1. Slow incremental growth of plutons by magmatic crack-seal
No big tank:
Slow incremental growth of plutons by
magmatic crack-seal
John M. Bartley
University of Utah
Drew S. Coleman
Allen F. Glazner
Univ. of North Carolina

4. McDoogle pluton
(Mahan et al., 2003)

5. Simplified after Mahan(2000)

7. Phantom internal contact in McDoogle granodiorite

9. Presence or absence of wall-rock screens in a composite pluton depends
on relative fracture resistance of
older increments and wall rock
Next fracture in
wall rock
Next fracture
in intrusion
The presence or absence of wall rock sheets in a sheeted pluton is governed by whether it is easier to crack the wall rock or previously emplaced granite. The McDoogle represents the case in which the wall rock was easier to crack.

11. Tuolumne Intrusive Suite, Coleman et al. (2004)
U/Pb dates,
Tuolumne Intrusive Suite, Coleman et al. (2004)
85.4 +/- 0.1
88.1 +/- 0.2
93.5 +/- 0.7
93.1 +/- 0.1
95 +/- 1
92.8 +/- 0.1
91.7 +/- 0.2
91.1 +/- 0.1
88.8 +/- 0.8
Protracted incremental emplacement may be the norm. Very few plutons have been examined in this way, but those that have are turning up similar stories, e.g.--
Jenny Matzel’s recent results from the Mt. Stuart tonalite in Washington state indicate growth of that complex over 5.5 Ma.
Susanne Meschter McDowell’s recent MS thesis work on the Mineral Mountains plutonic complex in SW Utah indicates construction over a duration of 8 Ma.
Sandy Cruden’s new U/Pb dates from several plutons in the Coastal Batholith in Chile indicate construction over 2-5 Ma.
So large plutons appear commonly to be constructed incrementally over millions of years.

12. Internal contact in Half Dome Granodiorite

13. Geologic map of May Lake-Murphy Creek area, Yosemite National Park, CA
Detailed field mapping in the TIS (left), focusing on the Half Dome (light blue mafic granodiorite and pink leucogranite), confirms the composite nature of the intrusion. Discrete units that show up at the map scale typically are several hundred meters thick (note that contacts dip moderately to steeply westward so that outcrop widths exceed actual thicknesses). What the geologic map does not readily show, but is illustrated by the magnetic susceptibility map of the same area (right), is that the map-scale units grade upward (westward) from mafic to more felsic compositions, with a sharp contact between the felsic top and the next mafic unit which commonly contains abundant schlieren and enclave swarms.

15. Phantom internal contact in Half Dome granodiorite