Extrusive Units Research Articles

U-Pb dating of Mesozoic igneous rocks from Hong Kong

Twenty-two new zircon and monazite U-Pb ages have been obtained for Mesozoic plutonic and volcanic rocks of Hong Kong. These have revealed the existence of four distinct periods of Mid-Jurassic to Lower Cretaceous volcano-plutonism: 164.6 ±0.2 to 159.3 ±0.3 Ma, 146.6 ±0.2 to 146.2 ±0.2 Ma, 143.7 ±0.3 to 142.5 ±0.3 Ma, and 140.9 ±0.2 to 140.4 ±0.2 Ma. A concordant monazite age of 236.3 ± 0.8 Ma has also been obtained from one intrusive body representing the first known occurrence of a Mid-Triassic granitoid in the Territory. Among these rocks, discordant U-Pb data projected to the concordia curve have given a broad range of inheritance ages from early Phanerozoic to late Archaean. A xenocrystic zircon from one sample has yielded a concordant age of 1872 ±3 Ma. The new U-Pb ages have refined the results of recent Rb-Sr whole-rock age dating of the granitoid rocks and have considerably improved correlation between intrusive and extrusive units. The occurrence of inherited components in many of the analysed rocks indicates that they were derived from, or erupted through, older crust containing a wide variety of age components, either as primary igneous or detrital material.

Retrospective cohort study of workers of a synthetic textiles plant in quebec: I. General mortality

This paper presents results from a retrospective cohort study of workers of a synthetic textiles plant in Quebec. This plant has been the subject of a previous case-control study, in which an excess risk of colorectal cancer was observed. The cohort consisted of 7,487 men and 2,724 women who had worked at least 1 year at the plant and who were either working in 1947 or were newly employed between 1947 and 1977. The period of follow-up was from 1947 to 1986, thus yielding 307,278 person-years of observation. Mortality rates for most causes of death were less than expected; the standardized mortality ratio (SMR) for all causes of death among men was 0.73 (95% confidence interval (CI): 0.70-0.77) and among women it was 0.77 (95% CI: 0.68-0.87). For men, the SMR for all neoplasms was 0.76 (95% CI: 0.69-0.83) and for colorectal cancer it was 0.69 (95% CI: 0.52-0.92). For women, the SMR for all neoplasms was 1.01 (95% CI: 0.83-1.22) and for colorectal cancer it was 1.02 (95% CI: 0.57-1.69). Among men, risks for ischemic heart disease increased with increasing length of service at the plant, even though the SMRs were less than unity (overall SMR = 0.76; 95% CI: 0.70-0.83); no trend was observed for women. SMRs and relative risks were also calculated according to duration of employment in each processing unit. Of the scores of associations tested, very few showed increased risks. Of those showing increased risks, the data were not persuasive in indicating a connection with work at the plant; nevertheless, some of these associations may be worth following up in future studies. The following associations of potential significance were observed: all cancers, liver and gallbladder cancers, non-Hodgkin's lymphomas, and reticulum cell sarcoma in the cellulose acetate fiber manufacturing unit; all cancers in the polypropylene and cellulose triacetate extrusion unit and in the janitor unit; leukemias in the unexposed unit; ischemic heart disease in the maintenance and janitor units; and cerebrovascular disease in the weaving unit.

Retrospective cohort study of workers of a synthetic textiles plant in Quebec: II. Colorectal cancer mortality and incidence.

This paper describes standard mortality and nested case-control analyses of colorectal cancer for a cohort of synthetic textiles workers in which a cluster of five cases was observed previously. The cohort consisted of 7,487 men and 2,724 women who had more than 1 year's experience at the plant and who were either working in 1947 or were newly employed between 1947 and 1977. The Standardized Mortality Ratio (SMR) for colorectal cancer for men was 0.69 (95% confidence interval (CI): 0.52-0.92; 50 deaths) and for women it was 1.02 (95% CI: 0.57-1.69; 15 deaths). Among men only there was a suggestion that risks increased according to the length of service at the plant. In the nested case-control study, incident cases as well as deceased cases were included. A variety of analyses were carried out according to duration of employment in the processing units. For men the risk of colon cancer increased with duration of employment in the polypropylene and cellulose triacetate extrusion unit (unadjusted odds ratio (OR) for > or = 5 years duration = 5.52; 95% CI: 1.12-27.26; 4 exposed cases). It was not possible, however, to provide an independent confirmation of this putative association because the case series included three of the original five cases. There was some evidence of increased risks in the cellulose acetate fiber manufacturing unit and in the dyeing and finishing unit, but the data were compatible with the null hypothesis of no effect. No associations were observed for employment in any of the other processing units.

Reconstruction of the tectonic, volcanic, and sedimentary setting of strongly deformed Zn-Cu massive sulfide deposits at Benambra, Victoria

The Wilga and Currawong massive sulfide deposits at Benambra are hosted by a strongly deformed, Upper Silurian rhyolite-dacite-andesite-basalt-sediment succession, dominated by silicic volcanics and sedimentary rocks. This host succession developed in an ensialic back-arc or intra-arc basin of limited extensional origin on the active Gondwana continental margin. Strong deformation is related to compressional and transcurrent crustal movements during closing of the depositional basin at the end of the Silurian and to subsequent steep faulting.The massive sulfide deposits are essentially synvolcanic because (1) they have the same three generations of tectonic structures and the same intensity of deformation as their host rocks, suggesting that the ores predate the earliest compressional deformation, (2) the ores are constrained to a Late Silurian age similar to their host rocks because both the host rocks and this earliest deformation are Late Silurian in age, (3) there is a tight stratigraphic control on the location of the massive sulfides, and (4) the stratigraphy, depositional setting, and alteration are similar to many other less-deformed volcanic-associated massive sulfide districts.Deformation and alteration inhibit detailed stratigraphic correlation and modify rock textures, but do not destroy facies relationships, which allow sedimentological and volcanological interpretation. Sedimentary facies associations indicate that mineralization occurred in the basin center after fault-controlled subsidence from a mixed subaerial and subaqueous environment with active rhyolitic volcanism, through a marine shelf environment with mixed limestone-volcaniclastic sedimentation, to a moderate- to deep-water environment with mudstone and turbidite sedimentation and rhyolitic to basaltic volcanism.Juvenile elastic and nonelastic volcanic rocks are closely associated, subequal in volume, and comprise 50 to 70 percent of the volcano-sedimentary sequence. Evaluation of fragmentation and emplacement processes indicates that the juvenile volcaniclastics are mainly in situ, resedimented and intrusive hyaloclastites, rather than pyroelastics. Classification of primary contact relationships of the volcanics into passive, slightly disruptive and highly disruptive types, and the internal facies organization of the volcanic units, indicate that the volcanics form extrusive domes, extrusive tabular flows, partially emergent cryptodomes and sills, and entirely intrusive sills. Shallow sills and cryptodomes emplaced into wet subsea-floor sediments are texturally similar to, but more abundant than, lavas in the environment of mineralization. These shallow intrusions are distinguished by disruptive upper contacts including sediment-veined and sediment-matrix hyaloclastite. Their recognition is critical because the mineralized stratigraphic interval can only be correlated using volcanics and sediments emplaced on the sea floor. Based on extrusive units, the massive sulfides occur within 100 m above the last major extrusive rhyolites and within 50 m below the first major basalt lava.Integrating the tectonic, structural, sedimentological, and volcanological data shows that the massive sulfides formed in siltstone, in the proximal (near-vent) facies association of a low profile, mainly nonexplosive, moderate to deep-water submarine volcano composed of turbiditic sediments interleaved with numerous rhyolitic to basaltic sills, lavas, and associated hyaloclastites. The ores can be related in space and time to the advanced stage of extension and subsidence in a continental margin incipient rift, when relatively deep-marine conditions were attained, generation of voluminous rhyolitic crustal melts waned, and basaltic magma started to penetrate up deep extensional faults to the sea floor. The spatial association of the last rhyolite extrusions, the massive sulfides, and the first basalt extrusions, suggests that these rocks are genetically related and that all were fed via the same extensional fault system. This setting of silicic volcanic-associated massive sulfides at Benambra provides a model for the many deposits related to mainly nonexplosive volcanism and an alternative to the recently popular submarine pyroclastic caldera model.

Full-scale prototype of the continuous pellet injector for ITER

Abstract A full-scale prototype of the continuous pellet injector with parameters adequate to the requirements of ITER (pellet size, 6 mm; pellet velocity, 1.2–2.0 km s−1; injection rate, up to 6 s−1) is described. The principal parts of the injector are: the extrusion unit where fuel condenses and solidifies and which provides the extrusion; the rotor transporting unit, cooled by the helium flow; and the injector unit consisting of six channel light gas accelerator. A special attention is paid to the injectors safety and reliability.

Cover gneisses of the Monashee Terrane: a record of synsedimentary rifting in the North American Cordillera

The Monashee Terrane of southeastern British Columbia is composed of Lower Proterozoic basement gneisses unconformably overlain by cover gneisses. The latter constitute a thick (> 2000 m) and laterally extensive (> 150 km) upper-amphibolite-grade succession of metasedimentary rocks, locally intercalated with minor intrusive and extrusive units. This succession is interpreted as reflecting initial broad, amagmatic subsidence and sedimentation on a cratonic platform (basement gneisses), most likely of North American affinity. Throughout most of the terrane, syndepositional magmatism is first marked by a laterally extensive (> 100 km) stratiform pyroclastic carbonatite, which is part of intermittent (long-lived?) alkaline magmatism. One alkaline body was intruded at ca. 740 ± 36 Ma (U–Pb zircon), suggesting that it may be part of initial Windermere rifting. Post-pyroclastic-carbonatite syndepositional extensional tectonism is further evidenced at the north end of the terrane by interlayered mature and immature siliciclastic sediments, with rapid facies changes, intercalated with ultramafic and mafic sills and flows, plus minor felsic pyroclastic deposits. All of these later deposits lie above stratigraphy correlated with strata hosting a stratiform Pb–Zn deposit with an Early Cambrian galena Pb-isotope age and, therefore, may be correlative with a rift–drift transition recorded in Hamill Group strata to the east. Rift tectonism recorded in cover gneisses may reflect one or more documented rift and rift–drift events recorded in Upper Proterozoic to Lower Cambrian strata of the western North American continental terrace prism.

Deep crustal structure and magmatic processes: the inception of seafloor spreading in the Norwegian-Greenland Sea

Summary An extensive investigation into the deep crustal structure of the conjugate Norwegian and E Greenland margins utilizing two-ship Expanded Spread and Wide Aperture CDP Profiling has led to the identification of a unique suite of crustal structures developed during the onset of seafloor spreading. On both margins, a region of low velocity crust was found proximal to marginal escarpments. Immediately seaward of the low velocity crustal block seaward-dipping reflectors occur within thick crust that is interpreted to be wholly igneous, a product of voluminous melt production enhanced by convective partial melting processes. This earliest formed oceanic crust is actually several kilometres thicker than the low velocity crust or the adjacent continental crust. Normal oceanic crustal thicknesses are not produced until about 5 My after the initiation of spreading when convective partial melting has abated and the magma budget has reduced. The extrusive unit reduces to normal thicknesses long before the intrusive and plutonic section, indicating that the thickening or thinning of oceanic crust is accomplished primarily by variations in the latter.

Origin of hybrid ferrolatite lavas from Magic Reservoir eruptive center, Snake River Plain, Idaho

The mineralogy and geochemical characteristics of intermediate composition ferrolatites and related lavas from the Magic Reservoir eruptive center (central Snake River Plain) have been investigated to evaluate the origin and petrologic significance of these hybrid lavas. The ferrolatites are chemically uniform, but contain a disequilibrium phenocryst/xenocryst assemblage derived in part from mixed rhyolitic and basaltic magmas that are closely represented by extrusive units in the area. The hybrid lavas also contain xenoliths of Archean granulites and have high 87Sr/ 86Sr and low 143Nd/144Nd ratios, all of which suggest significant magma-crust interaction. Quantitative models including magma mixing, minor crystal fractionation, and crustal contamination very closely reproduce the observed compositions of these ferrolatites; closed system fractionation and (or) simple bulk contamination models are not as successful and can be ruled out. It appears that preexisting mafic and silicic magmas from distinct sources (e.g., mantle and crust) encounter one another in crustal-level magma chambers under conditions where intimate mixing may occur despite wide differences in the physical properties of these liquids.

An early proterozoic ophiolite — the jormua mafic-ultramafic complex, Northeastern Finland

The 1.96 Ga old Jormua complex comprises a fault-bounded unit (2–5 × 20 km) in the centre of the Kainuu Schist Belt at the eastern margin of the early Proterozoic Svecokarelides. The complex was dismembered and metamorphosed to lower amphibolite facies during the Svecokarelian orogeny. In the intact complex the basal unit was formed by serpentinites, the middle unit of mafic dykes, and the uppermost extrusive unit of basaltic pillow lavas. Gabbroic rocks are present as intrusive bodies in the serpentinites and narrow interdyke screens within the dyke unit. Trondhjemitic rocks occur as minor segregations in the gabbroic rocks or as cross-cutting narrow dykes in the mafic dyke unit. The dyke unit is locally composed exclusively of subparallel metabasalt and metadolerite dykes; in places interdyke metagabbro and serpentinite screens are abundant. Well-preserved chilled margins are common and many dykes are split and intruded by younger dykes. The chemical composition of the serpentinites indicates derivation from harzburgitic-dunitic mantle peridotites. The gabbroic rocks, which represent obvious tholeiitic cumulates, show extreme differentiation similar to gabbros of the high-Ti ophiolites and gabbros dredged from present midocean ridges. In chemistry, the trondhjemitic rocks are close to granitic rocks from mid-ocean ridges and high-Ti ophiolites. The similar chemical composition of the dykes and lavas of the complex indicates that they are cogenetic, both being tholeiitic metabasalts with chondrite-normalized REE patterns ranging from slightly LREE depleted to slightly LREE enriched ones. Compared with modern rocks, the Jormua metabasalts most closely resemble (transitional) midocean ridge basalts. It is suggested that the Jormua complex and related mafic-ultramafic bodies in eastern Finland originated as divergent-margin ophiolites concomitantly with the break-up of the Archaean craton in the extensional stage of the Svecokarelian orogeny, which culminated in the opening of a major ocean after 1.97 Ga. The tectonic emplacement of these ophiolites into their present locations occurred in connection with thrusting during the compressional stage of the orogeny about 1.9 Ga ago.

Alteration of Late Middle Proterozoic Volcanics and its Relation to Stratabound Copper-Silver-Gold Mineralization Along the Margin of the Kalahari Craton in Swa/Namibia and Botswana

Summary Metavolcanic and metasedimentary rocks of late Middle Proterozoic age are preserved in basins along the northern margin of the Kalahari Craton. Three broad stratigraphic divisions are recognized: a lower acid extrusive unit, a basic extrusive/red bed unit and a shallow marine unit. The basalts of the basic extrusive/red bed unit are interpreted to be continental within-plate basalts. Copper mineralization occurs as native copper in amygdaloidal altered basalt, and as sediment-hosted, stratiform sulphide deposits at the base of the shallow marine unit. The rocks have been strongly altered during regional metamorphism which reached temperatures up to 350°C. Massive domains were metamorphosed to oligoclase-chlorite-epidote-haematite rocks. In permeable zones such as flow top breccias, amygdaloidal zones, and regions of secondary, tectonically induced fracturing, the basalts have been altered to epidote-quartz-chlorite-haematite-muscovite rocks. During this epidote alteration copper, zinc and cobalt were leached from the basalts, thus providing a possible metal source for the sediment-hosted deposits. A model for multiphase ore genesis is presented in which basalt alteration is the main metal source for epigenetic mineralization.

Paleomagnetic evidence bearing on the structural development of the Latir Volcanic Field near Questa, New Mexico

The mid‐Tertiary Latir volcanic field in northern New Mexico comprises intermediate‐composition volcanics overlain by a regional ash flow sheet and associated lavas of rhyolitic composition that are all cut by silicic‐alkalic granitic intrusives. Deeply exposed along the eastern flank of the Rio Grande rift, the silicic extrusive and intrusive rocks all yield radiometric ages of about 23 m.y., within analytical uncertainties. Flow banding and eutaxitic structures within the extrusive units indicate intense structural deformation, which for the most part increases toward the Questa caldera. Paleomagnetic study was undertaken to obtain information on the orientations of the eruptive and intrusive units and to employ the magnetic directions for an evaluation of the tectonic and igneous history of the caldera and enclosing volcanic field. Radiometric ages and the paleomagnetic data indicate that an episode of pronounced extension coincided with a major pulse of igneous activity in the region 23 m.y. ago. Eastward tilting of the volcanic units appears to have been closely followed by caldera collapse and then by resurgent doming of the caldera. Welded tuff units within the caldera were turned on end and together with older volcanic units were incorporated in a collapse megabreccia. The megabreccia and other nearby units were partially or entirely overprinted by a thermochemical aureole associated with the resurgent doming and attendant hydrothermal circulation. The stocks of the resurgent dome appear to have undergone no significant later tilting.

Regional geochemical studies in the Patagonia Mountains, Santa Cruz County, Arizona

The Patagonia Mountains in southern Arizona contain the deeply buried porphyry copper system at Red Mountain as well as a number of other base- and precious-metal mines and prospects. The range contains complex Basin and Range geology with units ranging in age from Precambrian to Holocene. Rock types present include igneous intrusive and extrusive units as well as sedimentary and metamorphic units, most of which have been tectonically disturbed. A total of 264 stream-sediment samples were collected and analyzed for 32 elements. Geochemical maps for Sb, Ag, Pb, Te, B, Mn, Au, Zn, Cu (total), Cu (cold-extractable), and Mo, as well as for Cu (cold-extractable)/Cu (total) and Fe/Mn, are presented. Anomaly patterns for these elements generally occur over the Red Mountain deposit and (or) along a north-northwest trend parallel to the major Harshaw Creek Fault. Much of the entire area sampled contains widespread anomalies for Pb, Te, and Cu; the other elements are only locally anomalous. Various plots of ratios of Cu (cold-extractable) to Cu (total) did not produce any new information not readily apparent on either one of the two copper maps. A plot of ratios of Fe to Mn delineated many areas of pyrite mineralization. Several of these areas may represent the pyritic halos around deeply buried porphyry copper systems. The best ore guide for the Red Mountain porphyry system is the coincidence of positive anomalies of Mo, Pb, and Te and a negative anomaly of Mn. Other areas with anomalies of the same suite of elements are present within the Patagonia Mountains. It is concluded that geochemical sampling, even in a highly contaminated area, can be useful in delineating major geologic features, such as porphyry copper belts and major faults. Multielement geochemical surveys on a regional scale can effectively locate large, deeply buried, zoned mineral systems such as that at Red Mountain. Plots of element ratios, where adequately understood, can provide geochemical information not readily discernible from plots of single elements alone.

Aspects of Caledonian palaeomagnetism and their tectonic implications

Palaeomagnetic results from the Lower Palaeozoic inliers of northern England cover the upper part of the (Middle Ordovician) Borrowdale Volcanic Series (palaeomagnetic pole 208°E, 18°S, A 95 = 9.4°), minor extrusive units relating to the Caradoc and Ashgill stages of Ordovician times, intrusive episodes of Middle Ordovician and Middle Silurian to Late Devonian age, and the Shap Granite of Devonian (393 m.y.) age (palaeomagnetic pole 313°E, 33°S, A 95 = 5.6°). A complete assessment of Ordovician to Devonian palaeomagnetic data for the British region shows that the pole was nearly static relative to this region for long intervals which were separated by shifts occupying no more than a few millions of years. The mean palaeomagnetic poles are: Ordovician (6°E, 16°S), Lower Silurian (58°E, 16°N), Middle Silurian/Lower Devonian (318°E, 6°N) and Middle/Upper Devonian (338°E, 26°S); the first two shifts separating these mean poles can be explained predominantly in terms of rotational movements of the crustal plate but the last involved appreciable movement in palaeolatitude. Comparison of Lower Palaeozoic palaeomagnetic data from the British region with contemporaneous data from continental Europe/North America on the Pangaean reconstruction reveals a systematic discrepancy in palaeolatitude between the two regions prior to Middle Devonian times. This discrepancy was eliminated during a few millions of years of Lower/Middle Devonian times (ca. 395 m.y.) and can be explained in terms of ca. 3500 km of sinistral strike-slip movement close to the line of the orthotectonic Caledonides. This motion is linked both in time and place to the impingement of the Gondwanaland and Laurentian supercontinents during the Acadian orogeny; this appears to have displaced the British sub-plate until it became effectively locked between the Baltic and Laurentian regions. Although movement of the dipole field relative to the British region in Lower Palaeozoic times is now well defined, nearly one fifth of the total data show that the geomagnetic field was more complex than dipolar during this interval. Until the significance of these anomalies is fully resolved, the tectonic model derived from the palaeomagnetic data cannot be regarded as unambiguous.

K‐Ar dating of Permian and Tertiary igneous activity in the Southeastern Sydney Basin, New South Wales

Abstract The southern part of the Sydney Basin of New South Wales is comprised mainly of Permian and Triassic marine to freshwater clastic sedimentary rocks. Within this sequence there are six latite extrusive units, several medium‐sized monzonite intrusions and a large number of small to medium‐sized basic to intermediate intrusions. Thin basaltic flows were extruded onto the Tertiary topographic surface. All of these rocks are relatively undeformed. Radiometric (K‐Ar) dating has previously been carried out on Mesozoic and Tertiary intrusions and flows of the southwestern portion of the Sydney Basin. However, relatively few Permian, and no post‐Permian, K‐Ar dates have been published for the southeastern portion of the basin. The present investigation provides nine K‐Ar dates from the latter area. Four extrusive and intrusive units have been confirmed as Permian in age (238 ± 6; 241 ± 4; 245 ± 6; and 251 ± 5 m.y.). Five post‐Permian (on stratigraphic criteria) intrusions yielded Tertiary ages (26.2 ± 3.0...

Analysis of emplacement mode in basalt from Deep-Sea Drilling Project Holes 319A and 321 using anisotropy of magnetic susceptibility

The anisotropy of magnetic susceptibility has been measured in three suites of basaltic specimens: (1) 312 specimens from known intrusives and extrusives, (2) 22 samples from a sill and underlying lava recovered from the 580-m depth in the Bermuda deep drill 1972 hole, and (3) 43 samples from hole 319A and 321 of the Deep-Sea Drilling Project. A new anisotropy of magnetic susceptibility parameter (F) is proposed, and it is shown, by using sample suites 1 and 2, that diagnosis of emplacement modes in basaltic materials can be made with 80% confidence. Measurements of F in samples from Deep-Sea Drilling Project leg 34 suggest that at least one intrusive and two extrusive units have been sampled between the 95- and 155-m depths in hole 319A and that material recovered between the 125- and 130-m depths in hole 321 is of an intrusive nature. Contrasts in morphology of the titanomagnetites, as observed in polished thin sections, support these interpretations.

Geology and Petrology of the Cambria Felsite, a New Oligocene Formation, West-Central California Coast Ranges

Geologic mapping in the Cambria and Black Mountain areas, western San Luis Obispo County, California, has led to recognition of the two stratigraphically and petrologically distinct extrusive felsic igneous units: (1) the Obispo Formation; and (2) a new unit, named here the Cambria Felsite. The Obispo Formation was deposited above the Rincon Shale and below the Monterey Formation during middle Miocene time. The Cambria Felsite rests with angular unconformity on Franciscan rocks of Jurassic and (or) Cretaceous age, and occurs as clasts in the nonmarine Lospe Formation of Oligocene age. Textural and mineralogic intergradations exist between the Cambria Felsite and the nearby upper Oligocene hypabyssal volcanic necks, plugs, lava domes, and dikes of the Morro Rock–Islay Hill complex; this suggests that the Cambria Felsite represents an effusive equivalent of the Morro Rock–Islay Hill. Cambria tuffs are enriched in alkalis plus silica and are depleted in ferromagnesian constituents relative to samples of the Morro Rock type. Chemical contrasts in the two units probably arose either because of a winnowing of airborne crystals from the glassy shards or because of magma fractionation in the conduits followed by pyroclastic eruption of the upper portions of the melt columns. No obvious correlation exists between the two-stage silicic volcanism, separated by a 10-m.y quiescent interval, and middle Tertiary plate tectonics. However, generation of these igneous rocks was probably a thermal response of the western margin of the continental crust-capped Americas plate to underflow of the Farallon plate, resulting in Morro Rock and Cambria units, followed by a complex encounter with the East Pacific Rise, resulting in the Obispo Formation.

Santiaguito Volcanic Dome, Guatemala

The Santiaguito volcanic dome insouthwestern Guatemala is a multiple extrusive dome that has shown constant volcanic activity since its birth in 1922. Fourteen extrusive units are mapped. Five of these are volcanic domes; the remaining nine are lava flows which generally cling to the sides of the domes. The volume of material extruded at Santiaguito since 1922 (0.7 km 3 ) is a small fraction of the volume of pyroclastic debris from the 1902 eruption of Santa Maria (5.5 km 3 ), Santiaguito9s parent composite cone. Extrusion of the dome began in the center of the explosion crater created by Santa Maria9s 1902 activity. This crater was volumetrically much smaller (0.5 km 3 ) than the amount of material erupted during the 1902 event, and local slumping near the crater has occurred and is continuing along a series of east-trending faults. The general westward growth of the dome complex and many of the structural featureson Santiaguito are controlled by these near-vertical faults. The domes are studded with Pelean spines. Twenty-five new chemical analyses are presented, showing Santiaguito9s eruptive products to be soda-rich dacite of the calc-alkaline suite. The dome lava has differentiated quite significantly from the overwhelmingly abundant pyroxene andesite magma which makes up Santa Maria and the older volcanic rocks in the area. Trace element analyses of the lavas, along with major element data suggest a differentiation by fractional crystallization under constant or increasing PO 2 . Sr isotope determinations could not detect contamination of the lavas by radiogenic crustal material; the Sr 87 /Sr 86 ratio in Santiaguito rock averages .7043. The Santiaguito lavas are tridymite-bearing hypersthene dacites, characterized by strongly zoned plagioclase phenocrysts and oxyhornblende. Compositional and petrographic comparisons are made with other modern volcanic domes and other Central American lavas.