The Diana Structural Complex, located in the NE of the Arnaud River area, was introduced by Madore and Larbi (2000, 2001) to describe an Archean lithological package reworked during the Proterozoic (Simard, 2008). The name comes from Diana Bay.

The Diana Structural Complex consists essentially of tonalitic orthogneiss with strips of paragneiss, amphibolite and ultramafic rocks. These rocks are generally mylonitized and migmatized (Madore and Larbi, 2000, 2001). Four informal units, APdia1 to APdia4, are recognized.

This unit consists of migmatitic metasedimentary rocks of unknown age comprising paragneiss, marbles and calcosilicate rocks that form strips transposed parallel to the structural fabric within unit APdia unit orthogneiss (Simard, 2008). In thin sections, a fine-grained quartzofeldspathic matrix is observed. Biotite porphyroblasts are parallel to foliation and represent 15 to 40% of the rock. Porphyroblastic garnet up to 5 mm is observed, which gives a poikolitic texture to the rock. Small amounts of epidote, calcite, muscovite, green hornblende, sphene and opaque minerals are visible (Madore and Larbi, 2000, 2001).

Diana Structural Complex 2 (APdia2): Amphibolite, Gneiss and Ultramafic Rocks

This informal unit consists of klippes of amphibolite, mafic gneiss and ultramafic rocks. Amphibolite is fine grained, homogeneous and well foliated. It is composed of hornblende (45-75%) and plagioclase (25-40%), but locally contains sphene, biotite or garnet (Madore and Larbi, 2000, 2001).

Diana Structural Complex 3 (APdia3): Mylonitized Gneissic Tonalite

The dominant facies of the Diana Structural Complex consists of tonalitic orthogneiss with associated diorite, trondhjemite and granodiorite. All of these rocks are highly deformed and exhibit a mylonitic foliation accompanied by a well-developed stretch lineation (Simard, 2008). In thin sections, orthogneiss has a well-developed fine-grained granoblastic texture. Relics of banded quartz and porphyroclastic plagioclase indicate ductile deformation. Porphyroclastic hornblende (<1-10%) and calcite (<1%) are present in places. Epidote (<1-3 %), muscovite (1-5 %) and biotite (5-15 %) are oriented according to the planar foliation (Madore and Larbi, 2000, 2001).

Diana Structural Complex 4 (APdia4): Monzonite

Porphyraceous monzonite and quartz monzonite, foliated or mylonitic (APdia4), are distributed in intrusive bodies a few kilometres long, elongated parallel to the regional fabric. These intrusive rocks, containing 20 to 40% orthose porphyroclasts, cut tonalitic gneiss (Simard, 2008). Evidence of intense recrystallization is visible in thin sections. The fine-grained quartzofeldspathic matrix gives a granoblastic texture to the rock, and banded quartz relics are present. Monzonite contains 20 to 40% porphyroclastic K-feldspar.

Thickness and distribution

The paragneiss unit (APdia1) forms strips up to ~5 km wide and 40 km long. This paragneiss unit is interstratified in strips of calcosilicate rocks that are no more than one metre thick and <50 m long. The ultramafic rock unit APdia2 is spatially associated with unit APdia1 paragneiss. With a surface area of <30 km² , ultramafic rocks forms small strips interstratified in paragneiss (Simard, 2008). Unit APdia3 is the dominant unit of the Diana Structural Complex. Unit APdia4 monzonite forms kilometric, tabular bodies parallel to the regional foliation. It is in sharp contact with the APdia3 host unit (Madore and Larbi, 2000, 2001).

Dating

Two isotopic ages were obtained in the Diana Structural Complex: an age of 2782 ±19 Ma in a tonalitic orthogneiss and an age of 2756 ±8 Ma in a porphyritic monzonite (David et al., 2009).