LITHOSTRATIGRAPHY OF INTRUSIVE ROCKS – CAN WE DO BETTER?

This session provides a forum for the discussion of both formal and informal approaches to Precambrian stratigraphy; emphasizing lithodemic, chronostratigraphic and geochronologic units. A critical area of discussion is the nomenclature of intrusive rocks, regardless of their age. Although stratigraphy and geological mapping are irrevocably linked, many workers are reluctant to create formal lithostratigraphic units for intrusive rocks, especially for individual intrusions. The current approach, using lithodemic units, was established in the 1983 North American Stratigraphic Code. In 2008, the British Geological Survey (BGS) introduced a lithodemic framework for the igneous rocks in the United Kingdom. Despite these advances, there are many challenges related to the application of the lithodemic units:

• What role should whole-rock and isotope tracer (e.g., Nd-Sm) chemistry play in the definition of intrusive stratigraphic units, especially at the suite and supersuite level; especially now that key elements can be determined on the outcrop using portable scintillometer and x-ray fluorescence instruments?
• Most stratigraphic codes or guides eschew the use of form terms for lithostratigraphic units (e.g., sill, pluton, stock, etc.) yet they continue to be used because many intrusions exhibit compositional ranges, even if slight, that make many workers reticent to use compositional terms.
• Do we need additional lithodemic units for detailed mapping purposes (3 levels in the Code versus 6 levels in the BGS proposal)?
• Current Stratigraphic Codes and Guides restrict the term complex to units that contain more than one class of rock. Yet the term is well entrenched for intrusive bodies with considerable textural and compositional heterogeneity, especially in alkaline systems. Should we embrace this usage through an amendment to the Code?
• We need to ensure that when new lithodemic or chronostratigraphic terms are created, that they get added to provincial and national stratigraphic lexicons quickly.
• Finally, with the availability of reliable high-precision geochronology, the need for lithostratigraphic units for intrusive rocks has been questioned - should chronostratigraphy be used exclusively?

Can we do better? In short - yes.