Funnel-shaped Intrusion Research Articles

Incremental growth of an upper crustal, A-type pluton, Argentina: Evidence of a re-used magma pathway

Carboniferous igneous activity in the Sierra de Velasco (NW Argentina) led to the emplacement of several magmas bodies at shallow levels (<2kbar). One of these, the San Blas intrusive complex formed over millions of years (≤2–3m.y.) through three periods of magma additions that are characterized by variations in magma sources and emplacement style. The main units, mostly felsic granitoids, have U-Pb zircon crystallization ages within the error range. From older to younger (based on cross-cutting relationships) intrusive units are: (1) the Asha unit (340±7Ma): a tabular to funnel-shaped intrusion emplaced during a regional strain field dominated by ~WSW-ENE shortening with contacts discordant to regional host-rock structures; (2) the San Blas unit (344±2Ma): an approximate cylindrical-shaped intrusion formed by multiple batches of magmas, with a roughly concentric fabric pattern and displacement of the host rock by ductile flow of about 35% of shortening; and (3) the Hualco unit (346±6Ma): a small body with a possible mushroom geometry and contacts concordant to regional host-rock structures. The magma pulses making up these units define two groups of A-type granitoids. The first group includes the peraluminous granitic rocks of the Asha unit generated mostly by crustal sources (εNdt=−5.8 and εHft in zircon=−2.9 to −4.5). The second group comprises the metaluminous to peraluminous granitic rocks of the youngest units (San Blas and Hualco), which were formed by a heterogeneous mixture between mantle and crustal sources (εNdt=+0.6 to −4.8 and εHft in zircon=+3 to −6). Our results provide a comprehensive view of the evolution of an intrusive complex formed from multiple non-consanguineous magma intrusions that utilized the same magmatic plumbing system during downward transfer of host materials. As the plutonic system matures, the ascent of magmas is governed by the visco-elastic flow of host rock that for younger batches include older hot magma mush. The latter results in ductile downward flow of older, during rise of younger magma. Such complexes may reflect the plutonic portion of volcanic centers where chemically distinct magmas are erupted.

Geochemistry and petrogenesis of Neoproterozoic Mylliem granitoids, Meghalaya Plateau, northeastern India

The Mylliem granitoids of the Meghalaya Plateau, northeastern India, represent one of the disharmonic Neoproterozoic igneous plutons, which are intrusive into low-grade Shillong Group of metasediments. Field studies indicate that the Mylliem granitoids cover an area of about 40 km2 and is characterized by development of variable attitude of primary foliations mostly marked along the margin of the pluton. Xenoliths of both Shillong Group of metasediments and mafic rocks have been found to occur within Mylliem granitoids. Structural study of the primary foliation is suggestive of funnel-shaped intrusion of Mylliem granitoids with no appreciable evidence of shearing. Petrographically, Mylliem granitoids are characterized by pink to white phenocrysts of prismatic microcline/perthite and lath-shaped plagioclase (An20–An29). Groundmass material is characterized by quartz, microcline, plagioclase, muscovite and biotite. Sphene and apatite occur as accessory minerals. Petrographically Mylliem granitoids have been discriminated as granite and granodiorite according to IUGS system of classification. Critical evaluation of geochemical data and variation trends of major oxides/trace elements suggests a significant role of fractional crystallization in the evolution of Mylliem pluton. Th/U ratios (3.22–6.77) indicate a relatively higher abundance of Th over U. Chondrite-normalized REE diagram characteristically shows an enriched LREE pattern and prominent negative Eu anomaly (Eu/Eu* = 0.16–0.42) indicating the significant role of plagioclase fractionation from the parent magma. An overall strong REE fractionation pattern has been envisaged for Mylliem granitoids. The strong REE fractionation of the Mylliem granitoids is depicted by (Ce/Yb) N values, which show a range of 1.39 to 1.65. The aluminium saturation index (ASI) (ranging from 1.0 to 1.3), A/CNK ratios (ranging from 1.4 to 2.11) and A/NK ratios (ranging from 1.75 to 2.43) provide evidences for the peraluminous, S-type nature of the Mylliem granitoids. The peraluminous, S-type character is further supported by geochemical parameters such as Fe* and MALI (modified alkali lime index). Normative corundum > 1.0 wt.% is suggestive of the S-type nature of Mylliem granitoids. This is indicative of parent melt-extraction from metasedimentary source rocks by partial melting. Distinct geochemical parameters suggest a post-orogenic tectonic environment for the Mylliem granitoids. The peraluminous, calc-alkalic to alkali-calcic, post-orogenic Mylliem granitoids are geochemically correlatable with the post-orogenic Caledonian granitoids of Ireland and Britain.

Origin of the giant Allard Lake ilmenite ore deposit (Canada) by fractional crystallization, multiple magma pulses and mixing

The late-Proterozoic Allard Lake ilmenite deposit is located in the Havre-Saint-Pierre anorthosite complex, part of the allochtonous polycyclic belt of the Grenville Province. Presently the world's largest Fe–Ti oxide deposit, it had a pre-mining amount in excess of 200 Mt at grades over 60 wt.% hemo-ilmenite. The main ore body is a funnel-shaped intrusion, measuring 1.03 × 1.10 km and 100–300 m-thick. Two smaller bodies are separated by faults and anorthosite. The ore is an ilmenite-rich norite (or ilmenitite) made up of hemo-ilmenite (Hem 22.6–29.4, 66.2 wt.% on average), andesine plagioclase (An 45–50), aluminous spinel and locally orthopyroxene. Whole-rock chemical compositions are controlled by the proportions of ilmenite and plagioclase ± orthopyroxene which supports the cumulate origin of the deposit. Ore-forming processes are further constrained by normal and reverse fractionation trends of Cr concentration in cumulus ilmenite that reveal multiple magma emplacements and alternating periods of fractional crystallization and magma mixing. Mixing of magmas produced hybrids located in the stability field of ilmenite resulted in periodic crystallization of ilmenite alone. The unsystematic differentiation trends in the Allard Lake deposit, arising from a succession of magma pulses, hybridisation, and the fractionation of hemo-ilmenite alone or together with plagioclase suggest that the deposit formed within a magma conduit. This dynamic emplacement mechanism associated with continuous gravity driven accumulation of Fe–Ti oxides and possibly plagioclase buoyancy in a fractionating ferrobasalt explains the huge concentration of hemo-ilmenite. The occurrence of sapphirine associated with aluminous spinel and high-alumina orthopyroxene (7.6–9.1 wt.% Al 2O 3) lacking exsolved plagioclase supports the involvement of a metamorphic overprint during the synchronous Ottawan orogeny, which is also responsible for strong textural equilibration and external granule of exsolved aluminous spinel due to slow cooling.

An Anorthosite Suite in a Ring-Complex: Crystallization and Emplacement of an Anorogenic Type from Abontorok, A r, Niger

Laminated anorthosite grading outwards into leucogabbro, gabbro, and monzogabbro occurs in a 2.6-km-diameter funnel-shaped intrusion, cut by a quartz alkali syenite plug and concentric syenite and granite ring-dykes. The anorthosite-gabbro series is laminated but not modally or otherwise texturally layered. The lamination, defined by large tabular plagioclase crystals, forms a set of inwarddipping cones, the dips of which decrease from 60–45° in the central anorthosite to < 25° in the outer gabbros. Rocks close to the outer contact are medium-grained isotropic gabbros. Plagioclase, forming >80% of the series, generally has homogeneous labradorite cores (An62–58 in the whole series) and thin strongly zoned rims, which follow progressively longer solidus paths from the anorthosites to the gabbros. All rocks contain a late-magmatic alkali feldspar. Plagioclase is the main or only cumulus phase, the anorthosites being ad- to mesocumulates and the gabbros orthocumulates. Olivine (FO49–41) is more abundant than clinopyroxene in most of the series. Depending on quartz content, the syenites and granites are hypersolvus or subsolvus and the depth of crystallization was calculated to be 5 ± 2 km. A Rb/Sr isochron for the syenites and granites gave an age of 399 ± 10 Ma with an initial strontium isotopic ratio of 0.7084 ± 0.0005. Ten samples from the anorthosite-gabbro scries have an average calculated initial ratio of 0.70582 ± 0-00004 at − 400 Ma, showing that the two series are not comagmatic. The anorthosite-gabbro series has parallel REE trends (LaN/YbN ˜ 7–10) with decreasing positive Eu anomalies and increasing total REE contents from anorthosite to gabbro; two monzogabbros have almost no Eu anomaly. The liquid calculated to be in equilibrium with the lowest anorthosite has almost no Eu anomaly and its normalized REE pattern lies just above those for the monzogabbros. The syenites and granites have complementary REE patterns with negative Eu anomalies. The inferred parental magma was alkalic and leucotroctolitic with high TiO2 P2O5, Sr and K/Rb and with low MgO, very similar to parental magmas in the Gardar province, South Greenland. It was probably produced at depth by settling of olivine and clinopyroxene but not of plagioclase, which accumulated by flotation. It is suggested that plagioclase crystals from this lower chamber were progressively entrained (from 0% in the gabbros to 30–40% in the anorthosites), giving rise to the flow lamination in the upper chamber. The magma in the lower chamber may have been layered, because the plagioclase cores in the anorthosite are considerably richer in Or than those in the leucogabbros or gabbros. Overall convection did not occur in the upper chamber, whereas compositional convection occurred in the more slowly cooled central anorthositic adcumulates.

The Kemi stratiform chromitite deposit, northern Finland

The Kemi chromite deposit is hosted by an early Proterozoic (2.44 Ga) layered intrusion. The lower part of the intrusion is composed of peridotitic and pyroxenitic cumulates and chromitite layers. Olivine and chromite have not been observed in the upper part, where plagioclase is the predominant cumulus phase occurring either alone or together with pyroxenes. The intrusion was metamorphosed during the Svecokarelidic orogeny. Mafic minerals of the lower and upper parts of the complex are completely altered to chlorite, serpentine, talc, amphiboles, and carbonates, whereas the middle part is well preserved. The chromite grains, too, suffered from alteration, but the cores of the grains still exhibit primary compositions.A continuous chromitite layer can be traced for the entire length of 15 km of the intrusion. It varies in thickness from a few millimeters to as much as 90 m in the central part of the intrusion. The mineable thicker part of the main chromitite layer is about 4.5 km long. The main chromitite layer is overlain by a sequence of thin chromite-rich layers which continues upward to the stratigraphic level of 500 m above the basal contact of the intrusion. The upper part of the main chromitite is layered, but the lower part is more massive and brecciated.The ore reserves for open-pit mining are ca. 40 million metric tons with an average grade of 26.6 percent Cr 2 O 3 and a Cr/Fe ratio of 1.53. The whole area has additional reserves of about 110 million metric tons of ore. The ore zone is intensely fractured. The host rocks have undergone alteration, which has degraded the quality of the ore.The present surface section of the Kemi intrusion is interpreted as a cross section typical of a funnel-shaped intrusion and plunging about 70 degrees to the northwest. The cumulate sequence, especially the main chromitite layer, is at its thickest in the middle of the intrusion, and the magmatic conduit which fed the intrusion is also thought to have been located just below this thickening.It is suggested that the chromitite was deposited when the input of magma into the Kemi intrusion was contaminated by salic material from the underlying basement complex. This magma mixed with the fresh input of primitive magma resulting in chromite saturation. The chromite crystals formed during mixing in a plume according to the model of Huppert et al. (1986) and accumulated preferentially around the magmatic conduit. Evidence of contamination is found in chromite grains in the form of small inclusions, rich in alkalies, which are thought to represent trapped droplets of the contaminant salic melt.

The Cuillin layered igneous complex—evidence for multiple intrusion and former presence of a picritic liquid

Synopsis Much of the western Cuillin is formed by part of a funnel-shaped intrusion comprising layered ultrabasic rocks which range upwards from dunite to peridotite to allivalite (feldspar-olivine cumulate). The funnel-shaped intrusion has a marginal border group on which allivalites are banked. Recent mapping indicates that a peridotite mass previously regarded as a sill, is part of this layered sequence, and that inner, ultrabasic cumulates to the E are merely a downthrown portion of the same funnel-shaped mass. The eastern rocks were cut by a later pyroxene-rich magma from which feldspar-olivine-pyroxene cumulates were formed. In the lower dunite-peridotite-allivalite series (Unit 1), cumulus olivine and feldspar range from Fo 90–86 and An 91–85 , respectively. In the upper unit (Unit 2) of feldspar-olivine-pyroxene cumulates cryptic variation is more complex; feldspar cores are An 84–83 , and olivines range from Fo 77–67 . The high level of the Cuillin complex suggests that there was insufficient room for a layered basic intrusion some 10,000 metres thick. Evidence is presented for successive pulses of ultrabasic magma, indicating that the complex as a whole comprises intrusions of tholeiitic affinity ranging broadly with time from ultrabasic to gabbroic, rather than a tectonically disrupted single, layered basic intrusion.

Paleomagnetic evidence for tectonic rotation of the Belchertown Pluton, west central Massachusetts

A paleomagnetic study of the Belchertown pluton, a syntectonic Devonian intrusion in the Bronson Hill anticlinorium, Massachusetts, indicates the presence in 23 samples from eight sites in primary quartz monzodiorites of a strong, stable, and consistently oriented remanent magnetization vector (D = 154.9°, I = −13.4°, K = 20.8° and α95 = 12.4°) with a high Q value (7.0). The remanence is inferred to be of thermal and/or thermochemical origin acquired during the original crystallization and cooling of the pluton. Recrystallized equivalents of the primary rocks, which occur as a surrounding border zone, yield weak and inconsistent results (four samples from two sites) due to the absence of opaque minerals. The mean Belchertown vector is discordant with respect to the mean North American Devonian vector (D = 170° and I = +32°) in such a direction as to suggest that the entire pluton has undergone some 30°–60° of rotation to the northwest about a NE-SW axis. The high Q value of the primary rocks and their isolation from magnetic country rocks by the surrounding non-magnetic border zone allow computer modeling of the three-dimensional shape of the pluton using the measured remanent vector and the total field aeromagnetic anomaly. The resulting model is consistent with the proposed amount and direction of tectonic rotation of a once roughly vertically symmetrical funnel-shaped intrusion.

The Foyers granitic complex, Inverness-shire, Scotland

The Foyers 'Newer Granite' is a composite pluton consisting of tonalite, granodiorite and adamellite emplaced in that order. The complex evolved by crystal-differentiation from a series of cooling magmas which were additionally modified by contamination with appinite, microdiorite and semi-pelite. A distensive foliation was progressively imparted to the units of the complex as they solidified, followed by post-magmatic alkali autometasomatism which extended into the envelope. Parts of the roofing envelope remain and the outer, largely tonalitic mantle to the inclined funnel-shaped intrusion conforms structurally with the envelope. After some uplift the adamellite was emplaced under brittle conditions, but was also subjected to some distension. Intense movements along ne–sw faults before and after the deposition of the local Old Red Sandstone complicate relations on the north-western margin of the complex, which is truncated by the Great Glen fault. The respectively high and low structural levels of exposure of the Foyers and Strontian intrusions complicate comparison, but there are sufficient chemical and structural similarities to confirm their former unity before being displaced laterally and vertically by the Great Glen fault.

Origin of the White Allivalite, Western Cuillin, Isle of Skye

SUMMARYTholeiitic contact rocks and a wide zone (White Allivalite) rich in primary precipitate calcic plagioclase are marginal to a layered allivalite series in a funnel-shaped intrusion. The high structural level apparently precludes the possibility that the ultrabasic layered rocks are the basal part of an accumulate from a tholeiitic magma. It is suggested, therefore, that a suspension of calcic plagioclase in a liquid more basic than basalt gave rise to an approximately equal volume of layered ultrabasic rocks by fractional crystallization. Emplacement was probably initiated as a tholeiitic cone-sheet complex, followed by the injection of the ultrabasic suspension in a volume sufficiently large to form a funnel-shaped body within the cone-sheet complex.

A Study of the Strathcona Mine and Its Bearing on the Origin of the Nickel-Copper Ores of the Sudbury District, Ontario

A Study of the Strathcona Mine and Its Bearing on the Origin of the Nickel-Copper Ores of the Sudbury District, Ontario

The Loon Lake Pluton, Bancroft Area, Ontario, Canada

The Loon Lake batholith, which is supposed to be part of larger granitic masses underlying the Canadian Shield, was found to be a small, funnel-shaped intrusion. Its structure and shape is in complete accordance with the structure of the surrounding sediments and metamorphics.