Structure and Petrology of the Partridge Island Block and its Role in the Tectonic Evolution of the Saint John Area, New Brunswick

Structure and Petrology of the Partridge Island Block and its Role in the Tectonic Evolution of the Saint John Area, New BrunswickRobert TreatMSc thesis defenseSpring, 2011

What is the Partridge Island block (PIB)? Location and background geology Field relations Petrography Structure Deformation history Geochemistry Regional implications Conclusions

Two areas of highly deformed (and previously largely ignored) rocks in the Saint John area of southern New Brunswick.

This study was undertaken to document the petrology and structure of these two areas in order better understand the history of these rocks.

Appalachian Orogen

Hibbard (2006)

Saint JohnCobequid Highlands

Southern New BrunswickSaint John area located in Caledonia terrane, near boundary with Meguma.

Boundary between Avalonia and Meguma along Cobequid-Chedabucto Fault Zone (CCFZ).

CCFZ has been inferred to be located just offshore, but its exact nature and extent in this area are largely uncertain.Wolczanski et al. (2006)

Southern New BrunswickSome problems: Deformed rocks ofunknown provenance

Nature of contacts

Unit ages

Southern New BrunswickChanges as a result of this study:

PIB extended andclassified

Contacts defined

Age and stratigraphic relationshipsredefined

Southern New BrunswickPIB divided into two areas

Tiner Point area

Red Head areaChanges as a result of this study:

PIB extended andclassified

Contacts defined

Age and stratigraphic relationshipsredefined

Field Relations - Tiner Point areaPIB in steeply faulted contact with the Taylors Island Formation and Saint John Group.

Internally, it consists of three units:

Two variably mylonitized meta-igneous units Alkali feldspar granite (ca. 346 Ma) Leucotonalite

Tectonic slivers of quartzite and phyllite along coast (Tiner Point formation).

Field Relations - Tiner Point area

Field Relations - Red Head areaPIB in shallowly faulted contact with Balls Lake Formation (alluvial fan complex). Distribution of Balls Lake Formation due to lateral facies changes and folding.

Internally, PIB consists of two units: Quartz monzodiorite Leucotonalite

Unconformably overlain by the Taylors Island Formation.

Field Relations - Red Head area

Petrography Tiner Point areaAlkali feldspar granite variably mylonitizedprotomylonitemyloniteultramylonite

Petrography Tiner Point areaLeucotonalite highly strained and recrystallized

Petrography Tiner Point areaTiner Point formation composite fabricBedding transposed with schistosity, which is in places folded.

Petrography Red Head areaQuartz monzodiorite variably mylonitized

Ca. 332 Ma 40Ar/39Ar muscovite cooling age

Petrography Red Head areaLeucotonalite variably mylonitized

ultramylonitephyllonite

Petrography Taylors Island FormationBasalt altered and variably deformedRed Head area: carbonate-epidote alteration.

Tiner Point area: epidote-chlorite-albite alteration.

Highly deformed.

Structure Tiner Point areaPartridge Island block: mylonitic foliation strikes NE and dips steeply SE, with a mineral stretching lineation that plunges steeply NE/SW.

Composite fabric of Tiner Pointformation has similar orientation,but has minor thrust-related foldingwith fold axes plunging shallowlyto the SE.

Poles to myloniticfoliation (n = 11)Poles to compositefoliation (n = 9)Fold axes(n = 5)Mineral lineations(n = 11)

Structure Tiner Point areaTaylors Island Formation: bedding strikes N/NE and dips E/SE, locally overturned.

Cleavage strikes NE and dips tothe SE/NW.

Fold axes plunge shallowly to moderately to the NE/SW. Poles to bedding(n = 35)Poles to cleavage(n = 35)Fold axes(n = 18)

Structure Red Head areaPartridge Island block: mylonitic foliation strikes NE and dips shallowly to the SE, with a mineral stretching lineation that plunges shallowly SE.

Cleavage in overlying Taylors Island Formation dips shallowly E/NE.

Poles to myloniticfoliation (n = 33)Mineral lineations(n=21)Poles to TIFCleavage (n = 18)

Structure Red Head areaBalls Lake Formation: bedding north ofcrystalline block dips SE whereasbedding south of block dips NW.

South of the crystalline block, way-upindicators show clusters of beds whichare both right-way up and overturned.

Cleavage dips uniformly SE, and foldaxes plunge shallowly to the NE/SW.Poles to bedding(n = 143)Poles to cleavage(n = 63)Fold axes (n = 42) andIntersection lineations (n=18)

Deformation HistoryStructural relationship of the internal components of Partridge Island block and its overall relationship with surrounding units can be explained by a sequence of three deformational events:

D1 early mylonitization of meta-igneous units in both Red Head and Tiner Point areas prior to ca. 332 Ma, followed by uplift and deposition of the Taylors Island Formation on top of the exposed crystalline rocks.

This attributes an Early Carboniferous age to the Taylors Island Formation.

Deformation HistoryD2 NE/SW directed thrusting emplaced the units of the Partridge Island block structurally above and beside younger rocks of theBalls Lake and Taylors Island Formation, and interlayered them as tectonic slices. Deformation in overlying Taylors Island Formation.

AA

Deformation HistoryD3 NW-directed compression folded the Balls Lake Formation into a syncline around the crystalline block in Red Head area, and also folded the Taylors Island Formation in Tiner Point area.

BBCC

Geochemistry PIB felsic rocksDiscrimination diagrams:

Alkali feldspar granite and leucotonalite from Red Head area plot as linear trend ranging from peralkaline to peraluminous, and as A-type and within-plate granite.

Leucotonalite from Tiner Point area and felsic layer in quartz monzodiorite plot in volcanic-arc fields.

Geochemistry PIB felsic rocksSpider diagrams normalized to averageA-type granite:

Alkali feldspar granite andleucotonalite from Red Head areaplot as near-normal, whereasleucotonalite from Tiner Point areaand felsic layer are depleted.

Near-normal samples depleted inMobile elements Rb, Ba, and K asThe result of albitic alteration.Leucotonalite from Red Head areaalso depleted in Sr, likely albitizedequivalent of the alkali feldspar granite.

Geochemistry Taylors Island FormationDiscrimination diagrams:

Compositionally, plot as basalt.

Samples of Taylors Island Formation basalt from the Tiner Point and Red Head areas plot closely together in within-plate tholeiite fields on a variety of diagrams.

Emplacement in extensional regime.

Geochemistry Quartz monzodioriteQuartz monzodiorite represents intermediate composition, likely formed as the result of hybridization of a felsic and mafic magma.

Compared to the hybridized West Moose River pluton of slightly older age (ca. 361 Ma) in the Cobequid Highlands, the alkali feldspar granite and quartz monzodiorite display similar trends in major and trace element oxide content with respect to SiO2.Major element oxides

West Moose River pluton data from Pe-Piper et al. (1989, 1991)

Geochemistry Quartz monzodioriteDiscrimination diagrams show similarranges and trends.

Taylors Island Formation correlates with mafic end member, alkali feldspar granite with felsic end member, and quartz monzodiorite with intermediate compositions.

Suggests Early Carboniferous age for quartz monzodiorite.

Regional ImplicationsThis study documents multiple periods of deformation in the Saint John area, which range in style from ductile to brittle, as well as bimodal magmatism in the Carboniferous.

This evidence suggests that magmatism and widespread deformation in the Saint John area is likely related to dextral transpression along the CCFZ, much like in the Cobequid Highlands.Wolczanski et al. (2006)

Regional ImplicationsDextral transpression along this restraining bend can be accommodated by the development of a series of right-stepping wrench faults along the coast of southern New Brunswick.

Conclusions PIB consists of the Tiner Point and Red Head areas; Tiner Point area: alkali feldspar granite, leucotonalite, and metasedimentary rocks. Meta-igneous rocks display A-type characteristics, emplaced during the Early Carboniferous. Red Head area: quartz monzodiorite and leucotonalite, unconformably overlain by Taylors Island Formation. Igneous rocks of intermediate composition the result of hybridization of felsic and mafic magma.

PIB involved in three stages of deformation: D1 ductile event prior to ca. 332 Ma. D2 - thrusting which emplaced the rocks of the PIB structurally above and beside younger rocks. D3 - later folding of younger rocks in response to NW-directed compression.

This history can be explained as the result of a restraining bend in the CCFZ, which lead to the development of a series of right-stepping wrench faults along the coast of southern New Brunswick.