussed. Aspects considered include contrasts between the host melt and enclaves in temperature, viscosity, chemistry, volatile content, deformation and recrystallisation. A liquid immiscibility model is discounted in favour of the magma mingling model. Subsequent intrusion of granite was followed by a porphyritic microgranodiorite. Microdiorite and microgranite dykes intrude all of the above rocks with the exception of the microgranodiorite. Coexisting microdiorite, microgranite, and composite microdiorite-microgranite dykes in the one dyke swarm also provide compelling evidence for magma mingling.

GRANITOGÊNESE BRASILIANA NO SERIDÓ: O MACIÇO DE ACARI (RN)

The Acari granitoid is a type example of the Brazilian magmatism in Serido, NE Brazil. Its country rocks are supracrustals and orthogneisses of early proterozoic age. The oldest unit in the massif is a suite differentiated from gabbro to granodiorite, with dominant quartz diorites and tonalites enclosed as small bodies in younger granites, specially coarse porphyritic ones, and succeeded by fined grained porphyritic and equigranular leucrogranites. The vast majority of these rocks is syntectonic with the F 3 regional deforrnation; some leucogranites are slightly latter, while the diorites are somewhat earlier. A balooning fabric or flow structures are sometimes folder or crenulated by the F 3 event. The geometry (trajectories and tripple points of the S 3 foliation points to a process of diapirism coeval with a regime of normal folds and subhorizontal extension. Micachists surrounding the granites display parageneses such as cordierite + andaluzite or cordierite + silllmanite. The diorite suite is akin to an expanded calc-alkaline (or type I) series, while the porphyritic granites are closer to restrict, potassic calc-alkaline affinities, intermediate between types I and S, and the leucogranites are of S type. The diorites are thought to be derived from mantle magmas or melting of amphibolite (consumed oceanic crust?) followed by variable crustal contamination and magma mixing. The granites appear to represent crustal anatexis in progressively shallower levels. These rocks are regarded as a distal manifestation of an aclive plate margin or Tibetan type plateau.

Cambro-Proterozoic volcanism near Buckingham, Quebec

Grenvillian gneiss and marble of the Buckingham area, Quebec were intruded by a syenite–monzonite–diorite suite, overlain by trachyandesite, and, finally, intruded by diabase. The three igneous events are tentatively dated as late Proterozoic, Cambro-Proterozoic, and Cambro-Ordovician.The trachyandesites were extruded subaerially from en echelon vents or fissures. These rocks are highly oxidized and locally enriched in potassium, thought to be the result of late volcanic, gaseous, or hydrothermal transfer. They are preserved in a small, northeast-trending graben, part of the St. Lawrence Valley Rift System. This occurrence of Cambro-Proterozoic volcanic rocks is the first reported for the Canadian Shield of central Canada.

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.

Time differences within a calc-alkaline association

A Strontium isotope study of a high-K diorite complex at Yeoval, N.S.W. (Australia) points to clear time differences (40 m.y.) between the diorite suite (gabbro-diorite-grano-diorite) and surrounding granites. Such a time interval, in association with other data, precludes a genetic relationship between these rock groups whereas the data support a common origin for the diorite suite and associated andesitic volcanics. The low (0.705) initial Sr ratios for the diorite suite and volcanics are similar to those of other diorite and andesite occurrences.