The Igneous, Sedimentary and Metamorphic History of Mount Royal
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Department of Earth and Planetary Sciences McGill University Understanding Planet Earth (186-201) by Venitia Bodycomb Geologic Field Trip 2 THE IGNEOUS, SEDIMENTARY AND METAMORPHIC HISTORY OF MOUNT ROYAL This document was originally developed by Venetia Bodycomb and David Dolejs. Their wisdom is much appreciated. (The following description was liberally modified from a previous 186-201 handout which was based on Clark, 1972 and Gelinas, 19721.) The Geology of Mount Royal Mount Royal is one of the Monteregian Hills and is the prominent topographic feature of the Montreal area with a maximum elevation of 254 m above sea level (just a little higher than the restaurant at the top of Place Ville Marie). Mount Royal was formed by magma intruding upward through the sandstone-limestone-shale sequence of the St. Lawrence Lowlands. The oldest rocks of the St. Lawrence Lowlands sedimentary sequence were deposited during Early Cambrian time almost 600 million years ago. The youngest rocks of this sequence that we can now see intruded by the Mount Royal intrusion are approximately 470 million years old. The magma came considerably later, approximately 125 million years ago, and the resulting igneous intrusion is called a stock (a small pluton). During the past 125 million years, erosion has removed at least 1.5 kilometres of rock, so what we are seeing today is only part of the story. Sedimentary rocks The sedimentary sequence of the St. Lawrence Lowlands is up to 2.6 km thick today and represents sediments laid down on the old North American passive continental margin beginning more than 500 million years ago. At the time, the eastern north American continent was bordered by the Iapetus Ocean, which has since closed and been re-opened as the Atlantic Ocean. Sedimentation in a shallow inland sea of the Iapetus Ocean began more than 500 million years ago with the deposition of fluvial to shallow marine sandstones (Potsdam Group). These rocks were subsequently overlain by shallow marine limestones (commencing with the Lower Ordovician Beekmantown Group), and finally the Utica black shales. The sequence of these rock types (sandstone -> limestone -> shale) indicate that marine conditions underwent a prolonged period of subsidence (transgression) in which water levels were rising over the land. Subsequent uplift and disappearance of the inland sea (regression) was caused by the formation of the Appalachian mountains to the southeast during ocean closure. Clastic sediments from the 1Clark, T.H., 1972. Montreal Area. Ministere de l’Energie et des Ressources du Quebec, Geol. Report 152, 244p. Gelinas, L., 1972. Geology of Mount Royal. 24th International Geological Congress, Montreal, Guidebook B-12, 40p. eroding new mountains filled the sedimentary basin, covering the shales, forming the Lorraine and Queenston rock groups. Erosion (by rivers, glaciers, etc.) has since removed most of these rocks from the Montreal area, except for the limestones and a small vestige of the shale. The sandstone is buried under all the limestone. How much of these rocks were already eroded away at the time of the magma intrusion 125 million years ago? We cannot tell with certainty, but we do have evidence that 400 million year old rocks were still present at the time of the Mount Royal intrusion. This evidence is in the form of limestone fragments in a diatreme breccia on St. Helen’s Island. The diatreme breccia is an explosion pipe related to the Monteregian intrusive event: after magma violently shot up through the area that is now St. Helen’s Island, pieces of ~400 Ma rock fell back into the explosion pipe where they have since been preserved. Igneous rocks The igneous rocks of Mount Royal comprise intrusive rocks of various compositions emplaced as a series of magmatic pulses 125 million years ago. The first and volumetrically most important event involved basaltic magma that cooled to form a gabbro stock that we call “Mount Royal”. Successive magmatic pulses gave rise to swarms of thin dykes around the margins of the stock. Some of the rocks are porphyritic (recall: big crystals in a fine-grained matrix). Some other rocks are explosive breccias that imply a violent (rapid) rise of magma. Both of these rock types provide evidence that the intrusive complex we now see was formed under a 1.5 km sedimentary rock cover which has since been eroded (this depth corresponds to a pressure of about 0.5 kilobars). Metamorphism The sedimentary rocks in the vicinity of the intrusion have been thermally metamorphosed (recrystallized by heat) to form what we call a contact aureole some 800 m wide. Contact metamorphism means that if a rock at temperature X is suddenly heated up to temperature Y by a nearby magma, then the rock’s minerals that were stable at temperature X will have to change (recrystallize) into other minerals that are stable at the new, elevated temperature Y. At Mount Royal, we would therefore expect that the sedimentary rocks closest to the intrusion will show the greatest change and contain the higher temperature mineral assemblages, and that the metamorphic aureole will diminish in intensity moving away from the intrusion. In fact, it is only within the innermost 100 m of the contact metamorphic aureole that we can observe the effects of thermal change with the naked eye. The results are “bleaching” of the dark grey limestone to form white marble, and metamorphism of the soft and fissile shale to a slightly coarser-grained and harder hornfels. The Big Picture The rocks of the St. Lawrence Lowlands represent continental shelf sequences deposited on the western passive continental margin of the Iapetus Ocean - the ocean that existed before the Atlantic Ocean. Part of the Iapetus Ocean system included widespread inland seas that covered much of what is now known as North America (Lake Champlain is a remnant feature of those seas). The shelf deposits were uplifted during Middle Ordovician time, causing the inland seas to retreat and sea levels to drop relative to the continent. This uplift was caused by the onset of mountain building as the Iapetus began to close as the African and North America collided around 350 million years ago. The mountains that formed during this closure are known as the Appalachians, and as other continents collided around the world, the supercontinent Pangea was created. The rocks comprising the Appalachians were intensely deformed, but the St. Lawrence Lowlands rocks to the northwest of the mountain chain were too far away from the convergent plate boundary to have suffered extensive deformation. Instead, they were only slightly deformed into large broad open folds (km scale), the most prominent of which is the Chambly-Fortierville Syncline (Mount Royal is located at the southern end of this structure). Folds are useful in that they tilt the original horizontal layers allowing erosional processes to effectively expose a cross section of the underlying sedimentary rocks. Mount Royal is only one of the Monteregian Hills formed by similar intrusions. The hills extend in an approximate west-east direction from Oka to Lac Megantic in the Appalachians. They are all alkaline in nature, which means that they are enriched in some of the alkali elements (like K and Na) with respect to other igneous rocks. Alkaline rocks tend to be associated with continental rift zones, and it has been suggested that the Monteregian province is similar to the East African Rift Zone (Kumarapeli & Saull, 1966). Long after the emplacement of the Mount Royal intrusion, an estimated 1.5 to 2 km of overlying sedimentary and igneous rocks were removed by running water and ice (during the last glaciation) to form the present surface. Vocabulary: basement: older rock that underlies younger rock and is separated from the younger rock by an unconformity (erosional event); typically metamorphosed and deformed. carbonates: carbonate-rich rock (typically limestone) derived from chemically precipitated calcite (=calcareous) mud. Limestone composition = calcite = CaCO3. diatreme breccia: a volcanic pipe formed by a gaseous explosion and filled with large angular fragments in an igneous matrix. dyke: thin, tabular intrusive body that cross-cuts the rock layers it intrudes. fluvial: of or pertaining to rivers. gabbro: mafic intrusive rock composed of Ca-feldspar and pyroxene; typically dark coloured. Iapetus Ocean: the ocean that disappeared about 350 Ma when Africa collided with North America during the formation of Pangea - later rifting during the breakup of Pangea produced the Atlantic Ocean in its place, leaving remnants of Africa and other smaller continental masses accreted onto N. America. marine: of or pertaining to seas/oceans. porphyritic: igneous rock texture whereby bigger crystals are surrounded by a finer-grained matrix. 1 sandstone: rock formed from clastic (detrital) sediments (typically quartz-bearing) with grain size averaging /16 mm to 2 mm in diameter. shale: a clastic rock formed from clay (mud) with minor silt component. (black shale: shale containing organic matter to give it a blackish colour) stratigraphic section: a column displaying a chronologic succession of sedimentary rock from older (bottom) to younger (top). stock: irregularly shaped intrusive igneous body <100km2; type of intrusive igneous body. QUESTIONS RELATED TO SPECIFIC FIELD TRIP STOPS: Stop 1: Meeting Place (corner Peel and Pine avenue) Most of Montreal is underlain by limestone, however the rock used to build the flagstone steps leading up the mountain is not limestone. a) What rock type is it? What criteria did you use to determine this? b) Using your stratigraphic section in the handout, make a guess as to the stratigraphic name for this rock (i.e., Trenton, Utica, Potsdam, etc.). c) On the top step of the first set of steps, there is a flagstone with a distinctive sedimentary structure (somewhat worn down). What is that structure and what does it signify? Notice that the wall to your right as we climb higher is made of grey limestone blocks.