1. Kehoe Beach Salinian Outcrop, Point Reyes, California
KHB-01: Hornblende Biotite Granodiorite with ~ 20% mafic minerals
KHB-02: Biotite Granodiorite with ~ 25% mafic minerals
KHB-03: Biotite Hornblende Tonalite with ~50% mafic minerals
KHB-04:Alaskite with ~3% mafic minerals
KHB-01
KHB-02
KHB-03
KHB-04
Fig. 1: Studied outcrop. See sample compositions in figure’s legend.
20
40
60
80
100
Age(Ma)
Kehoe Beach Intrusives Single Zircon 206Pb/238U ages box heights are 2σ
Host Rock KHB01
KHB02
KHB04
Maximum Age
of KHB-03
KHB03
Interpretation of Findings
Methods
Motivation
Abstract
Dating Intrusions in the Salinian Block using Single Zircon
U-Pb CA-TIMS Analysis
Sam DeNicola1, Roland Mundil2, Chi-Yuen Wang1, Courtney Sprain1,2, Michael Manga1
AGU Fall Meeting, San Francisco, December 2015
The Salinian block (SB) consists largely of metasedimentary basement intruded by suites of
compositionally heterogeneous Early to Late Cretaceous rocks. Exposed throughout Central
and Northern California, it is bordered to the northwest by the Nacimiento fault, to the
southwest by the Big Pine fault and to the east by the San Andreas fault. These faults are
thought to have caused a dextral offset of the SB; however, its original position is still
debated. Published radioisotopic ages from various techniques on SB intrusives suggest that
the initial magmatic activity of the Salinian arc began at around 100 Ma and concluded some
40 myr later, but the timing of individual magmatic pulses are yet to be fully understood
because arguably most of the available geochronological data are compromised with respect
to precision and accuracy. Previous studies include U-Pb zircon ages from different
lithologies and areas using multi-crystal TIMS as well as ICPMS, and SIMS techniques.
Whereas the former technique is now known to be prone to producing inaccurate, although
precise, ages due to averaging effects from older inheritance and open system behavior, the
latter are often associated with analytical uncertainty at the several percent level as well as
unrecognized open system behavior, making the interpretation of age data notoriously
difficult. Complementary geochemical, petrological and structural studies have been
conducted, but are also difficult to interpret in absence of precise and accurate
geochronological data because geochemical trends and intrusion rates cannot be
reconstructed. Here, we examine a group of intrusive rocks from the Northern California
SB that include a biotite-hornblende granodiorite host rock which is intruded by a complex
array of igneous dikes of varying lithology ranging.We perform closed system single zircon
U-Pb CA-TIMS analysis and obtain a range of 206Pb/238U ages (with permil level resolution)
for the host rock and a crosscutting intrusions. Our findings show variability in the origin of
each intrusion, revealing a complex geological story visible at the outcrop scale.
Ø Four distinct samples selected from formation at Kehoe Beach for laboratory analysis.
Ø Thin sections made for optical microscope compositional characterization.
CA-TIMS U-Pb Single Crystal Zircon Geochronology
Ø Before TIMS analysis, selected zircons are thermally annealed at 850° C for 48 hours and
then undergo chemical abrasion using concentrated hydrofluoric acid in pressurized
dissolution capsules at 220° C for 8 hours (Mattinson 2005, Mundil et al. 2004a).
u The range of measured zircon crystallization age for the basement rock and first
intrusion is of typical timescales of zircon crystallization within intrusive units (Memeti
et al. 2014).
u We interpret the gap between KHB-01 and KHB-02 as evidence for independent
and distinct magmatic events.
u We interpret the full range of zircon crystallization ages of KHB-03 as (1) a result of
the intrusion pulling in older material, (2) of younger zircons crystallizing around older
inherited cores, or (3) the combination of the two processes.
u Thus, only the youngest KHB-03 ages are geologically meaningful.The
maximum zircon crystallization age of 32.70 Ma suggests a relationship to the ridge
trench encounter of Southern California ~ 32 Ma.
u KHB-04 yielded few zircons and we submit that all found are xenocristic.
u Field observed cross-cutting relationships show that the intrusives vary in age
with oldest to youngest being KHB-01, KHB-02, KHB-03, KHB-04.
u Optical microscope thin section characterization gives rough composition (see
figure 5 legend).
u Host rock KHB-01 206Pb/238U zircon crystallization ages show a range of ~900
ky from 98.19 Ma to 97.34 Ma.
u KHB-02 206Pb/238U zircon crystallization ages show a range from 98.57 Ma to
90.06 Ma with a clustering around 93 Ma.
u KHB-03 has a 206Pb/238U maximum zircon crystallization age of 32.70 Ma.
u KHB-04 has a 206Pb/238U maximum zircon crystallization age of 95.27 Ma.
u All samples are analytically concordant (in the sense of 206Pb/238U, 207Pb/235U
equivalence).
References
Atwater,T. (2007), Northeast Pacific and Western North American Plate Tectonic History, animation,
Available from: http://emvc.geol.ucsb.edu/2_infopgs/IP4WNACal/bNEPacWNoAmer.html
(Accessed 9 December 2015).
Mattinson, J.M., (2005), Zircon U-Pb chemical abrasion (“CA-TIMS”) method: Combined annealing
and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages,
Chemical Geology, v. 220, no. 1–2, p. 47–66.
Memeti,V., Paterson, S., Mundil, R., (2014), Day 4: Magmatic evolution of the Tuolumne Intrusive
Complex, Field Guides, 34, p. 43-74.
Mundil, R., Ludwig, K.R., Metcalfe, I., and Renne, P.R., (2004a),Age and timing of the Permian mass
extinctions: U/Pb dating of closed-system zircons, Science, v. 305, no. 5691, p. 1760–1763.
V33D-3136
Future Work
u Geochemical analysis (XRF, EMP) will potentially provide evidence to determine
the depth of emplacement for each intrusive body as well as aid in the search
for the origin location of KHB-03.
u LA-ICP-MS analysis on KHB-03 zircons will allow for greater spatial
analysis, as well as selection of zircons to determine maximum age zircons to
be analyzed with CA-TIMS analysis.
u High-resolution single zircon U-Pb CA-TIMS analysis throughout the Salinian
complex outcrops have the potential to reveal further complexities of the
Salinian complex.
Timing of Intrusions
Figure 4: U-Pb emplacement ages for KHB-01 and KHB-02. Note the
presence of ~98 Ma xenocrystic zircons in KHB-02 and then a ~93 Ma
autocryst age.
Figure 3: U-Pb emplacement ages for Kehoe Beach outcrop. Order of
samples here is oldest to youngest as determined by proposed field
cross-cutting relationships.
1 Department of Earth and Planetary Sciences, University of California, Berkeley, CA 94720, USA
2 Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709, USA
Fig. 5: Western
North America
~32 Ma, Plate
reconstruction
slides from
Atwater, 2007.
Shows
approximate
outcrop location.
Fig. 6: Western
North America
~15 Ma. Motion
along the San
Andreas has
begun to
transport the
Salinian block
north west.
Fig. 7: Present
Day Western
North America.
Salinian
transported ~
500km north
west.
Tectonic Reconstruction
Fig. 2: Salinian Block in California
(Mattinson, 1978).
Summary of Ages
Cretaceous
Volcanism
15 Ma
Present
86
88
90
92
94
96
98
100
Age(Ma)
KHB01 and KHB02 Single Zircon 206Pb/238U ages box heights are 2s
Host Rock KHB01
KHB02
32 Ma
We study a host granodiorite rock of the Salinian block and three igneous dikes that intrude
it.We seek to resolve the following questions:
1. What are the age relations between the host rock and the intrustions?
2. Are the tectono-magmatic events they are recording related or are they distinctly
different?
We will resolve these questions using high-resolution geochronological techniques to
determine the absolute ages of this as yet poorly studied intrusive suite.