Background

Description

Pingasualuit Complex 1 (nApgs1): Mesocratic Gabbronorite and Hypersthene Diorite

Unit nApgs1 consists of gabbronorite and hyperstene diorite, more locally of jutonite, dark greenish grey to dark brownish. The rock is even grained and at the limit between fine and medium grained. The rock is commonly homogeneous and foliated, but also banded. Banding is produced by varying modal contents of ferromagnesian minerals (40-70%)or by the prsence of diffuse leucosomes (<10%), which forms millimetric to centimetric clusters or bands. They are slightly coarser grained than the matrix. They are partially recrystallized, oriented along foliation and contain clinopyroxene, orthopyroxene and hornblende isolated crystals or clusters. Locally, garnet clusters or millimetric crystals are observed. In the Cape Wolstenholme area, Charette and Beaudette (2018) also assigned speckled rocks to unit nApgs1. This appearance is due to millimetric to centimetric clusters of ferromagnesian minerals and recrystallized plagioclase clustersf <1 cm. Rocks displaying this texture in the Sirmiq Lake area were instead assigned to unit nApgs5 (Vanier and Lafrance, 2020).

In thin sections, the rock texture is polygonal to interlobed. Hornblende and clinopyroxene are the main ferromagnesian minerals, followed by orthopyroxene and biotite. Clinopyroxene is commonly amphibolitized and orthopyroxene is partially replaced by serpentine and oxides. Hornblende is slightly coarser than other minerals and marks foliation. It occurs as poikiloblastic crystals in some samples. Accessory minerals are sparse, the main ones being opaque minerals, apatite, garnet, quartz and carbonates.

 

Pingasualuit Complex 2 (nApgs2): Hypersthene Quartz Diorite and Hypersthene Diorite

In diffuse contact with the banded phase, a second phase of unit nApgs2 is homogeneous, sligtly coaser-grained and speckled. This phase is characterized by millimetric clusters of ferromagnesian minerals and slightly coarser plagioclase crystals producing the speckled appearance. The modal proportion of ferromagnesian minerals (10-20%) is generally lower than in the main phase. 

In thin sections, grains exhibit an interlobed to partially granoblastic fabric. Plagioclase locally forms rounded millimetric crystals (3-7 mm) coarser than the matrix, while ferromagnesian minerals are in discontinuous millimetric laminae, in clusters aligned in foliation, and disseminated in the matrix. If present, K-feldspar is distributed heterogeneously as fine crystals disseminated in the matrix, as inclusions in plagioclase or as fine laminae. Hornblende and pyroxene are the main ferromagnesian minerals, followed by biotite. The modal proportion of these minerals varies by band. In some cases where pyroxene is not uralitized, hornblende can be absent. The latter either occurs as recrystallized even-grained crystals or as crystals with a poikilitic core and a rim without inclusions. Pyroxene crystals are commonly elongated in the primary foliation. Specifically, orthopyroxene is fractured and partially replaced by serpentine and oxides. Biotite forms rosettes including quartz intergrowth and is observed partially replacing orthopyroxene or hornblende. Locally, garnet is present in small stocky crystals in equilibrium contact with hornblende. Epidote locally forms small stocky crystals associated with ferromagnesian laminae or clusters. In places, chlorite is replacing hornblende, biotite and garnet in fractures.

 

 

 

 

 

Pingasualuit Complex 3 (nApgs3): Migmatized Garnet Gabbro and Gabbronorite

Unit nApgs3 differs from other units of the Pingasualuit Complex by the greater presence of garnet, significant compositional variations and higher migmatization. Outcrops commonly exhibit alternating decametric to centimetric melanocratic, mesocratic and leucocratic gabbro (or gabbronorite), as well as local ultramafic rock layers. The garnet content also varies widely with composition, with some layers not even containing garnet. This banding is now transposed into foliation. It possibly reflects original variations of the unit, indicating contemporaneous intrusion of different magmas. Indeed, zones of magma mixing have been observed locally.

Gabbro and gabbronorite are also migmatized to varying degrees. The leucosome most commonly forms coarser and discontinuous clusters and bands millimetre to centimetre wide. They contain garnet, clinopyroxene or hornblende porphyroblasts. Stromatic areas were also observed locally. Garnet may be present in all phases of unit nApgs3, but the highest contents are associated with the leucosome, where it forms porphyroblasts up to 4 cm in diameter. They are commonly rimmed by clinopyroxene ± hornblende.

In thin sections, the general fabric is mostly even grained and granoblastic to interlobed (partial recrystallization), although coarser clinopyroxene and plagioclase crystals are still present. Plagioclase usually forms monomineralic clusters, locally lenticular, between ferromagnesian mineral clusters. These systematically include clinopyroxene, hornblende and fine magnetite grains. One sample contains orthopyroxene in smaller amounts. The latter can form large crystals or occur as highly serpentinized relics. Hornblende replaces clinopyroxene; it commonly forms a thin rim around it. In some layers, uralitization is more extensive and hornblende is the main ferromagnesian mineral.

Garnet occurs as millimetric grains and fractured subhedral poikiloblasts containing some plagioclase, clinopyroxene and orthopyroxene inclusions. Garnet is locally rimmed by hornblende-plagioclase symplectites. Locally, such symplectites are also observed without any residual garnet, or a very fine core. Except for magnetite (1-3%), accessory minerals are not abundant and include apatite, biotite and quartz.

 

Pingasualuit Complex 3a (nApgs3a): Garnetite and Metasomatic rocks

Subunit 3a comprises two main phases of metasomatized rocks within the Pingasualuit Complex. The first alteration phase consists of garnetite containing quartz (20-35%), clinopyroxene (<20%) and plagioclase (5-15%) in transitional contact with a garnet gabbro from unit nApgs3. On the same outcrop, garnet (20-60%) may be purplish pink or brown; these two types of crystals are in sharp contact. It occurs as fine grains aligned in banding or locally fractured porphyroblasts. Quartz occurs commonly coarse bands having subgrain extinction. Biotite (<8%) is chloritized to varying degrees. Garnetite layers also contain chalcopyrite (<1%) disseminated and filling fractures, as well as malachite and azurite coatings, and carbonate veinlets.

The second alteration phase is also composed of garnet, clinopyroxene, quartz and plagioclase in varying amounts. It also includes K-feldspar (10-40%) and magnetite (1-5%). K-feldspar and quartz locally form interconnected centimetric bands parallel to foliation. Garnet is porphyroblastic.

Pingasualuit Complex 4 (nApgs4): Foliated Ultramafic Rocks

Ultramafic rocks are generally foliated, black or dark grey in fresh exposure and brown or buff in patina. They form klippes decimetre to >350 m thick within various units of the Narsajuaq Domain. Locally, decimetric to metric rusyt layers, containing disseminated or nodular sulphides (<12% pyrrhotite ± chalcopyrite ± pentlandite) and malachite were observed. Unit nApgs4 consists of three distinct phases: olivine websterite, websterite, and more locally, lherzolite. For these three phases, the rock can contain up to 5% plagioclase in interstitial clusters and be cut by carbonate-serpentine veinslet. If present, olivine is fractured, variably serpentinized (antigorite or iddingsite) and appears to be more highly affected by deformation than other mineral phases. Locally, olivine can also be replaced by a carbonate-talc-tremolite assemblage.

Olive websterite is composed of pyroxene crystals of various sizes and 15-30% olivine. The proportions of clinopyroxene and orthopyroxene also vary widely from sample to sample, although they are generally similar. Magnetite (2-5%), green to brown spinel (picotite, <2%) and phlogopite (<4%) are the main accessory minerals observed in thin sections. Almost half of the olivine websterite samples also contain 15-50% brown hornblende, the origin of which (primary or metamorphic) is difficult to determine. Where such hornblende is present, olivine and magnetite are less abundant.

Similar to olive websterite, websterite also contains pyroxene as large crystals or partially recrystallized clusters distributed heterogeneously. It is varoiusly urallitized in a mixture of actinolite and green hornblende concentrated at the edge of pyroxene crystals. Websterite also contains significantly less olivine (<5%). Some samples are almost completely amphibolitalized and are characterized by a greenish colour, and by the presence of large, poorly defined actinolite zones with darker green crystals of pseudomorph hornblende on pyroxene. Secondary minerals observed are carbonate, apatite and opaque minerals. Magnetism seems to diminish as rock amphibolitalization increases.

Finally, the last phase, lherzolite, is less abundant. The rock contains a higher content in olivine (50-70%) than clinopyroxene and orthopyroxene, which have varying contents. Picotite (2-3%), magnetite (<5%), phlogopite (<7%) and carbonate (filling) are the main accessory phases.In thin sections, plagioclase is partially recrystallized with large interlobed anhedral grains and granoblastic zones, commonly rimming them. Ferromagnesian mineral clusters are lenticular, usually stretched in deformation. These clusters are composed of green hornblende and clinopyroxene in all cases, but may also include varying amounts of orthopyroxene and biotite (<5%). Orthopyroxene is locally serpentinized and biotite occurs as miscellaneous and randomly oriented flakes. Green hornblende replaces clinopyroxene, which locally forms preserved cores within it. Pseudomorphs of anatectic liquid are present as inclusions in hornblende. Some samples also contain some quartz (<4%) and dark red garnet forming millimetric grains and porphyroblasts (1-4 cm in diameter). The latter is locally rimmed by clinopyroxene or hornblende-plagioclase symplectites. Opaque minerals (<2%) and apatite (<1%) are present in all cases. Other accessory phases are epidote, carbonate, scapolite, chlorite and allanite.

Pingasualuit Complex 5 (nApgs5): Speckled Gabbro and Gabbronorite

Gabbro and gabbronorite have a granular, speckled appearance caused by alternating millimetric to centimetric clusters of ferromagnesian and quartzofeldspathic minerals. These clusters are, however, partially recrystallized into finer grains. The rock is homogeneous, foliated, usually lineated, and typically contains 20-45% (locally 70%) ferromagnesian minerals.

Unit nApgs 5 is spatially associated with other units of the Pingasualuit Complex, but appears to have been less affected by migmatization (it may also be later). Centimetric to decimetric boudinaged gabbronorite layers similar to that of unit nApgs1 were observed in unit nApgs5. Where the speckled rock of unit nApgs5 is intruded by centimetric to decimetric granite dykes, garnet preferentially develops at the edges of intrusions.

In thin sections, plagioclase is partially recrystallized with large interlobed anhedral grains and granoblastic zones, commonly rimming them. Ferromagnesian mineral clusters are lenticular, usually stretched in deformation. These clusters are composed of green hornblende and clinopyroxene in all cases, but may also include varying amounts of orthopyroxene and biotite (<5%). Orthopyroxene is locally serpentinized and biotite occurs as miscellaneous and randomly oriented flakes. Green hornblende replaces clinopyroxene, which locally forms preserved cores within it. Pseudomorphs of anatectic liquid are present as inclusions in hornblende. Some samples also contain some quartz (<4%) and dark red garnet forming millimetric grains and porphyroblasts (1-4 cm in diameter). The latter is locally rimmed by clinopyroxene or hornblende-plagioclase symplectites. Opaque minerals (<2%) and apatite (<1%) are present in all cases. Other accessory phases are epidote, carbonate, scapolite, chlorite and allanite.

 

Thickness and Distribution

The pingasualuit Complex is the dominant unit of the Narsajuaq Domain. It forms kilometre-thick strips and klippes, elongated and folded in the regional fabric. In the Cape Wolstenholme area (Charette and Beaudette, 2018), the hypersthene diorite unit predominates, while it is subordinate in the Sirmiq Lake area (Vanier and Lafrance, 2020). Units nApgs3 to nApgs5 were only mapped in this area.

 

Dating

Two hypersthene quartz diorite samples were collected by Charette and Beaudette (2018) in the Cape Wolstenholme area. In both cases, U-Pb analyses give an age range from Neoarchean to Paleoproterozoic. Davis and Sutcliffe (2018) separatedzircons into two populations on the basis of Th/U ratios. The protolith is Neoarchean, and Paleoterozoic metamorphism resulted in Pb loss via diffusion.

 

Stratigraphic Relationship(s)

Intermediate to mafic granulitic rocks of the Pingasualuit Complex seems to form one of the oldest units of the Narsajuaq Domain. These rocks are injected in alternating layers and cut by leucocratic enderbite intrusions assigned to the Navvaataaq Suite and by foliated to massive granitoids of the Sanningajualuk Suite. They are also observed as enclaves in these suites. Contacts with units of the Estre Complex are poorly defined, but it is plausible that mafic rocks of the Pingasualuit Complex intruded gneiss of the Estre Complex and were transposed into gneissosity.

Paleontology

Does not apply.

References

 

Publications Available Trough SIGÉOM Examine

Other Publications

Suggested Citation

Contributors

First Publication	Benoit Charette, P. Geo., M.Sc. benoit.charette@mern.gouv.qc.ca; Mélanie Beaudette, GIT, B.Sc. melanie.beaudette@mern.gouv.qc.ca (redaction) Mehdi A. Guemache, P. Geo., Ph.D. (coordination); Mélina Langevin, GIT, B.Sc. (critical review); Simon Auclair, P. Geo., M.Sc. (editing); Ricardo Escobar Moran (HTML editing); Céline Dupuis, P. Geo., Ph.D. (English version).
Révision(s)	Isabelle Lafrance, P. Geo., M.Sc. isabelle.lafrance@mern.gouv.qc.ca; Marc-Antoine Vanier, Jr. Eng. marc-antoine.vanier@mern.gouv.qc.ca (redaction) Mehdi A. Guemache, P. Geo., Ph.D. (coordination); Benoit Charette, P. Geo., M.Sc. (critical review); Simon Auclair, P. Geo., M.Sc. (editing);  André Tremblay (HTML editing); Céline Dupuis, P. Geo., Ph.D. (English version).