|Author:||Charette and Beaudette, 2018|
|Geological province:||Churchill Province|
|Geological subdivision:||Ungava Orogen / Narsajuaq Arc|
|Lithology:||Intermediate to mafic granulitic rocks|
Table des matières
The Pingasualuit Complex was introduced by Charette and Beaudette (2018) to describe intermediate to mafic foliated granulitic rocks forming a large part of the Narsajuaq Arc. These were previously included in the older plutonic suite (pPNAR1) and unit pPNAR2 of the younger plutonic suite, described by St-Onge and Lucas (1992) and St-Onge et al. (1992).
The Pingasualuit Complex consists primarily of banded or locally gneissic hypersthene quartz diorite and hypersthene diorite (pPpgs2), as well as metric to kilometric horizons of mesocratic gabbronorite and hypersthene diorite (pPpgs1). The majority of this complex is characterized by a paragenesis characteristic of the granulite facies. In places, mafic to ultramafic klippes at the amphibolite facies, possibly retromorphosed, are included in subunit pPpgs1a.
Unit pPpgs1 consists of klippes and hectometric to kilometric horizons of mesocratic and granoblastic gabbronorite and hyperstene diorite usually present in the main unit of the Pingasualuit Complex (pPpgs2). These mafic rocks are dark greenish grey to brownish black in altered surface. The rock is heavily recrystallized, fine to medium grained and contains 50 to 65% mafic minerals. It also has a homogeneous, speckled or foliated to banded appearance. Banding is produced by a variation in the modal proportion of mafic minerals (35-75%). On the other hand, the speckled appearance is due to millimetric to centimetric clusters of mafic minerals and recrystallized plagioclase clusters of less than one centimetre. Leucosomes occuring as 2 mm to 10 cm diffuse clusters form up to 10% of the rock. They are slightly coarser grained than the matrix. They are recrystallized, oriented to foliation and include isolated crystals or clusters of clinopyroxene, orthopyroxene and hornblende. Locally, garnet clusters are observed. In thin section, minerals are strongly recrystallized. Hornblende and clinopyroxene are the principal mafic minerals, followed by orthopyroxene and biotite. Clinopyroxene is commonly amphibolitized and orthopyroxene is partially replaced by serpentine and oxides. Hornblende marks the foliation and occurs as poikiloblastic crystals in some samples. Very few accessory minerals are observed (opaque minerals, apatite and carbonates).
Pingasualuit Complex 1a (pPpgs1a): Gabbro, Diorite and Associated Ultramafic Rocks
Subunit pPpgs1a consists of klippes of intermediate to mafic rocks without orthopyroxene in the vicinity or within units of the Pingasualuit Complex. Unlike rocks of unit pPpgs1, rocks of subunit pPpgs1a do not have the brownish colour typical of altered orthopyroxene rocks. Their alteration patina ranges from greenish black to dark green. In thin section, the presence of serpentine clusters and orthopyroxene cores locally preserved in hornblende crystals suggests that they are granulitic rocks retromorphosed to the amphibolite facies. With the exception of their colour, the appearance of these rocks in outcrop differs little from the rest of unit pPpgs1. They are banded or homogeneous and foliated. A speckled facies with clusters of mafic minerals and small centimetric plagioclase crystals is also observed. In some places, in this facies, mafic mineral clusters are centimetric and joint, giving a recrystallized cumulate appearance. Ultramafic horizons and boudins are locally present in gabbro and diorite of subunit pPpgs1a. A few fractured olivine crystals (less than 10%) partially replaced by iddingsite and magnetite are observed in thin section in a websterite.
No orthopyroxene was observed in the area north of the Naujaat Shear Zone. Lithologies in this area are foliated, homogeneous and have, in places, discontinuous leucosomes indicating partial melting. Microscopic observations identify hornblende, actinolite and clinopyroxene as the main mafic minerals. Clinopyroxene is variably amphibolitized and biotite replaces hornblende in fracture and cleavage planes.
Unit pPpgs2 includes hypersthene quartz diorite and hypersthene diorite, fine to medium grained, usually banded and granoblastic. They form the majority of the Pingasualuit Complex. These rocks are greenish to brownish grey in altered surface and have a foliated to gneissic fabric, as well as banding marked by the presence of mafic mineral laminae and millimetric to centimetric leucocratic bands. These are brownish to whitish beige in altered surface and generally vary from a quartz dioritic to tonalitic composition, even granitic locally. Some of these bands are fine grained, conformable to foliation and in diffuse contact in the matrix. They occur as lenticular clusters in places and commonly include orthopyroxene, clinopyroxene and magnetite crystals. These thin bands, interpreted as leucosomes derived from melting of intermediate granulitic rocks of the Pingasualuit Complex, account for 5 to 20% of the rock. Other bands are slightly coarser grained than the fine-grained matrix, and they are subconformable to or cutting foliation and in sharp contact. In places, these coarser-grained bands include quartz bands. At the outcrop scale, banding is also marked by compositional variations produced by variation of mafic mineral contents (10-40%) within quartz diorite or hypersthene diorite. Centimetric to metric horizons of pPpgs1 gabbronorite are present in sharp or diffuse contact with granulitic intermediate rocks. These horizons are usually more competent and less banded than host rocks.
In diffuse contact with the banded main facies, a second facies of hypersthene quartz diorite is medium grained, homogeneous and speckled. This facies is characterized by millimetric clusters of mafic minerals and slightly coarser plagioclase crystals producing the speckled appearance. The modal proportion of mafic minerals (10-20%) is generally lower in this facies than in the main facies and the granoblastic texture is less developed.
Petrographic studies and geochemical analyses indicate that the composition of unit pPpgs2 varies from quartz dioritic, dioritic and quartz monzodioritic to tonalitic or monzodioritic locally. In places, these variations are observed in diffuse centimetric bands within the same sample. When present, K-feldspar is distributed heterogeneously as fine crystals disseminated in the matrix, as inclusions in plagioclase or as fine laminae. Plagioclase forms, in places, millimetric (3 to 7 mm) rounded crystals that are coarser than the fine-grained matrix, while mafic minerals occur as discontinuous millimetric laminae and are disseminated in the matrix. In thin section, minerals are moderately to well recrystallized in general. Hornblende and pyroxene are the main mafic minerals followed by biotite. The modal proportion of these minerals varies by band. Hornblende either occurs as recrystallized even-grained crystals or as crystals with a poikilitic core and a rim without inclusions. Pyroxene crystals are commonly elongated along primary foliation. Specifically, orthopyroxene is fractured and partially replaced by iddingsite and oxides. Biotite forms rosettes with quartz intergrowth and is observed as a partial replacement for orthopyroxene or hornblende. Locally, garnet is present in small stocky crystals in equilibrium contact with hornblende. Epidote locally forms small stocky crystals associated with mafic laminae or clusters. In places, chlorite is replacing hornblende, biotite and garnet in fractures.
Pingasualuit Complex 2a (pPpgs2a): Intermediate Granulitic Gneiss
Subunit pPpgs2a consists of granulitic banded sequences of felsic to intermediate composition whose fine grain size and compositional variations make the origin of the protolith uncertain. These banded sequences may be paragneiss, although they may include orthogneiss. In outcrop, these rocks have a brownish to rusty alteration patina and are friable. In fresh exposure, they are medium to medium-dark greenish grey. Straight-lined banding is produced by the presence of 5 to 20% millimetric to centimetric leucosomes with biotite ± garnet ± orthopyroxene rims. Also, several outcrops are characterized by alternating horizons showing variations in the proportion of mafic minerals and the presence or absence of garnet. Some horizons are very rich in quartz (over 70%), while others are very rich in garnet (up to 40%). Locally, horizons of diatxite with biotite-garnet schlierens are observed within granulitic gneiss.
Like unit pPpgs2, rocks of subunit pPpgs2a exhibit variations in the composition of diffuse centimetric bands of tonalite, granite, quartz diorite and locally quartz monzodiorite. Mafic minerals are disseminated or occur as fine laminas and account for 10 to 30% of the mineralogy. Biotite and orthopyroxene are the principal mafic minerals. In places, the latter is partially replaced by iddingsite and oxides or hornblende. Hornblende and garnet crystals are present locally. Garnet occurs as porphyroclasts and is sometimes poikilitic. Several indications of partial melting were observed in thin section such as irregular quartz films in the matrix and inclusions in garnet, the shape of which is dictated by its crystalline structure.
Hypersthene quartz diorite and hypersthene diorite of unit pPpgs2 account for most of the granulitic rocks present in the Narsajuaq Arc. They form kilometre thick bands and klippes, elongated and folded in the E-W regional fabric. They include centimetric to metric horizons or enclaves of mesocratic gabbronorite and hypersthene diorite. They also form kilometric bands and klippes that have been grouped into unit pPpgs1. A significant number of these intermediate to mafic rocks is present near the contact with the Kovik Antiform. The orthopyroxene-free facies of subunit pPpgs1a is present in significant proportion north of the Naujaat Shear Zone, where it forms kilometric klippes in association with the Saint-Hélène Complex.
A homogeneous, medium-grained, foliated quartz monzodiorite present in a sequence of intermediate granulitic rocks (pPpgs2) and a fine-grained granoblastic quartz diorite were sampled by Charette and Beaudette (2018) to date the Pingasualuit Complex. The analytical results are on hold. No other datings have been made previously.
The intermediate to mafic granulitic rocks of the Pingasualuit Complex appear to be one of the oldest units in the Narsajuaq Arc. These rocks are injected bed by bed and cut by leucocratic enderbite intrusions assigned to the Navvaataaq Suite (pPnav) and by foliated to massive granitoids of the Sanningajualuk Suite (pPsnn). They are also observed as enclaves in these suites.
Does not apply.